diff --git a/ucalc/Constants.cs b/ucalc/Constants.cs
index d850bf3..c44ebbc 100644
--- a/ucalc/Constants.cs
+++ b/ucalc/Constants.cs
@@ -2,6 +2,7 @@
 using System.Collections.Immutable;
 using System.Linq;
 using System.Windows.Media;
+using UCalc.Controls;
 using UCalc.Data;
 
 namespace UCalc
@@ -11,9 +12,15 @@ namespace UCalc
         public static readonly SolidColorBrush MainColor = Brushes.White;
         public static readonly SolidColorBrush SubMainColor = new SolidColorBrush(Color.FromRgb(0x00, 0x7A, 0xCC));
 
-        public const string DecimalFormat = "0.00";
+        public const int InternalPrecision = 6;
+        public const int DisplayPrecision = 2;
         public const string DateFormat = "dd.MM.yyyy";
 
+        public static readonly string InternalPrecisionFormat =
+            Helpers.PrecisionToFormat(InternalPrecision, InternalPrecision - 2);
+
+        public static readonly string DisplayPrecisionFormat = Helpers.PrecisionToFormat(DisplayPrecision);
+
         public static readonly ImmutableList<string> SalutationStrs =
             ((Salutation[]) Enum.GetValues(typeof(Salutation))).Select(value => value.AsString()).ToImmutableList();
 
