The conversion
The formula is pixels = (centimetres ÷ 2.54) × DPI. So a 10 × 15cm photo at 300 DPI is 1181 × 1772 pixels.
DPI is the part people miss. The same 10 × 15cm target produces wildly different pixel counts at different resolutions, and using the wrong one is why images come out of the printer blurry or absurdly large.
| Physical size | At 150 DPI | At 300 DPI | At 600 DPI |
|---|---|---|---|
| 3.5 × 4.5cm (passport) | 207 × 266 | 413 × 531 | 827 × 1063 |
| 10 × 15cm (6×4in print) | 591 × 886 | 1181 × 1772 | 2362 × 3543 |
| A4 (21 × 29.7cm) | 1240 × 1754 | 2480 × 3508 | 4960 × 7016 |
| A5 (14.8 × 21cm) | 874 × 1240 | 1748 × 2480 | 3496 × 4960 |
Which DPI to choose
Use 300 DPI for anything printed and viewed at normal reading distance — photos, documents, passport photos, flyers. It is the standard print resolution and the usual minimum specified by printers and government offices.
Use 150 DPI for large-format prints viewed from a distance, such as posters and banners, where the eye cannot resolve more detail anyway. Use 600 DPI only when explicitly required, and only if your source image genuinely contains that much detail — upscaling adds pixels without adding information.
Upscaling cannot add detail
If your source photo is 800 × 600 and you ask for A4 at 300 DPI (2480 × 3508), the tool will produce those pixels by interpolating between existing ones. The file gets bigger; the image does not get sharper. It gets softer.
Check that your source has at least as many pixels as your target before resizing up. If it does not, print smaller or start from a higher-resolution original.
Frequently asked questions
- How many pixels is 1 cm?
- It depends on DPI. At 300 DPI, 1cm is about 118 pixels; at 150 DPI it is about 59. Without a DPI figure the question has no answer.
- What DPI should I use for printing?
- 300 DPI for normal viewing distances. 150 DPI is fine for posters and banners seen from further away.
- Can I enlarge a small image for printing?
- You can add pixels, but not detail — the result will look soft. Print at a smaller physical size, or start from a higher-resolution original.