nested weight in linear layout

You have to add weightSum to the linearlayout and remove the weight of the listview:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical" >

    <ListView
        android:id="@+id/listWeighings_weighing"
        android:layout_width="match_parent"
        android:layout_height="0dp"
        android:layout_marginBottom="5dp"
        android:background="#000000" >
    </ListView>

    <LinearLayout
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:orientation="horizontal"
        android:weightSum="100" >

        <Button
            android:id="@+id/btnInsertMutation_weighing"
            android:layout_width="fill_parent"
            android:layout_height="wrap_content"
            android:layout_weight="50"
            android:text="Mutatie invoeren" />

        <Button
            android:id="@+id/btnExecuteWeighing_weighing"
            android:layout_width="fill_parent"
            android:layout_height="wrap_content"
            android:layout_weight="50"
            android:text="weging uitvoeren" />
    </LinearLayout>

</LinearLayout>

3 things to remember:

• set the android:layout_width of the children to "0dp"

• set the android:weightSum of the parent

• set the android:layout_weight of each child proportionally (e.g. weightSum = "5", three children: layout_weight="1", layout_weight="3", layout_weight="1")