diff --git a/ucalc/Controls/Helpers.cs b/ucalc/Controls/Helpers.cs
index c3cfab3..bc974e7 100644
--- a/ucalc/Controls/Helpers.cs
+++ b/ucalc/Controls/Helpers.cs
@@ -37,5 +37,10 @@ namespace UCalc.Controls
         {
             return start1.IsBetween(start2, end2) || start2.IsBetween(start1, end1);
         }
+
+        public static string PrecisionToFormat(int precision, int optional = 0)
+        {
+            return $"0.{new string('0', precision - optional)}{new string('#', optional)}";
+        }
     }
 }
\ No newline at end of file
diff --git a/ucalc/CostEntryDetailsWindow.xaml b/ucalc/CostEntryDetailsWindow.xaml
new file mode 100644
index 0000000..d098d48
--- /dev/null
+++ b/ucalc/CostEntryDetailsWindow.xaml
@@ -0,0 +1,207 @@
+<Window x:Class="UCalc.CostEntryDetailsWindow"
+        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
+        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
+        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
+        xmlns:local="clr-namespace:UCalc"
+        xmlns:controls="clr-namespace:UCalc.Controls"
+        mc:Ignorable="d"
+        Title="Details und Hilfswerkzeuge"
+        Width="340"
+        SizeToContent="Height"
+        ResizeMode="NoResize"
+        WindowStyle="ToolWindow"
+        WindowStartupLocation="CenterOwner"
+        ShowInTaskbar="False"
+        Closed="OnClosed">
+
+    <StackPanel>
+        <TabControl x:Name="TabControl">
+            <TabItem Header="Taschenrechner">
+                <StackPanel Margin="8"
+                            Height="250">
+                    <TextBox Name="ResultTextBox"
+                             MinHeight="22"
+                             KeyUp="OnResultTextBoxKeyUp" />
+
+                    <Grid Margin="0, 8, 0, 0">
+                        <Grid.RowDefinitions>
+                            <RowDefinition Height="30" />
+                            <RowDefinition Height="30" />
+                            <RowDefinition Height="30" />
+                            <RowDefinition Height="30" />
+                            <RowDefinition Height="30" />
+                        </Grid.RowDefinitions>
+
+                        <Grid.ColumnDefinitions>
+                            <ColumnDefinition Width="1*" />
+                            <ColumnDefinition Width="1*" />
+                            <ColumnDefinition Width="1*" />
+                            <ColumnDefinition Width="1*" />
+                        </Grid.ColumnDefinitions>
+
+                        <Button Grid.Row="0" Grid.Column="0" Content="CE" Click="OnCEClick" />
+                        <Button Grid.Row="0" Grid.Column="1" Content="(" Click="OnButtonClick" />
+                        <Button Grid.Row="0" Grid.Column="2" Content=")" Click="OnButtonClick" />
+                        <Button Grid.Row="0" Grid.Column="3" Content="/" Click="OnButtonClick" />
+                        <Button Grid.Row="1" Grid.Column="0" Content="1" Click="OnButtonClick" />
+                        <Button Grid.Row="1" Grid.Column="1" Content="2" Click="OnButtonClick" />
+                        <Button Grid.Row="1" Grid.Column="2" Content="3" Click="OnButtonClick" />
+                        <Button Grid.Row="1" Grid.Column="3" Content="*" Click="OnButtonClick" />
+                        <Button Grid.Row="2" Grid.Column="0" Content="4" Click="OnButtonClick" />
+                        <Button Grid.Row="2" Grid.Column="1" Content="5" Click="OnButtonClick" />
+                        <Button Grid.Row="2" Grid.Column="2" Content="6" Click="OnButtonClick" />
+                        <Button Grid.Row="2" Grid.Column="3" Content="-" Click="OnButtonClick" />
+                        <Button Grid.Row="3" Grid.Column="0" Content="7" Click="OnButtonClick" />
+                        <Button Grid.Row="3" Grid.Column="1" Content="8" Click="OnButtonClick" />
+                        <Button Grid.Row="3" Grid.Column="2" Content="9" Click="OnButtonClick" />
+                        <Button Grid.Row="3" Grid.Column="3" Content="+" Click="OnButtonClick" />
+                        <Button Grid.Row="4" Grid.Column="0" Content="Aufrunden" Click="OnRoundClick" />
+                        <Button Grid.Row="4" Grid.Column="1" Content="0" Click="OnButtonClick" />
+                        <Button Grid.Row="4" Grid.Column="2" Content="," Click="OnButtonClick" />
+                        <Button Grid.Row="4" Grid.Column="3" Content="=" Click="OnEvalClick" />
+                    </Grid>
+                </StackPanel>
+            </TabItem>
+
+            <TabItem Header="Einheitenrechner">
+                <StackPanel Margin="8"
+                            Height="250">
+                    <TextBlock
+                        TextWrapping="Wrap"
+                        Text="Dieser Rechner kann benutzt werden, um den Verbrauch von einem Preis pro Einheit umzurechnen (z.B. Wasserverbrauch)."
+                        Foreground="{x:Static local:Constants.SubMainColor}" />
+
+                    <DockPanel Margin="0, 12, 0, 0">
+                        <Label DockPanel.Dock="Left"
+                               Content="Gesamtbetrag:"
+                               Width="160"
+                               Foreground="{x:Static local:Constants.SubMainColor}"
+                               VerticalAlignment="Center" />
+
+                        <controls:ErrorIcon Margin="12, 0, 0, 0"
+                                            DockPanel.Dock="Right"
+                                            VerticalAlignment="Center"
+                                            Property="{Binding Path=Details.TotalAmount, RelativeSource={RelativeSource AncestorType=local:CostEntryDetailsWindow}}" />
+
+                        <TextBox
+                            VerticalAlignment="Center"
+                            MinHeight="22"
+                            Text="{Binding Path=Details.TotalAmount.Value, RelativeSource={RelativeSource AncestorType=local:CostEntryDetailsWindow}, UpdateSourceTrigger=PropertyChanged}" />
+                    </DockPanel>
+
+                    <DockPanel Margin="0, 4, 0, 0">
+                        <Label DockPanel.Dock="Left"
+                               Content="Gesamtverbrauch:"
+                               Width="160"
+                               Foreground="{x:Static local:Constants.SubMainColor}"
+                               VerticalAlignment="Center" />
+
+                        <controls:ErrorIcon Margin="12, 0, 0, 0"
+                                            DockPanel.Dock="Right"
+                                            VerticalAlignment="Center"
+                                            Property="{Binding Path=Details.UnitCount, RelativeSource={RelativeSource AncestorType=local:CostEntryDetailsWindow}}" />
+
+                        <TextBox
+                            VerticalAlignment="Center"
+                            MinHeight="22"
+                            Text="{Binding Path=Details.UnitCount.Value, RelativeSource={RelativeSource AncestorType=local:CostEntryDetailsWindow}, UpdateSourceTrigger=PropertyChanged}" />
+                    </DockPanel>
+
+                    <controls:HighlightButton Margin="0, 8, 0, 0"
+                                              HighlightForeground="{x:Static local:Constants.SubMainColor}"
+                                              HighlightBackground="{x:Static local:Constants.MainColor}"
+                                              Click="OnAddDiscountClick">
+                        <Viewbox Width="16"
+                                 Height="16"
+                                 Margin="12, 8, 4, 8"
+                                 Stretch="Uniform">
+                            <Canvas Width="512" Height="512">
+                                <Canvas.RenderTransform>
+                                    <TranslateTransform X="0" Y="0" />
+                                </Canvas.RenderTransform>
+                                <Path>
+                                    <Path.Data>
+                                        <PathGeometry
+                                            Figures="M256 8C119 8 8 119 8 256s111 248 248 248 248-111 248-248S393 8 256 8zm144 276c0 6.6-5.4 12-12 12h-92v92c0 6.6-5.4 12-12 12h-56c-6.6 0-12-5.4-12-12v-92h-92c-6.6 0-12-5.4-12-12v-56c0-6.6 5.4-12 12-12h92v-92c0-6.6 5.4-12 12-12h56c6.6 0 12 5.4 12 12v92h92c6.6 0 12 5.4 12 12v56z"
+                                            FillRule="NonZero" />
+                                    </Path.Data>
+                                </Path>
+                            </Canvas>
+                        </Viewbox>
+
+                        <Label Content="Abzug hinzufügen"
+                               Margin="0, 8, 12, 8" />
+                    </controls:HighlightButton>
+
+                    <ScrollViewer HorizontalScrollBarVisibility="Hidden"
+                                  VerticalScrollBarVisibility="Visible"
+                                  Height="80">
+                        <ItemsControl
+                            ItemsSource="{Binding Path=Details.DiscountsInUnits, RelativeSource={RelativeSource AncestorType=local:CostEntryDetailsWindow}}">
+                            <ItemsControl.ItemTemplate>
+                                <DataTemplate>
+                                    <DockPanel Margin="0, 0, 0, 4">
+                                        <Label DockPanel.Dock="Left"
+                                               Content="Abzug vom Verbrauch:"
+                                               Width="160"
+                                               Foreground="{x:Static local:Constants.SubMainColor}"
+                                               VerticalAlignment="Center" />
+
+                                        <controls:HighlightButton DockPanel.Dock="Right"
+                                                                  HighlightForeground="Red"
