The cm to pixels formula
An image file stores pixels, not centimetres. A size in cm only becomes a pixel count once you choose how many pixels go into each inch: the DPI (dots per inch, also called PPI). One inch is exactly 2.54 cm, so:
- Pixels from cm: cm ÷ 2.54 × DPI
- Pixels from mm: mm ÷ 25.4 × DPI
- Pixels from inches: inches × DPI
Take 3.5 cm at 300 DPI: 3.5 ÷ 2.54 × 300 = 413.386, which rounds to 413 px. The height, 4.5 cm, gives 531.496, which rounds to 531 px.
Round once, at the end. If you round 1 cm to 118 px first and then multiply, larger sizes drift: 21 × 118 = 2478, but the exact width of A4 at 300 DPI is 2480 px. The calculator always works from the full size.
Pixels per centimetre at each DPI
| DPI | Pixels per cm | 1 cm | 5 cm | 10 cm |
|---|---|---|---|---|
| 72 | 28.35 | 28 px | 142 px | 283 px |
| 96 | 37.8 | 38 px | 189 px | 378 px |
| 150 | 59.06 | 59 px | 295 px | 591 px |
| 200 | 78.74 | 79 px | 394 px | 787 px |
| 300 | 118.11 | 118 px | 591 px | 1181 px |
| 600 | 236.22 | 236 px | 1181 px | 2362 px |
For millimetres, divide the pixels per cm by 10: at 300 DPI, 1 mm is 11.81 pixels.
Common photo and paper sizes in pixels
Worked out with the same rounding as the calculator. Width comes first, then height.
| Size | At 200 DPI | At 300 DPI | Often used for |
|---|---|---|---|
| 3.5 × 4.5 cm (35 × 45 mm) | 276 × 354 px | 413 × 531 px | Passport-size photos |
| 3 × 4 cm (30 × 40 mm) | 236 × 315 px | 354 × 472 px | ID and student cards |
| 5 × 7 cm (50 × 70 mm) | 394 × 551 px | 591 × 827 px | Canadian passport photos |
| 6 × 2 cm | 472 × 157 px | 709 × 236 px | SSC signature preset |
| 3.5 × 2 cm | 276 × 157 px | 413 × 236 px | RRB signature box (35 × 20 mm) |
| 10 × 15 cm | 787 × 1181 px | 1181 × 1772 px | Photo prints |
| A5 (14.8 × 21 cm) | 1165 × 1654 px | 1748 × 2480 px | Half an A4 page |
| A4 (21 × 29.7 cm) | 1654 × 2339 px | 2480 × 3508 px | Documents and certificates |
| A3 (29.7 × 42 cm) | 2339 × 3307 px | 3508 × 4961 px | Posters and drawings |
A form's own numbers come first. When a notice lists pixels, such as the 200 × 230 px photo for IBPS and SBI forms, use those pixels and its KB range: the cm size and DPI no longer decide the upload. These presets are already set up: 3.5 × 4.5 cm, 3 × 4 cm, IBPS and SBI photo and SSC signature.
DPI changes the print size, not the pixels
A pixel has no fixed size. DPI is a number stored in the file that tells printing software how closely to pack the pixels on paper. Change it and the same pixels print larger or smaller:
| 413 × 531 px printed at | Print size |
|---|---|
| 72 DPI | 14.57 × 18.73 cm |
| 96 DPI | 10.93 × 14.05 cm |
| 150 DPI | 6.99 × 8.99 cm |
| 200 DPI | 5.25 × 6.74 cm |
| 300 DPI | 3.5 × 4.5 cm |
| 600 DPI | 1.75 × 2.25 cm |
Screens ignore DPI altogether. A phone or a website shows an image by its pixels and the space it is given, so 413 × 531 px looks the same on screen whether the file says 72 DPI or 300 DPI. That is why upload portals mostly check pixels and KB, and why a size in cm only matters once you print.
To change only the stored DPI, without touching the pixels, use the DPI Converter. Adding a 300 DPI tag to a photo with the wrong pixel size will not make it print at 3.5 × 4.5 cm.
Which DPI to choose
- 300 DPI is the usual figure for sharp photo prints and for forms that give a photo size in cm. The calculator starts here.
- 200 DPI gives a lighter file at the same print size. The RRB signature preset uses it: 3.5 × 2.0 cm at 200 DPI is 276 × 157 px, above the 140 × 60 px minimum in the notices.
- 150 DPI suits uploads with a small KB limit. A 3.5 × 4.5 cm photo becomes 207 × 266 px.
- 600 DPI doubles the pixels on each side compared with 300 DPI, for very detailed prints, at a much larger file size.
- 72 and 96 DPI are screen conventions. They only matter when software asks you for a DPI to turn cm into pixels.
If your form states a DPI, use that one. If it gives pixels, use the pixels and leave the DPI aside.
From the numbers to a finished photo
The calculator only does the arithmetic: it never opens or changes an image. Once you know the size you need:
- Make the image with Resize Image. Choose cm / inch, enter the size and DPI, and it crops or fits your photo and writes the DPI into the JPG or PNG.
- For the most common form photo, the 3.5 × 4.5 cm resizer opens already set to 413 × 531 px at 300 DPI.
- If the form also caps the file in KB, turn on Limit file size in the resizer. The pixels stay fixed and only the JPG quality drops.
- Check the finished file with the Image Size Checker before you upload it.
For an official passport or visa photo, the size is only part of the rules. The Passport Photo Maker adds head-size guides for each country.