Calculate number of pixels = 3cm

Good morning!

I know this is a beginners question (and I should be ashamed of myself) but how do I calculate the amount of (horizontal) pixels which corresponds to 3 centimeters.

(I want an image to be ~3cm wide, no matter which phone, tablet, toaster, screen or Android TV!)

Can anyone help me out with a formula and/or B4A code?

Many many thanks in advance!

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