Guide
KB and pixels
Upload forms give sizes in three different units, often in the same sentence: kilobytes, pixels and centimetres at a DPI. They measure different things, and getting one right does not get the others right.
Three units, three questions
- KB is how many bytes the file takes. It depends on the pixels, the quality, and how busy the picture is.
- Pixels are how many dots the picture has across and down. A form that says 200 × 230 px wants exactly that.
- Centimetres at a DPI is a print size. Pixels = centimetres ÷ 2.54 × DPI. The DPI itself is a label in the file and costs no bytes.
From centimetres to pixels
| 3.5 × 4.5 cm at 200 DPI | 276 × 354 px |
|---|---|
| 3 × 3 cm at 200 DPI (thumb impression) | 236 × 236 px (forms round to 240 × 240) |
| 10 × 5 cm at 200 DPI (declaration) | 787 × 394 px (forms round to 800 × 400) |
| 2 × 2 in at 300 DPI (US visa) | 600 × 600 px |
| A4 page at 200 DPI | 1654 × 2339 px |
Why a small KB limit needs fewer pixels
A JPEG at a readable quality spends very roughly 0.5 to 1.5 bits per pixel on a photo. Twenty kilobytes is 163,840 bits, so it holds a few hundred thousand pixels at most, about 500 × 400. That is why compress photo to 20 KB reduces the pixels after the quality search, and why 200 KB usually does not need to.
When the form gives all three
Set the pixels first (an exact width × height under Dimensions), then the KB range, then the DPI. The exam form presets do all three from the notice and check the result. If a file is too small for the minimum, increase photo size in KB explains what can and cannot be done honestly.