KB, pixels, cm and DPI: photo size rules on application forms, explained
Application forms describe photos in four different units — centimetres, pixels, DPI and kilobytes — often all in one sentence: "Upload a recent photograph, 3.5 × 4.5 cm, 200 DPI, between 20KB and 50KB, JPG." They measure different things, and a photo can pass one rule while failing another. Here is what each one means and how they fit together.
Resize a photo →Pick a preset or type the pixels, drag the frame onto your face and save under the KB limit.
The four units
| Unit | What it measures | What the upload check sees |
|---|---|---|
| Pixels (px) | The width and height of the image in dots, e.g. 200 × 230 | Exactly this — portals read the pixel size directly |
| cm / mm / inches | The printed size of the photo | Nothing on its own; it becomes pixels via the DPI |
| DPI (dots per inch) | How many pixels go into one printed inch | A conversion factor between cm and pixels |
| KB (kilobytes) | How much storage the file takes | The file size — checked against a minimum and/or maximum |
The short version: pixels and KB are what get checked. Centimetres plus DPI are just another way of describing pixels.
Converting cm to pixels
One inch is 2.54 cm. So:
For a 3.5 cm wide photo at 300 DPI: 3.5 ÷ 2.54 × 300 = 413 pixels. Common sizes worked out:
| Printed size | 100 DPI | 200 DPI | 300 DPI |
|---|---|---|---|
| 3.5 × 4.5 cm (passport, most ID photos) | 138 × 177 | 276 × 354 | 413 × 531 |
| 3 × 4 cm (Korean half-size ID photo) | 118 × 157 | 236 × 315 | 354 × 472 |
| 2 × 2 inch (US visa) | 200 × 200 | 400 × 400 | 600 × 600 |
| 6 × 2 cm (signature) | 236 × 79 | 472 × 157 | 709 × 236 |
| 3.5 × 2 cm (signature box) | 138 × 79 | 276 × 157 | 413 × 236 |
If a form gives centimetres but no DPI, there is no single right pixel size. In practice 200–300 DPI looks good and stays well inside typical KB limits. If the form gives pixels, use the pixels and ignore the centimetres.
Why DPI alone doesn't change anything
A JPEG stores pixels plus a small note saying how many of them to print per inch. Changing that note from 72 to 300 DPI in an editor doesn't add or remove a single pixel, so the file looks identical and its KB stays the same. Portals that ask for "200 DPI" are really asking for enough pixels for the printed size — work out the pixels with the table above.
What decides the KB
For a JPEG, three things set the file size:
- Pixel count. 413 × 531 has nearly five times as many pixels as 200 × 230 and usually needs several times the KB.
- Detail in the picture. A plain white background and an evenly lit face compress well; a patterned shirt, a busy background or camera noise take more bytes.
- JPEG quality. The quality setting trades sharpness for size. Between about 70% and 90% the difference is hard to see but the size can double.
That is why "resize to 200 × 230" doesn't guarantee "under 50KB", and why two people's photos at the same pixel size can differ by 20KB. A good resizer sets the pixels first, then searches for the highest quality that lands inside the KB range.
When the file is too small
Ranges like "20–50KB" or "30–49KB" have a minimum, and simple images — especially signatures on white paper — often come out below it. The options, from best to worst:
- Raise the JPEG quality. More detail is kept, KB goes up, nothing looks worse.
- Pad the file. JPEG and PNG files can carry extra data that viewers ignore (a comment block). Adding it raises the KB without touching the picture. SlimIO does this automatically when a result is still under the minimum.
- Enlarge the pixels. Works, but stretching a small image makes it softer, and it can break a pixel limit.
KB: 1000 or 1024 bytes?
Windows shows file sizes in units of 1024 bytes and calls them KB; macOS, phones and most websites use 1000. The same file of 50,500 bytes shows as 50.5KB on a Mac and 49.3KB on Windows. You can't know which one a portal uses, so stay safely inside the range: for "under 50KB" keep the file under 50,000 bytes, and for "at least 20KB" make it at least 20,480 bytes. SlimIO's limits work this way, so its results pass under either definition.
Shape: crop, don't stretch
3.5 × 4.5 cm is a 7 : 9 shape; 200 × 230 px is a little wider; a phone photo is usually 3 : 4 or 9 : 16. Squeezing one shape into another makes faces look wide or thin, and portals that check face proportions may reject it. Crop to the right shape first — drag the frame onto your face on the ID photo page — and then scale down.
A checklist before you upload
- Pixel size matches the form (or the cm converted at 200–300 DPI).
- File size is inside the KB range, with a little margin on both ends.
- Format is what's asked for — usually JPG/JPEG.
- The face or signature fills the frame; the background is plain.
For exam-specific numbers see photo and signature sizes for Indian exams; for signatures, how to make a clean signature image.
Pick a preset or type the pixels, drag the frame onto your face and save under the KB limit.