Passport photo size in cm

Centimetres versus millimetres is only notation

There is no technical distinction here, just editorial preference. Indian, Vietnamese and some European authorities write 3.5 x 4.5 cm. The ICAO specification and most European passport offices write 35 x 45 mm. Both describe an identical photo, and unlike imperial conversions there is no rounding involved at all.

Millimetres are used throughout the rest of this site because head height rules are always published in millimetres. Saying a head must be 31 to 36 mm is clearer than 3.1 to 3.6 cm, especially at that precision.

The 4x6 confusion

This one causes real mistakes. A 4x6 cm photo is a Vietnamese, Saudi, Egyptian or Greek passport photo, 40 by 60 mm. A 4x6 inch print is a standard sheet of photo paper roughly 10 by 15 cm, which is what you order from a lab to hold several passport photos tiled with cut guides.

If someone tells you to get a "4 by 6", establish which they mean. Ordering a 4x6 inch print when you needed a 4x6 cm photo gives you a sheet with one enormous portrait on it.

Converting to pixels

Divide centimetres by 2.54 for inches, then multiply by the DPI. At 300 DPI, 3.5 x 4.5 cm becomes 413 by 531 pixels, and at 600 DPI it becomes 827 by 1063. The slightly odd numbers in official digital requirements, such as IRCC's 420 by 540 minimum, are rounded results of the same calculation.