KB, MB, pixels and DPI: what a form’s size limit really means

Online forms describe pictures in two different ways, and mixing them up is the reason many uploads fail. One is the file size, measured in KB or MB. The other is the picture size, measured in pixels. A third word, DPI, appears on some instructions and matters much less than people think.

KB and MB measure the file

A kilobyte (KB) and a megabyte (MB) measure how much storage a file takes. One MB is about 1,000 KB. When FPSC says “less than 30 KB” or NTS says “5 MB”, they are talking about this number and nothing else.

Strictly, some software counts 1,000 bytes as a KB and some counts 1,024. For a 30 KB limit the difference is under 1 KB, but it can matter when a file sits exactly on the line. The resizer on this site stops a little below each limit so the file passes under either way of counting.

Pixels measure the picture

A digital picture is a grid of coloured dots called pixels. “413 × 531 pixels” means 413 dots across and 531 dots down. More pixels mean more detail, and also a larger file.

A phone camera takes pictures of around 4,000 × 3,000 pixels. A photo on a form is shown at the size of a postage stamp, so it needs only a few hundred pixels each way. Reducing the pixels is the main way a picture is brought down from megabytes to kilobytes.

Why two files with the same pixels differ in KB

A JPG file is compressed. When it is saved, a quality setting decides how much fine detail is thrown away to save space. Two things then decide the file size:

• The quality setting. Lower quality gives a smaller file and, if pushed too far, a blocky picture.
• What is in the picture. A plain wall compresses to almost nothing. Hair, patterned cloth and printed text take much more space.

This is why a form may state a file size and no pixel size at all. FPSC, PPSC, ASF and the Pak Navy form each give a KB limit and no pixel size. Any pixel size is acceptable as long as the file fits and the picture is clear.

DPI matters for printing, not for uploading

DPI means dots per inch. It tells a printer how large to print a picture. It does not change the pixels in the file or the size of the file. A picture of 413 × 531 pixels is the same upload whether it is labelled 72 DPI or 300 DPI.

DPI is useful for one calculation. A passport photo is 35 × 45 mm. Printed at 300 DPI, that is 413 × 531 pixels, which is why you will see that pair of numbers on many photo studio files.

How to check your file

• Windows: right-click the file and choose Properties for the size in KB. The Details tab shows the pixels.
• Android: open the photo in the gallery and choose Details or Info.
• iPhone: open the photo and tap the info button to see the size and the pixels.

What to do with a limit

1. Find out whether the form gives a KB limit, a pixel size, or both.
2. If it gives only KB, crop the picture to a sensible shape and let the pixels fall wherever they need to.
3. If it gives pixels as well, the picture must match those pixels exactly and also fit the KB limit.
4. Always check the result with your own eyes before uploading.

Each form page on this site quotes the official wording, so you can see which kind of limit you are dealing with before you start.