Converting a physical photo size to pixels

There is no single correct pixel size for a millimetre spec

A notification that states "3.5 cm × 4.5 cm" is describing a printed size, not a digital one. Converting it to pixels needs a resolution (DPI — dots per inch) as well, and portals almost never state one. At 300 DPI, 35×45mm becomes roughly 413×531 pixels; at 600 DPI it becomes roughly 827×1063 pixels. Both print at the identical physical size — the higher-DPI file is just sharper and larger.

This is why two applicants can both submit a technically correct "35×45mm" photo at very different pixel dimensions, and both be accepted, as long as the portal validates only the printed size or the aspect ratio rather than exact pixels.

The formula

pixels = (millimetres ÷ 25.4) × DPI. So 35mm at 300 DPI is (35 ÷ 25.4) × 300 ≈ 413px, and 45mm at 300 DPI is (45 ÷ 25.4) × 300 ≈ 531px.

300 DPI is the de facto standard for exam and passport photos — it is sharp enough for a portal to display and print, without producing a needlessly large file. Use 300 DPI unless your notification states an exact pixel figure instead of a physical size, in which case follow the pixel figure exactly.

Common conversions at 300 DPI

• 3.5 cm × 4.5 cm (35×45mm) — 413 × 531 px, the SSC/Railway/most-exam standard
• 2 × 2 inch (51×51mm) — 600 × 600 px, the Indian passport and several other countries' standard
• 6 cm × 2 cm (60×20mm) — 709 × 236 px, a common signature size
• 4 × 6 cm — 472 × 709 px, used by some Gulf-country passport photos

When the pixel figure matters more than the physical one

Some portals — UPSC, several banking exams, several countries' passport authorities — publish an exact pixel figure directly (for example, IBPS's 200×230px) rather than a physical size. When that is the case, that number is the requirement; do not re-derive it from millimetres, since the two can legitimately differ from what a straight conversion would produce.