+                                                                  HighlightBackground="White"
+                                                                  Click="OnDiscountDeleteClick">
+                                            <Viewbox Width="16"
+                                                     Height="16"
+                                                     Margin="8, 6, 8, 6"
+                                                     Stretch="Uniform">
+                                                <Canvas Width="448" Height="512">
+                                                    <Canvas.RenderTransform>
+                                                        <TranslateTransform X="0" Y="0" />
+                                                    </Canvas.RenderTransform>
+                                                    <Path>
+                                                        <Path.Data>
+                                                            <PathGeometry
+                                                                Figures="M32 464a48 48 0 0 0 48 48h288a48 48 0 0 0 48-48V128H32zm272-256a16 16 0 0 1 32 0v224a16 16 0 0 1-32 0zm-96 0a16 16 0 0 1 32 0v224a16 16 0 0 1-32 0zm-96 0a16 16 0 0 1 32 0v224a16 16 0 0 1-32 0zM432 32H312l-9.4-18.7A24 24 0 0 0 281.1 0H166.8a23.72 23.72 0 0 0-21.4 13.3L136 32H16A16 16 0 0 0 0 48v32a16 16 0 0 0 16 16h416a16 16 0 0 0 16-16V48a16 16 0 0 0-16-16z"
+                                                                FillRule="NonZero" />
+                                                        </Path.Data>
+                                                    </Path>
+                                                </Canvas>
+                                            </Viewbox>
+                                        </controls:HighlightButton>
+
+                                        <controls:ErrorIcon Margin="12, 0, 8, 0"
+                                                            DockPanel.Dock="Right"
+                                                            VerticalAlignment="Center"
+                                                            Property="{Binding Path=.}" />
+
+                                        <TextBox
+                                            VerticalAlignment="Center"
+                                            MinHeight="22"
+                                            Text="{Binding Path=Value, UpdateSourceTrigger=PropertyChanged}" />
+                                    </DockPanel>
+                                </DataTemplate>
+                            </ItemsControl.ItemTemplate>
+                        </ItemsControl>
+                    </ScrollViewer>
+                </StackPanel>
+            </TabItem>
+        </TabControl>
+
+        <DockPanel Margin="8">
+            <Button x:Name="ApplyButton"
+                    DockPanel.Dock="Right"
+                    Content="Übernehmen"
+                    MinWidth="100"
+                    MinHeight="24"
+                    Click="OnApplyClick"
+                    IsEnabled="False" />
+
+            <Label VerticalAlignment="Center"
+                   Content="{Binding Path=ResultStr, RelativeSource={RelativeSource AncestorType=local:CostEntryDetailsWindow}}"
+                   Foreground="{x:Static local:Constants.SubMainColor}" />
+        </DockPanel>
+    </StackPanel>
+
+</Window>
\ No newline at end of file
diff --git a/ucalc/CostEntryDetailsWindow.xaml.cs b/ucalc/CostEntryDetailsWindow.xaml.cs
new file mode 100644
index 0000000..f0e404a
--- /dev/null
+++ b/ucalc/CostEntryDetailsWindow.xaml.cs
@@ -0,0 +1,346 @@
+using System;
+using System.Collections.Generic;
+using System.Collections.Specialized;
+using System.ComponentModel;
+using System.Runtime.CompilerServices;
+using System.Windows;
+using System.Windows.Controls;
+using System.Windows.Input;
+using UCalc.Annotations;
+using UCalc.Controls;
+using UCalc.Models;
+
+namespace UCalc
+{
+    public partial class CostEntryDetailsWindow : INotifyPropertyChanged
+    {
+        public CostEntryDetailsProperty Details { get; }
+        private decimal? _result;
+
+        public decimal? Result
+        {
+            get => _result;
+            private set
+            {
+                if (_result == value)
+                {
+                    return;
+                }
+
+                _result = value;
+                ApplyButton.IsEnabled = value != null;
+                OnPropertyChanged();
+                OnPropertyChanged("ResultStr");
+            }
+        }
+
+        public string ResultStr =>
+            _result != null ? $"Betrag: {_result.Value.ToString(Constants.DisplayPrecisionFormat)} €" : "";
+
+        public CostEntryDetailsWindow(CostEntryDetailsProperty details)
+        {
+            Details = details;
+            InitializeComponent();
+
+            Details.TotalAmount.PropertyChanged += DetailsPropertyChanged;
+            Details.UnitCount.PropertyChanged += DetailsPropertyChanged;
+            Details.DiscountsInUnits.CollectionChanged += DetailsDiscountChanged;
+
+            DetailsPropertyChanged(null, null);
+
+            if (Details.TotalAmount.ConvertedValue != 0 || Details.UnitCount.ConvertedValue != 0 ||
+                Details.DiscountsInUnits.Count != 0)
+            {
+                TabControl.SelectedIndex = 1;
+            }
+        }
+
+        private void DetailsPropertyChanged(object sender, PropertyChangedEventArgs e)
+        {
+            var totalAmount = Details.TotalAmount.ConvertedValue;
+            var unitCount = Details.UnitCount.ConvertedValue;
+
+            if (totalAmount == null || unitCount == null)
+            {
+                Result = null;
+                return;
+            }
+
+            decimal totalDiscount = 0;
+            foreach (var discount in Details.DiscountsInUnits)
+            {
+                var n = discount.ConvertedValue;
+                if (n == null)
+                {
+                    Result = null;
+                    return;
+                }
+
+                totalDiscount += n.Value;
+            }
+
+            Result = unitCount != 0
+                ? totalAmount / unitCount * (unitCount - totalDiscount)
+                : null;
+        }
+
+        private void DetailsDiscountChanged(object sender, NotifyCollectionChangedEventArgs e)
+        {
+            foreach (var discount in Details.DiscountsInUnits)
+            {
+                discount.PropertyChanged -= DetailsPropertyChanged;
+                discount.PropertyChanged += DetailsPropertyChanged;
+            }
+        }
+
+        private void OnCEClick(object sender, RoutedEventArgs e)
+        {
+            ResultTextBox.Text = "";
+        }
+
+        private void OnButtonClick(object sender, RoutedEventArgs e)
+        {
+            ResultTextBox.Text += (string) ((Button) sender).Content;
+        }
+
+        private void OnRoundClick(object sender, RoutedEventArgs e)
+        {
+            ResultTextBox.Text = "ceil(" + ResultTextBox.Text + ")";
+        }
+
+        private void OnEvalClick(object sender, RoutedEventArgs e)
+        {
+            try
+            {
+                Result = Calc(ResultTextBox.Text);
+
+                ResultTextBox.Text = Result.Value.ToString(Constants.InternalPrecisionFormat);
+            }
+            catch
+            {
+                Result = null;
+
+                ResultTextBox.Text = "Ungültig!";
+            }
+        }
+
+        private void OnResultTextBoxKeyUp(object sender, KeyEventArgs e)
+        {
+            if (e.Key == Key.Enter)
+            {
+                OnEvalClick(sender, null);
+            }
+        }
+
+        private static decimal Calc(string str)
+        {
+            var tokens = Tokenize(str);
+            var pos = 0;
+
+            var result = CalcPlus(tokens, ref pos);
+            if (pos != tokens.Count)
+            {
+                throw new Exception();
+            }
+
+            return result;
+        }
+
+        private static decimal CalcPlus(List<string> tokens, ref int pos)
+        {
+            var result = CalcMul(tokens, ref pos);
+
+            while (true)
+            {
+                // + or - might follow
+                if (pos >= tokens.Count)
+                {
+                    break;
+                }
+
+                var op = tokens[pos];
+                if (op != "+" && op != "-")
+                {
+                    break;
+                }
+
+                ++pos;
+
+                var result2 = CalcMul(tokens, ref pos);
+                if (op == "+")
+                {
+                    result += result2;
+                }
+                else
+                {
+                    result -= result2;
+                }
+            }
+
+            return result;
+        }
+
+        private static decimal CalcMul(List<string> tokens, ref int pos)
+        {
+            var result = CalcSimple(tokens, ref pos);
+
+            while (true)
+            {
+                // * or / might follow
+                if (pos >= tokens.Count)
+                {
+                    break;
+                }
+
+                var op = tokens[pos];
+                if (op != "*" && op != "/")
+                {
+                    break;
+                }
+
+                ++pos;
+
+                var result2 = CalcSimple(tokens, ref pos);
+                if (op == "*")
+                {
+                    result *= result2;
+                }
+                else
+                {
+                    result /= result2;
+                }
+            }
+
+            return result;
+        }
+
+        private static decimal CalcSimple(List<string> tokens, ref int pos)
+        {
+            var token = tokens[pos];
+            ++pos;
+
+            decimal result;
+
+            switch (token)
+            {
+                case "(":
+                    result = CalcPlus(tokens, ref pos);
