Formula for pixel to millimeter and vice versa

Millimeter to Pixel
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$pixels = ($mm * $dpi) / 25.4

Pixel to Millimeter
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$mm = ($pixels * $25.4) / dpi


ex : milli meter to pixel
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Let me explain a little bit.

1 inch = 25.4 mm

If we assume a resolution of e.g. 300 dpi (that's 300 dots or pixels per inch)
then 100 mm (being approx. 4 inches) is 300*4 = 1200 pixels

The formula gives:
100 * 300 / 25.4 = 1181 pixels