KB, pixels and DPI: what the form is asking for
Upload forms describe a photo with three different kinds of number. They measure different things, and a file has to pass each one.
KB is the weight of the file
KB (kilobytes) and MB (megabytes) say how much space the file takes. 1 MB is about 1,000 KB. A photo straight from a phone camera is usually 2 to 8 MB. Many forms accept only 20 to 200 KB, so the file has to be made much lighter.
Pixels are the width and height
A picture is a grid of tiny dots called pixels. "413 × 531 pixels" means 413 dots across and 531 down. A form that names pixels wants exactly that grid, or one inside a stated range.
cm and DPI describe a printed size
Some forms give the size in centimetres, like 3.5 × 4.5 cm. A file has no centimetres of its own, so a second number is needed: DPI, dots per inch. It says how many pixels are printed in each inch. One inch is 2.54 cm.
Pixels = centimetres ÷ 2.54 × DPI. When a form gives cm but no DPI, 300 is the usual choice.
| Size on the form | At 200 DPI | At 300 DPI |
|---|---|---|
| 3.5 × 4.5 cm (passport size) | 276 × 354 pixels | 413 × 531 pixels |
| 2 × 2 inches (5.1 × 5.1 cm) | 400 × 400 pixels | 600 × 600 pixels |
| 5 × 7 cm | 394 × 551 pixels | 591 × 827 pixels |
| 8.6 × 5.4 cm (ID card) | 677 × 425 pixels | 1016 × 638 pixels |
Why a file can pass one and fail another
The numbers are linked but not the same. A picture with the right pixels can still be too heavy, and a light file can have the wrong pixels. Some forms also set a smallest file size, so a file can be rejected for being too light.
The quickest fix is to type the form's own numbers into Fit a form's rules. It makes the file match all of them at once and shows a pass or fail for each.
Tools for this
- Resize a picture to exact pixels and KB
- Make your photo fit any online form
- Find out why a form rejects your photo
More guides
- Why is my file too large?Why phone photos are heavy, and the safe way to shrink them.
- JPG, JPEG, PNG and HEICWhich file type a form wants, and what to do with iPhone photos.
- Passport photo at homeLight, distance and background, so the photo is not rejected.
- Scan documents with a phoneA sharp, flat, small scan without a scanner.
- Sign a PDF without printingDraw your signature and place it on the page.
- Share an ID card copy safelyMark it, hide what is not needed, and send less.