Why a document photo is measured in millimetres
Every other photo tool asks how many pixels you want. A document photo does not work that way: the office that receives it measures it with a ruler. 35 × 45 mm is 35 × 45 mm, and how many pixels that is depends entirely on the resolution it will be printed at. At the usual 300 DPI it is 413 × 531 pixels; at 600 DPI it is double that and the same size on paper.
What the print sheet is for
Handing a print shop a single image is where the size usually gets lost: the shop scales it to fill the paper and what comes back is the wrong size. The sheet solves that by deciding the layout here. It places as many copies as fit on 10 × 15 cm or A4, each at the exact measurement, with cut marks in the margins so the scissors have a line to follow. The shop only has to print it at 100%.
The guides, and what they are not
The oval and the eye line show roughly where a head sits in an accepted photo: head filling most of the height, a little space above it, eyes around the upper third. They are a sanity check, not a rule book — every country has its own tolerances, and some require specific expressions, backgrounds or glasses rules this tool knows nothing about. The guides are drawn on screen only and never appear in the file.
When the warning appears
Zooming in uses fewer pixels of the original to fill the same physical size, and past a point the print looks soft. The warning appears when the crop has less than nine tenths of the pixels the chosen size needs. Enlarging does not create detail, so the honest fix is to zoom out or start from a bigger photo.
Why it runs in your browser
A document photo is a picture of your face attached to an identity process. Uploading it to a free website means handing a biometric image to a service with no reason to keep it. The crop is arithmetic on a canvas; your own machine does it instantly.
Want to go deeper? Read our guide: Passport and ID photo sizes, and how to print them at home