+                    if (tokens[pos] != ")")
+                        throw new Exception();
+                    ++pos;
+                    return result;
+                case "ceil":
+                    result = CalcPlus(tokens, ref pos);
+                    return Math.Round(result, 2);
+                default:
+                    return decimal.Parse(token);
+            }
+        }
+
+        private static List<string> Tokenize(string str)
+        {
+            var tokens = new List<string>();
+            var token = "";
+            var i = 0;
+
+            while (i < str.Length)
+            {
+                var c = str[i];
+                ++i;
+
+                switch (c)
+                {
+                    case '0':
+                    case '1':
+                    case '2':
+                    case '3':
+                    case '4':
+                    case '5':
+                    case '6':
+                    case '7':
+                    case '8':
+                    case '9':
+                    case ',':
+                        token += c;
+                        break;
+                    case '+':
+                    case '-':
+                    case '*':
+                    case '/':
+                    case '(':
+                    case ')':
+                        if (token != "")
+                        {
+                            tokens.Add(token);
+                        }
+
+                        tokens.Add(c.ToString());
+                        token = "";
+                        break;
+                    default:
+                        // Must be ceil
+                        if (token != "")
+                        {
+                            tokens.Add(token);
+                        }
+
+                        if (str.Substring(i - 1, 4) == "ceil")
+                        {
+                            tokens.Add("ceil");
+                            i += 3;
+                        }
+                        else
+                        {
+                            throw new Exception();
+                        }
+
+                        break;
+                }
+            }
+
+            if (token != "")
+            {
+                tokens.Add(token);
+            }
+
+            return tokens;
+        }
+
+        public event PropertyChangedEventHandler PropertyChanged;
+
+        [NotifyPropertyChangedInvocator]
+        private void OnPropertyChanged([CallerMemberName] string propertyName = null)
+        {
+            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
+        }
+
+        private void OnApplyClick(object sender, RoutedEventArgs e)
+        {
+            DialogResult = true;
+        }
+
+        private void OnAddDiscountClick(object sender, RoutedEventArgs e)
+        {
+            Details.DiscountsInUnits.Add();
+        }
+
+        private void OnDiscountDeleteClick(object sender, RoutedEventArgs e)
+        {
+            var discount = (DiscountProperty) ((HighlightButton) sender).DataContext;
+
+            Details.DiscountsInUnits.Remove(discount);
+        }
+
+        private void OnClosed(object sender, EventArgs e)
+        {
+            Details.TotalAmount.PropertyChanged -= DetailsPropertyChanged;
+            Details.UnitCount.PropertyChanged -= DetailsPropertyChanged;
+            Details.DiscountsInUnits.CollectionChanged -= DetailsDiscountChanged;
+
+            foreach (var discount in Details.DiscountsInUnits)
+            {
+                discount.PropertyChanged -= DetailsPropertyChanged;
+            }
+        }
+    }
+}
\ No newline at end of file
diff --git a/ucalc/CostWindow.xaml b/ucalc/CostWindow.xaml
index f351449..e00efc2 100644
--- a/ucalc/CostWindow.xaml
+++ b/ucalc/CostWindow.xaml
@@ -177,28 +177,61 @@
                     <ItemsControl.ItemTemplate>
                         <DataTemplate>
                             <DockPanel>
-                                <controls:HighlightButton DockPanel.Dock="Right"
-                                                          HighlightForeground="Red"
-                                                          HighlightBackground="White"
-                                                          Click="OnCostEntryDeleteClick">
-                                    <Viewbox Width="24"
-                                             Height="24"
-                                             Margin="8, 12, 8, 12"
-                                             Stretch="Uniform">
-                                        <Canvas Width="448" Height="512">
-                                            <Canvas.RenderTransform>
-                                                <TranslateTransform X="0" Y="0" />
-                                            </Canvas.RenderTransform>
-                                            <Path>
-                                                <Path.Data>
-                                                    <PathGeometry
-                                                        Figures="M32 464a48 48 0 0 0 48 48h288a48 48 0 0 0 48-48V128H32zm272-256a16 16 0 0 1 32 0v224a16 16 0 0 1-32 0zm-96 0a16 16 0 0 1 32 0v224a16 16 0 0 1-32 0zm-96 0a16 16 0 0 1 32 0v224a16 16 0 0 1-32 0zM432 32H312l-9.4-18.7A24 24 0 0 0 281.1 0H166.8a23.72 23.72 0 0 0-21.4 13.3L136 32H16A16 16 0 0 0 0 48v32a16 16 0 0 0 16 16h416a16 16 0 0 0 16-16V48a16 16 0 0 0-16-16z"
-                                                        FillRule="NonZero" />
-                                                </Path.Data>
-                                            </Path>
-                                        </Canvas>
-                                    </Viewbox>
-                                </controls:HighlightButton>
+                                <Grid DockPanel.Dock="Right">
+                                    <Grid.RowDefinitions>
+                                        <RowDefinition Height="*" />
+                                        <RowDefinition Height="*" />
+                                    </Grid.RowDefinitions>
+
+                                    <controls:HighlightButton
+                                        HighlightForeground="{x:Static local:Constants.SubMainColor}"
+                                        HighlightBackground="{x:Static local:Constants.MainColor}"
+                                        Grid.Row="0"
+                                        ToolTip="Details und Hilfswerkzeuge zur Berechnung"
+                                        Click="OnCostEntryDetailsClick">
+
+                                        <Viewbox Width="24"
+                                                 Height="24"
+                                                 Margin="8, 12, 8, 12"
+                                                 Stretch="Uniform">
+                                            <Canvas Width="512" Height="512">
+                                                <Canvas.RenderTransform>
+                                                    <TranslateTransform X="0" Y="0" />
+                                                </Canvas.RenderTransform>
+                                                <Path>
+                                                    <Path.Data>
+                                                        <PathGeometry
+                                                            Figures="M328 256c0 39.8-32.2 72-72 72s-72-32.2-72-72 32.2-72 72-72 72 32.2 72 72zm104-72c-39.8 0-72 32.2-72 72s32.2 72 72 72 72-32.2 72-72-32.2-72-72-72zm-352 0c-39.8 0-72 32.2-72 72s32.2 72 72 72 72-32.2 72-72-32.2-72-72-72z"
+                                                            FillRule="NonZero" />
+                                                    </Path.Data>
+                                                </Path>
+                                            </Canvas>
+                                        </Viewbox>
+                                    </controls:HighlightButton>
+
+                                    <controls:HighlightButton HighlightForeground="Red"
+                                                              HighlightBackground="White"
+                                                              Grid.Row="1"
+                                                              Click="OnCostEntryDeleteClick">
+                                        <Viewbox Width="24"
+                                                 Height="24"
+                                                 Margin="8, 12, 8, 12"
+                                                 Stretch="Uniform">
+                                            <Canvas Width="448" Height="512">
+                                                <Canvas.RenderTransform>
+                                                    <TranslateTransform X="0" Y="0" />
+                                                </Canvas.RenderTransform>
+                                                <Path>
+                                                    <Path.Data>
+                                                        <PathGeometry
+                                                            Figures="M32 464a48 48 0 0 0 48 48h288a48 48 0 0 0 48-48V128H32zm272-256a16 16 0 0 1 32 0v224a16 16 0 0 1-32 0zm-96 0a16 16 0 0 1 32 0v224a16 16 0 0 1-32 0zm-96 0a16 16 0 0 1 32 0v224a16 16 0 0 1-32 0zM432 32H312l-9.4-18.7A24 24 0 0 0 281.1 0H166.8a23.72 23.72 0 0 0-21.4 13.3L136 32H16A16 16 0 0 0 0 48v32a16 16 0 0 0 16 16h416a16 16 0 0 0 16-16V48a16 16 0 0 0-16-16z"
+                                                            FillRule="NonZero" />
+                                                    </Path.Data>
+                                                </Path>
+                                            </Canvas>
+                                        </Viewbox>
+                                    </controls:HighlightButton>
+                                </Grid>
 
