Android DIP size wrong

This one is bedeviling me. What am I overlooking? I want a button exactly one inch wide, on any density screen. I declare it in the layout like this:

<Button  
android:layout_width="wrap_content" 
android:layout_height="wrap_content" 
android:width="160dip"
android:开发者_开发技巧text="How wide?"
/>

What I get is a button that is exactly 7/8ths of an inch wide on all screens, not the expected 1 inch. Is the difference maybe the margin or padding around the button? If so, how to set that to zero?

What about this ?

android:layout_width="1in"

dip are based on virtual densities, not actual physical densities. For instance, on a T-Mobile G1, the device's display physical density is around 180dpi, but we treat it as a 160dpi display. Android currently uses 3 "density buckets," 120, 160 and 240. Each device uses whatever is most appropriate. If you want accurate physical measurements, use physical units like in, mm, etc.