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.
Know the unit you are converting
Millimetres describe a physical print. Pixels describe a digital image. DPI describes a relationship between pixels and physical size for printing. Mixing those units is the source of many “correct-looking” conversion errors.
Use the formula only when it fits the task
For a print calculation, pixels = millimetres × DPI ÷ 25.4. If a portal gives its own pixel dimensions, use those values directly instead of reverse-engineering a DPI value.
When you need another version
Start again from the master rather than resizing the finished JPEG. Different applications can require different crops, dimensions or file limits, and a clean source makes the second output much easier.
Why two correct-looking photos can be different
Two portraits can show the same person and still be different deliverables because the receiving systems may measure the face or frame differently. A size conversion alone does not reconcile those differences.
Create each output from the clean master and label it for the destination.
Practical detail to carry forward
Save the rule with the file when the job is important. A screenshot or note of the size and upload limit can prevent a later re-edit when a portal changes its wording.
Passport Photo Size Converter
Start with a clean source and use FastIDPhoto for the practical preparation steps, while the official authority remains the final acceptance decision.