                                 <StackPanel Margin="0, 8, 8, 8">
                                     <DockPanel Margin="0, 0, 0, 2">
diff --git a/ucalc/CostWindow.xaml.cs b/ucalc/CostWindow.xaml.cs
index 402ac07..d313ad7 100644
--- a/ucalc/CostWindow.xaml.cs
+++ b/ucalc/CostWindow.xaml.cs
@@ -51,5 +51,16 @@ namespace UCalc
         {
             Cost.Entries.Add();
         }
+
+        private void OnCostEntryDetailsClick(object sender, RoutedEventArgs e)
+        {
+            var entry = (CostEntryProperty) ((HighlightButton) sender).DataContext;
+
+            var detailsWindow = new CostEntryDetailsWindow(entry.Details) {Owner = this};
+            if (detailsWindow.ShowDialog() == true)
+            {
+                entry.Amount.Value = detailsWindow.Result!.Value.ToString(Constants.DisplayPrecisionFormat);
+            }
+        }
     }
 }
\ No newline at end of file
diff --git a/ucalc/Models/CostEntriesProperty.cs b/ucalc/Models/CostEntriesProperty.cs
index 35f7c7b..0f84bc9 100644
--- a/ucalc/Models/CostEntriesProperty.cs
+++ b/ucalc/Models/CostEntriesProperty.cs
@@ -17,6 +17,8 @@ namespace UCalc.Models
 
