Millimetres to pixels and DPI for passport photos

Print rules are written in millimetres or inches and upload rules in pixels, and the bridge between them is DPI, dots per inch. One inch is 25.4 mm, so a 35 x 45 mm photo at 300 DPI is 413 x 531 pixels and at 600 DPI it is 827 x 1063 pixels. This guide shows the arithmetic, the sizes it produces for the documents in our dataset, and why kilobytes are a separate limit that pixels do not control.

Person holding a phone up with their own picture on the screen

The formula

Pixels equal millimetres divided by 25.4, multiplied by the DPI, rounded to the nearest whole pixel. For 35 mm at 300 DPI: 35 / 25.4 x 300 = 413.4, so 413 pixels. For 45 mm: 45 / 25.4 x 300 = 531.5, so 531 pixels. At 600 DPI the same photo is 827 x 1063 pixels. Going the other way, 600 pixels at 300 DPI is 600 / 300 x 25.4 = 50.8 mm, which is why the 2 x 2 inch US passport photo, stored as 51 x 51 mm, prints at 602 x 602 pixels while the DS-160 visa photo is defined directly as 600 x 600 pixels.

DPI changes nothing about the physical size; it only sets how many pixels describe each inch. A 413 x 531 file and an 827 x 1063 file both print as 35 x 45 mm if the printer is told the right DPI. Our print sheet is built at 300 DPI and the single photo is exported at the DPI the authority prefers, 300 or 600, with that value written into the JPEG header.

Wooden folding rules marked in centimetres on a yellow cloth

The sizes in our dataset

Ten print sizes cover all 128 documents. 35 x 45 mm is used by 78 of them, from the UK printed passport to the Schengen visa. 51 x 51 mm (2 x 2 in) is used by eight, 50 x 70 mm by Canada, 33 x 48 mm by China, 36 x 47 mm by Finland, 50 x 60 mm by Turkey, 40 x 60 mm by Greece and Vietnam, 35 x 50 mm by Malaysia, 26 x 32 mm by Spain and 30 x 40 mm by Japan's residence applications.

Twenty exact upload sizes appear on the digital side. Several are the same 35 x 45 mm at a stated DPI: 413 x 531 pixels for the Korean passport at 300 DPI, 827 x 1063 pixels for the Japanese passport and the Schengen visa at 600 DPI. Others are chosen by the portal and do not come from any DPI: Singapore's 400 x 514 pixels for the Singapore passport is the only figure ICA publishes, and Finland's 500 x 653 pixels for the Finnish passport must be met exactly, with deviations as small as one pixel refused.

Minimums, maximums and exact sizes

Portals state pixel rules three ways. Some give a minimum only: gov.uk wants at least 600 pixels wide and 750 pixels tall for the UK digital photo, and we render 900 x 1125 so HMPO's own crop has room. Some give a range: Korea accepts 395 to 431 by 507 to 550 pixels and recommends 413 x 531. Some give an exact size: the DV lottery wants exactly 600 x 600 pixels and Finland exactly 500 x 653. Our spec data stores a minimum and a target for every digital document, and a maximum where the authority sets one; the file we export is always the target; the pixel dimensions guide explains the three patterns.

Wooden ruler showing inches on one edge and centimetres on the other

Aspect ratio is the part people miss

Two files can both be big enough and still be the wrong shape. 35 x 45 mm is 7:9; 2 x 2 in is 1:1; the US online renewal wants a 2:3 portrait around 1200 x 1800 pixels for the online renewal photo, and users who upload the square visa file get 'photo isn't big enough'. China's 33 x 48 mm is a different shape again, so a 35 x 45 crop is refused or re-cut for the Chinese passport. Our validation checks that each document's print and upload sizes share the same aspect ratio to within one percent, and the format pages list which documents share each size.

Kilobytes are a different limit

File size in kilobytes depends on JPEG compression, not on pixels, so a 600 x 600 file can be 80 KB or 400 KB. The limits vary widely: 240 KB for the CEAC visa upload, 250 KB for Finland, 500 KB for Korea, 600 KB for Japan's マイナポータル, 2.5 MB for Poland's online child passport, between 50 KB and 10 MB for the UK, and up to 8 MB for Singapore. Japan's portal compresses files over its limit, which can soften the image; others simply refuse the file or, in CEAC's case, silently reload the page. We export each file under its portal's limit, and the checklist shows the measured pixels and kilobytes so you can confirm it before uploading.

Quick conversions

Common values, rounded to the nearest pixel.

1.35 x 45 mm: 413 x 531 px at 300 DPI, 827 x 1063 px at 600 DPI.
2.51 x 51 mm (2 x 2 in): 602 x 602 px at 300 DPI; the digital visa standard rounds this to 600 x 600.
3.50 x 70 mm: 591 x 827 px at 300 DPI.
4.33 x 48 mm: 390 x 567 px at 300 DPI.
5.36 x 47 mm: 425 x 555 px at 300 DPI; Finland's upload uses 500 x 653 instead.
6.600 x 600 px at 300 DPI: 50.8 x 50.8 mm.