Android: trying to understand android:layout_weight

Is there a way to get a percentage value (0..100%) out of android:layout_weight?

Sure. Make them add up to 100.

For your "percentage value", you want the android:layout_height of the individual items within the LinearLayout to be 0px.

When you use android:layout_height="fill_parent" for the bars, you are not leaving any available space. (Since they are filling the parent.) Because all 3 are set to fill, the requested height is actually 3x the parent height, so the weights are being applied to a negative remaining space. This explains the weird behavior you are seeing, and why setting layout_height to 0px solves it.