Exam portals throw four different units at you and assume you know all of them. Most people do not, and that is not a failing. Here is what each one means.
Pixels: how big the picture is
A digital image is a grid of tiny coloured squares called pixels. When a form says 200 x 230 px, it means the grid must be 200 squares across and 230 down.
This is about the shape and detail of the image, not how much space the file takes up.
KB and MB: how much space the file takes
KB means kilobyte and MB means megabyte. One MB is about 1,000 KB.
A photo straight from a phone camera is usually 2 MB to 5 MB, which is 2,000 to 5,000 KB. A form asking for 20 to 50 KB is asking for something roughly a hundred times smaller.
Why a photo can be right and wrong at the same time
This is the part that confuses everyone.
You can have an image that is exactly 200 x 230 pixels but still 400 KB, because it is stored at very high quality. And you can have a 30 KB image that is 3000 pixels wide but very blurry.
Dimensions and file size are two separate requirements. A form usually sets both, and you have to satisfy both at once.
Making the file smaller without changing the dimensions means reducing the image quality slightly. Reduce it too much and the photo looks blocky. Not enough and the file stays too large. This is why hitting an exact KB range by hand is so fiddly.
DPI: the bridge between print and screen
DPI means dots per inch. It only matters when a form gives you a physical measurement like centimetres or inches, because you need to know how many pixels fit into each inch to convert.
The standard is 300 DPI. So a 6 cm wide signature at 300 DPI is 6 ÷ 2.54 x 300, which is about 709 pixels.
If your form gives pixels directly, you can ignore DPI entirely.
Why the minimum file size exists
Many forms set a minimum as well as a maximum, such as 20 KB to 50 KB. The minimum exists because a file that is too small has usually been compressed so hard that the face is no longer clear enough to verify.
If your image comes out under the minimum, the usual cause is a low quality original. Take a sharper photograph rather than trying to pad the file.
The short version
- Pixels = the size of the picture
- KB and MB = the size of the file
- DPI = how to convert between physical and digital sizes
- You usually have to satisfy pixels and KB at the same time
Our resizer handles both together, and accepts cm, inches or pixels so you can type whatever your notification says.