Image size explained: KB, pixels and DPI
Forms ask for "a 3.5 × 4.5 cm photo under 50 KB at 200 DPI", and it is not obvious how those numbers fit together. This guide explains each one and how to meet all of them.
Pixels: the real size of a digital image
A digital photo is a grid of coloured squares called pixels. A photo "1200 × 1600" is 1200 pixels wide and 1600 tall, nearly two million pixels in all. On a screen, the pixel size is the only size that matters: it decides how sharp the photo can look and how big it appears at 100 %.
KB: how much storage the file takes
KB (kilobytes) is the size of the file, not of the picture. The same 1200 × 1600 photo can be 80 KB or 800 KB depending on how strongly it is compressed and how much detail it contains. JPG and WebP throw away detail the eye is unlikely to notice to make files smaller; the more they throw away, the smaller the file and the more visible the blocks and smudges become.
Forms usually say "up to" a KB limit. A file just under the limit at the best quality that fits is ideal. Some forms also have a minimum KB; this is to stop people uploading tiny, unreadable images.
DPI: only for printing
DPI (dots per inch) tells a printer how many pixels to put in each inch of paper. It turns pixels into centimetres: at 300 DPI, 413 pixels make 3.5 cm, and 531 pixels make 4.5 cm. On a screen, changing the DPI does nothing at all. Many forms ask for a DPI only because the photo will be printed on an admit card or ID.
Putting it together
So "3.5 × 4.5 cm under 50 KB at 300 DPI" means: a photo of about 413 × 531 pixels, with 300 DPI written into the file, compressed to under 50 KB. The resize tool converts centimetres to pixels for you, and the compressor makes the file fit the KB limit. The exam tool does all of it at once for common exams.
Common traps
- 1 KB is 1000 or 1024 bytes, depending on the website. If a file is right at the limit, it may pass one portal and fail another. Stay a little inside the range.
- Changing DPI does not change KB. To make a file smaller, reduce pixels or quality, not DPI.
- A screenshot of a photo is not the same as the photo: it has the screen's pixel size and often a lower quality. Use the original file.
Last reviewed October 2026.
Frequently asked questions
What is a good size for a website image?
For most websites, 1200 to 1600 pixels wide and 100 to 200 KB in WebP or JPG looks sharp on phones and laptops and loads quickly.