MainWindow.xaml

<Window x:Class="Calculator.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:Calculator"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="247">
    <Grid>
        <Grid>
            <Grid.RowDefinitions>
                <RowDefinition Height="0.40*"/>
                <RowDefinition/>
            </Grid.RowDefinitions>

            <Grid Grid.Row="0">
                <Grid.ColumnDefinitions>
                    <ColumnDefinition/>
                    <ColumnDefinition Width="0.2*"/>
                </Grid.ColumnDefinitions>

                <TextBlock Name="Output" VerticalAlignment="Center" FontSize="12" Margin="10"
                           Text="{Binding Display, UpdateSourceTrigger=PropertyChanged}"
                           TextWrapping="Wrap"/>

                <Button Grid.Column="1" 
                        Name="Clean" Content="C" Click="Clear_OnClick"
                        Background="WhiteSmoke" BorderThickness="0"/>
            </Grid>
            <Grid Grid.Row="1">
                <Grid.RowDefinitions>
                    <RowDefinition/>
                    <RowDefinition/>
                    <RowDefinition/>
                    <RowDefinition/>
                </Grid.RowDefinitions>
                <Grid.ColumnDefinitions>
                    <ColumnDefinition/>
                    <ColumnDefinition/>
                    <ColumnDefinition/>
                    <ColumnDefinition/>
                </Grid.ColumnDefinitions>

                <Button Grid.Row="0" Grid.Column="0" Content="1" Click="Numpad_OnClick"/>
                <Button Grid.Row="0" Grid.Column="1" Content="2" Click="Numpad_OnClick"/>
                <Button Grid.Row="0" Grid.Column="2" Content="3" Click="Numpad_OnClick"/>
                <Button Grid.Row="1" Grid.Column="0" Content="4" Click="Numpad_OnClick"/>
                <Button Grid.Row="1" Grid.Column="1" Content="5" Click="Numpad_OnClick"/>
                <Button Grid.Row="1" Grid.Column="2" Content="6" Click="Numpad_OnClick"/>
                <Button Grid.Row="2" Grid.Column="0" Content="7" Click="Numpad_OnClick"/>
                <Button Grid.Row="2" Grid.Column="1" Content="8" Click="Numpad_OnClick"/>
                <Button Grid.Row="2" Grid.Column="2" Content="9" Click="Numpad_OnClick"/>
                <Button Grid.Row="3" Grid.Column="1" Content="0" Click="Numpad_OnClick"/>

                <Button Name="Equals" Grid.Row="3" Grid.Column="0" Content="=" Background="DarkGray" Click="Equals_OnClick"/>
                <Button Grid.Row="3" Grid.Column="2" Content="." Background="LightGray" Click="Numpad_OnClick"/>
                <Button Grid.Column="3" Grid.Row="0" Content="/" Background="LightGray" Click="Numpad_OnClick"/>
                <Button Grid.Column="3" Grid.Row="1" Content="*" Background="LightGray" Click="Numpad_OnClick"/>
                <Button Grid.Column="3" Grid.Row="2" Content="-" Background="LightGray" Click="Numpad_OnClick"/>
                <Button Grid.Column="3" Grid.Row="3" Content="+" Background="LightGray" Click="Numpad_OnClick"/>
            </Grid>
        </Grid>
    </Grid>
</Window>