         public void Add()
         {
+            using var validator = Model.BeginValidation(true);
+
             var entry = new CostEntryProperty(Model, this, new CostEntry());
             entry.StartDate.Value = null;
             entry.EndDate.Value = null;
diff --git a/ucalc/Models/CostEntryProperty.cs b/ucalc/Models/CostEntryProperty.cs
index a224b00..2bd21bc 100644
--- a/ucalc/Models/CostEntryProperty.cs
+++ b/ucalc/Models/CostEntryProperty.cs
@@ -1,4 +1,5 @@
 using System;
+using System.Collections.Generic;
 using System.Linq;
 using UCalc.Controls;
 using UCalc.Data;
@@ -69,19 +70,148 @@ namespace UCalc.Models
         }
     }
 
+    public class TotalAmountProperty : PositiveDecimalProperty
+    {
+        public TotalAmountProperty(Model model, Property parent, string name, decimal value) : base(model, parent, name,
+            value)
+        {
+        }
+
+        protected override string ValidateValue()
+        {
+            var entry = (CostEntryDetailsProperty) Parent;
+
+            try
+            {
+                var error = base.ValidateValue();
+                if (error != "")
+                {
+                    return error;
+                }
+
+                if (((entry.UnitCount.ConvertedValue ?? 1) != 0 || entry.DiscountsInUnits.Count > 0) &&
+                    ConvertedValue == 0)
+                {
+                    return $"{Name}: Der Wert muss größer als 0 sein.";
+                }
+
+                return "";
+            }
+            finally
+            {
+                using var validator = Model.BeginValidation();
+                validator.Validate(entry.UnitCount);
+            }
+        }
+    }
+
+    public class UnitCountProperty : PositiveMoreExactDecimalProperty
+    {
+        public UnitCountProperty(Model model, Property parent, string name, decimal value) : base(model, parent, name,
+            value, Constants.InternalPrecision)
+        {
+        }
+
+        protected override string ValidateValue()
+        {
+            var entry = (CostEntryDetailsProperty) Parent;
+
+            try
+            {
+                var error = base.ValidateValue();
+                if (error != "")
+                {
+                    return error;
+                }
+
+                if (((entry.TotalAmount.ConvertedValue ?? 1) != 0 || entry.DiscountsInUnits.Count > 0) &&
+                    ConvertedValue == 0)
+                {
+                    return $"{Name}: Der Wert muss größer als 0 sein.";
+                }
+
+                return "";
+            }
+            finally
+            {
+                using var validator = Model.BeginValidation();
+                validator.Validate(entry.TotalAmount);
+            }
+        }
+    }
+
+    public class DiscountProperty : PositiveDecimalProperty
+    {
+        public DiscountProperty(Model model, Property parent, string name, decimal value) : base(model, parent, name,
+            value)
+        {
+        }
+
+        protected override string ValidateValue()
+        {
+            var error = base.ValidateValue();
+
+
+            return error;
+        }
+    }
+
+    public class DiscountsProperty : MultiProperty<DiscountProperty>
+    {
+        public DiscountsProperty(Model model, Property parent, IEnumerable<decimal> data) : base(model, parent)
+        {
+            foreach (var discount in data)
+            {
+                Add(new DiscountProperty(Model, this, "Abzug", discount));
+            }
+        }
+
+        public void Add()
+        {
+            using var validator = Model.BeginValidation();
+
+            Add(new DiscountProperty(Model, this, "Abzug", 0));
+
+            validator.Validate(((CostEntryDetailsProperty) Parent).UnitCount);
+        }
+
+        public new void Remove(DiscountProperty discount)
+        {
+            using var validator = Model.BeginValidation();
+
+            base.Remove(discount);
+
+            validator.Validate(((CostEntryDetailsProperty) Parent).UnitCount);
+        }
+    }
+
+    public class CostEntryDetailsProperty : NestedProperty
+    {
+        public TotalAmountProperty TotalAmount { get; }
+        public UnitCountProperty UnitCount { get; }
+        public DiscountsProperty DiscountsInUnits { get; }
+
+        public CostEntryDetailsProperty(Model model, Property parent, CostEntryDetails data) : base(model, parent)
+        {
+            TotalAmount = Add(new TotalAmountProperty(model, this, "Gesamtbetrag", data.TotalAmount));
+            UnitCount = Add(new UnitCountProperty(model, this, "Gesamtverbrauch", data.UnitCount));
+            DiscountsInUnits = Add(new DiscountsProperty(model, this, data.DiscountsInUnits));
+        }
+    }
+
     public class CostEntryProperty : NestedProperty
     {
         public DateProperty StartDate { get; }
         public EndDateProperty EndDate { get; }
         public PositiveDecimalProperty Amount { get; }
-
-        // TODO: Details
+        public CostEntryDetailsProperty Details { get; }
 
