The formula
Pixels = centimetres × DPI ÷ 2.54
For millimetres divide by 25.4, and for inches simply multiply by the DPI. DPI means dots per inch: how many pixels are printed in each inch of paper.
Common sizes at 200 DPI
| Size on the form | Pixels |
|---|---|
| 3.5 × 4.5 cm (passport-style photo) | 276 × 354 |
| 3.5 × 1.5 cm (signature) | 276 × 118 |
| 4 × 2 cm (signature) | 315 × 157 |
| 3 × 3 cm (thumb impression) | 236 × 236 |
| 10 × 5 cm (declaration) | 787 × 394 |
Which DPI should I use?
If the form states a DPI, use that. If it does not, 200 is a common choice for application forms and 300 for anything that will be printed. A higher DPI gives more pixels for the same printed size, and a larger file.
Questions and answers
Does DPI change how a picture looks on screen?
No. On screen only the pixel count matters. DPI matters when the picture is printed or when a form calculates physical size from it.
My form gives both centimetres and pixels and they do not match. Which wins?
Follow the pixel figure. Upload checks are done on pixels and file size.