A document-photo page becomes useful when it answers the exact question the applicant is facing, not just when it repeats a size chart.
Keep the units straight
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.
Review the authority page one more time
Even a carefully prepared photo can become outdated when an application changes its technical rules. A final visit to the live requirement page is a small step with real value for time-sensitive forms.
One image, several possible outputs
A good original portrait can support several legitimate deliverables. What changes is the final crop, dimensions or file package required by the destination.
What to remember from this page
For recurring applications, keep a tiny photo record: date taken, destination, dimensions and final format. It is more reliable than remembering a size from a previous application.
Visa Photo Compression Guide
For print workflows, create the measured sheet and verify the physical output before producing the final batch.