         public CostEntryProperty(Model model, Property parent, CostEntry data) : base(model, parent)
         {
             StartDate = Add(new DateProperty(model, this, "Startdatum", data.StartDate));
             EndDate = Add(new EndDateProperty(model, this, "Enddatum", data.EndDate));
             Amount = Add(new PositiveDecimalProperty(model, this, "Betrag", data.Amount));
+            Details = Add(new CostEntryDetailsProperty(model, this, data.Details));
         }
     }
 }
\ No newline at end of file
diff --git a/ucalc/Models/Model.cs b/ucalc/Models/Model.cs
index dbff67b..a15d0b4 100644
--- a/ucalc/Models/Model.cs
+++ b/ucalc/Models/Model.cs
@@ -14,6 +14,7 @@ namespace UCalc.Models
         {
             private readonly Model _model;
             private int _counter;
+            private int _postPoneCounter;
             private readonly HashSet<Property> _validated;
             private readonly HashSet<Tuple<Property, string>> _notifications;
 
@@ -49,7 +50,7 @@ namespace UCalc.Models
                     return;
                 }
 
-                if (_validated.Add(property) && !_model._loading)
+                if (_validated.Add(property) && _postPoneCounter == 0)
                 {
                     property.Validate();
                 }
@@ -63,15 +64,33 @@ namespace UCalc.Models
                 }
             }
 
