CM to Pixels — the DPI Math Behind Photo Size Requirements
Forms that state photo sizes in centimetres — "3.5cm × 4.5cm", "minimum 4.5cm high" — create a question they never answer: how many pixels is that? A digital image has no physical size until it's printed; the bridge between centimetres and pixels is DPI (dots per inch), and choosing the wrong DPI is why a "correctly sized" photo can still be rejected.
The formula
Pixels = (centimetres ÷ 2.54) × DPI. So 3.5×4.5cm becomes: at 300 DPI (the print-quality standard most forms assume) 413×531 pixels; at 200 DPI, 276×354; at 96 DPI (screen resolution), 132×170. Same physical size, wildly different digital files.
Which DPI do forms actually want?
When a form states only centimetres, 300 DPI is the safe assumption — it's the printing standard, and government systems that check dimensions typically validate against the 300 DPI equivalent. Some portals state it explicitly ("3.5×4.5cm at 300 DPI"); when one specifies pixels directly, the pixel figure always wins over any cm interpretation.
Common conversions at 300 DPI
3.5×4.5cm → 413×531px (the standard South Asian form photo). 35×45mm → the same 413×531px (millimetres, same size). 2×2 inches → 600×600px (the US visa format). 6×2cm → 709×236px (a common signature box). 54×86mm → 638×1016px (Pakistani CNIC card scan).
The PixKB Dimensions tool does this conversion live: enter the size in cm, mm or inches, pick the DPI, and it shows the exact pixel result before resizing — no manual arithmetic, no rejected uploads from a 96-DPI misunderstanding.
Frequently asked questions
What DPI should I use if the form doesn't say?
300 DPI. It's the standard assumption for printed-photo equivalents and what most validation systems expect for cm-specified sizes.
Does changing DPI change my file's KB size?
Indirectly — higher DPI means more pixels for the same physical size, and more pixels means more kilobytes before compression. Dimensions and compression together determine the final size.
Is 35×45mm the same as 3.5×4.5cm?
Yes — identical size in different units. Both convert to 413×531 pixels at 300 DPI.