Passport photo size in inches

Two situations. The first is US and Indian documents, where the specification itself is written in inches: a 2 by 2 inch photo with a 1 to 1 3/8 inch head height. The second is printing, because photo paper and lab print sizes are sold in inches almost everywhere, including in metric countries. A 4x6 print is a 4x6 print in Toronto and in Berlin.

That second point is why the print sheet option on this site offers 4x6 and 5x7 inch sheets alongside A4 and Letter. You will be handing the file to a kiosk that thinks in inches even if your photo specification is in millimetres.

The one real advantage of an imperial specification is that DPI stands for dots per inch, so pixels follow directly. A 2 inch photo at 300 DPI is 600 pixels. At 600 DPI it is 1200 pixels. No arithmetic required, which is why US digital requirements are stated as neat round numbers while metric ones are not.

Compare that with 35 mm, which needs dividing by 25.4 first, giving 413 pixels at 300 DPI. The awkward numbers you see in metric pixel requirements, like IRCC's 420 by 540, are rounded versions of exactly this calculation.

The most common mistake with imperial conversions is compounding a rounding error. If a 35x45 mm photo becomes "about 1.4 by 1.8 inches" and a lab then prints to that, you have 0.6 mm of error on the width and 0.7 mm on the height before the crop is even considered. Photo tolerances do not leave room for that, so quote the published unit.