-            public void IncValidationCounter()
+            public void IncValidationCounter(bool postPone)
             {
                 ++_counter;
+
+                if (postPone || _postPoneCounter != 0)
+                {
+                    ++_postPoneCounter;
+                }
             }
 
             public void Dispose()
             {
                 --_counter;
 
+                if (_postPoneCounter > 0)
+                {
+                    --_postPoneCounter;
+
+                    if (_postPoneCounter == 0)
+                    {
+                        var queue = new List<Property>(_validated);
+                        _validated.Clear();
+
+                        ValidateRange(queue);
+                    }
+                }
+
                 if (_counter == 0)
                 {
                     _validated.Clear();
@@ -140,7 +159,6 @@ namespace UCalc.Models
         }
 
         private readonly Validator _validator;
-        private readonly bool _loading;
         public DateTime StartDate { get; }
         public DateTime EndDate { get; }
         public BillingProperty Root { get; }
@@ -151,17 +169,13 @@ namespace UCalc.Models
             StartDate = billing.StartDate;
             EndDate = billing.EndDate;
 
-            _loading = true;
+            using var validator = BeginValidation(true);
             Root = new BillingProperty(this, null, billing);
-            _loading = false;
-
-            using var validator = BeginValidation();
-            validator.Validate(Root);
         }
 
-        public Validator BeginValidation()
+        public Validator BeginValidation(bool postPone = false)
         {
-            _validator.IncValidationCounter();
+            _validator.IncValidationCounter(postPone);
             return _validator;
         }
 
diff --git a/ucalc/Models/Properties.cs b/ucalc/Models/Properties.cs
index aa33532..a6c92b9 100644
--- a/ucalc/Models/Properties.cs
+++ b/ucalc/Models/Properties.cs
@@ -6,6 +6,7 @@ using System.ComponentModel;
 using System.Linq;
 using System.Runtime.CompilerServices;
 using UCalc.Annotations;
+using UCalc.Controls;
 
 namespace UCalc.Models
 {
@@ -181,30 +182,49 @@ namespace UCalc.Models
         }
     }
 
-    public class PositiveDecimalProperty : ValueProperty<string>
+    public class PositiveMoreExactDecimalProperty : ValueProperty<string>
     {
-        public PositiveDecimalProperty(Model model, Property parent, string name, decimal value) : base(model, parent,
-            name,
-            value.ToString(Constants.DecimalFormat))
+        private readonly int _decimals;
+        public decimal? ConvertedValue { get; private set; }
+
+        public PositiveMoreExactDecimalProperty(Model model, Property parent, string name, decimal value, int decimals)
+            : base(model, parent, name, value.ToString(Helpers.PrecisionToFormat(decimals, decimals - 2)))
         {
+            _decimals = decimals;
         }
 
         protected override string ValidateValue()
         {
-            if (!decimal.TryParse(Value, out var n) || n < 0)
+            ConvertedValue = null;
+
+            if (!decimal.TryParse(Value, out var n))
             {
-                return $"{Name}: Der eingegebene Wert ist kein gültiger Betrag.";
+                return $"{Name}: Der eingegebene Wert ist ungültig.";
             }
 
-            if (Math.Round(n, 2) != n)
+            if (n < 0)
             {
-                return $"{Name}: Der eingegebene Wert besitzt mehr als 2 Nachkommastellen.";
+                return $"{Name}: Der eingegebene Wert darf nicht negativ sein.";
             }
 
+            if (Math.Round(n, _decimals) != n)
+            {
+                return $"{Name}: Der eingegebene Wert besitzt mehr als {_decimals} Nachkommastellen.";
+            }
+
+            ConvertedValue = n;
             return "";
         }
     }
 
+    public class PositiveDecimalProperty : PositiveMoreExactDecimalProperty
+    {
+        public PositiveDecimalProperty(Model model, Property parent, string name, decimal value) : base(model, parent,
+            name, value, Constants.DisplayPrecision)
+        {
+        }
+    }
+
     public abstract class NestedProperty : Property
     {
         private readonly List<Property> _properties;
diff --git a/ucalc/ucalc.csproj b/ucalc/ucalc.csproj
index 7e626fb..48118e6 100644
--- a/ucalc/ucalc.csproj
+++ b/ucalc/ucalc.csproj
@@ -46,6 +46,9 @@
         <Page Update="CostWindow.xaml">
             <Generator></Generator>
         </Page>
+        <Page Update="CostEntryDetailsWindow.xaml">
+            <Generator></Generator>
+        </Page>
     </ItemGroup>
 
     <ItemGroup>
@@ -88,6 +91,9 @@
         <Compile Update="CostWindow.xaml.cs">
             <DependentUpon>CostWindow.xaml</DependentUpon>
         </Compile>
+        <Compile Update="CostEntryDetailsWindow.xaml.cs">
+            <DependentUpon>CostEntryDetailsWindow.xaml</DependentUpon>
+        </Compile>
     </ItemGroup>
 
     <ItemGroup>