Converting centimetres to pixels for application forms

Pixels equal physical size in inches multiplied by DPI. One inch is 2.54 centimetres, so at 300 DPI one centimetre is about 118 pixels, and at 200 DPI about 79 pixels.

The common Indian and European passport format of 3.5 by 4.5 centimetres therefore works out to roughly 413 by 531 pixels at 300 DPI. The US 2 by 2 inch format is 600 by 600 pixels at the same resolution.

DPI is a printing instruction stored as a metadata tag. It describes how large the image should appear on paper. It has no effect on the image data itself.

An online upload form checks pixel dimensions and file size. A 600 by 600 image tagged 72 DPI and one tagged 300 DPI are the same image and both pass a pixel check.

DPI becomes relevant when you scan a printed photograph, because the scanner resolution determines how many pixels you end up with. Scanning a 2 by 2 inch print at 300 DPI produces exactly the 600 by 600 minimum the US digital standard requires.

Where a form states both a centimetre size and a pixel size, use the pixel size. Where the two disagree, the pixel figure is what the automated validator reads.

Where a form states only centimetres, convert at 300 DPI unless it says otherwise, and confirm the result is accepted before assuming the conversion was right.