The safest starting point is the application itself: identify the document, the country, and whether the photo is being uploaded, printed or both.
The measurement first
Millimetres and inches describe physical output; pixels describe digital dimensions; DPI connects pixels to physical size during printing. If an authority already states the required pixel dimensions, use them directly. If you are building a print file from millimetres, the basic relationship is pixels = millimetres × DPI ÷ 25.4.
Where calculations stop being enough
A mathematically correct file can still fail because the face is the wrong size inside the frame, the background is unsuitable, the compression introduces visible artefacts, or the application expects a different file type. Finish every conversion with a visual and application-specific check.
When the rule is strict
Prefer a fresh, controlled capture over aggressive software repair. A clean original is easier to defend and easier to reuse for another legitimate application route.
Why FastIDPhoto uses document profiles
A profile-based editor reflects the way real applications work: the destination determines the target. The same original portrait can feed several document profiles without making the applicant guess which “passport size” is meant.
That separation is especially useful for families and frequent travellers.
Practical detail to carry forward
When you are done, open the final file once more from disk rather than from the editor. That confirms the exported file—not just the preview—contains the intended crop and dimensions.
How Many Pixels Should a Passport Photo Have?
Prepare the file, inspect it at full size and save the exact version that belongs to your application.