Formula px to dp, dp to px android

Question

I am trying to calculate a variable amount of pixels to density independent pixels and vice-versa.

This formula (px to dp): dp = (int)(px / (displayMetrics.densityDpi / 160)); does not work on small devices because it is divided by zero.

This is my dp to px formula:

px = (int)(dp * (displayMetrics.densityDpi / 160));

Could someone give me some pointers?

Answer 1

Note: The widely used solution above is based on displayMetrics.density. However, the docs explain that this value is a rounded value, used with the screen 'buckets'. Eg. on my Nexus 10 it returns 2, where the real value would be 298dpi (real) / 160dpi (default) = 1.8625.

Depending on your requirements, you might need the exact transformation, which can be achieved like this:

[Edit] This is not meant to be mixed with Android's internal dp unit, as this is of course still based on the screen buckets. Use this where you want a unit that should render the same real size on different devices.

Convert dp to pixel:

public int dpToPx(int dp) {
    DisplayMetrics displayMetrics = getContext().getResources().getDisplayMetrics();
    return Math.round(dp * (displayMetrics.xdpi / DisplayMetrics.DENSITY_DEFAULT));     
}

Convert pixel to dp:

public int pxToDp(int px) {
    DisplayMetrics displayMetrics = getContext().getResources().getDisplayMetrics();
    return Math.round(px / (displayMetrics.xdpi / DisplayMetrics.DENSITY_DEFAULT));
}

Note that there are xdpi and ydpi properties, you might want to distinguish, but I can't imagine a sane display where these values differ greatly.