How to convert millimeters to pixels?

Question

ImageMagick

There is a task, knowing the diameter of a circle in millimeters (for example, 100), to create an image using ImageMagick (DPI by default 72), the actual size of which on any screen will be 100mm. The image parameters are specified in pixels. By what formula can I correctly convert millimeters to pixels?
So far I've tried this: 72/2.54 = 28.35 dots/cm, so 1mm = 0.2835px. It turned out 353px = 100mm
Meryl on the screen with a tape measure) 12mm is missing
What did I do wrong?

Answer 1

1. Wrongly calculated. 100mm = 285px.
2. You are taking 72dpi, not the actual dpi of the output device.