"Upload a 35 × 45 mm photo." Your image editor asks for pixels. These are different kinds of units, and converting between them needs one extra piece of information: DPI (dots per inch — how densely the pixels will be printed).
The One Formula
pixels = millimetres ÷ 25.4 × DPI
pixels = centimetres ÷ 2.54 × DPI
pixels = inches × DPIThere are exactly 25.4 mm in an inch, so everything follows from that. At the print standard of 300 DPI: 1 inch = 300 px, 1 cm ≈ 118 px, 1 mm ≈ 11.8 px.
Worked example: 35 × 45 mm at 300 DPI → 35 ÷ 25.4 × 300 ≈ 413 px, 45 ÷ 25.4 × 300 ≈ 531 px. So 413 × 531 pixels.
Which DPI Should You Use?
- 300 DPI — the default for anything printed: ID photos, flyers, photo prints. Use this unless told otherwise.
- 200 DPI — acceptable for large prints viewed from a distance (posters); often indistinguishable from 300.
- 72 / 96 DPI — a legacy screen convention. Physical units are meaningless on screens (a "1 cm" box renders at different physical sizes on different displays), so for web work, work in pixels directly and ignore DPI.
If a portal states both a pixel size and a physical size, the pixel numbers win — that's what actually gets validated.
ID Photo Table (300 DPI)
| Size | Millimetres | Pixels |
|---|---|---|
| 1-inch (一寸) | 25 × 35 | 295 × 413 |
| Large 1-inch | 33 × 48 | 390 × 567 |
| 2-inch (二寸) | 35 × 49 | 413 × 579 |
| Visa / Schengen | 35 × 45 | 413 × 531 |
| US passport | 51 × 51 (2 × 2 in) | 600 × 600 |
Note: "2-inch" is defined as 35 × 49 mm by most portals but as 35 × 53 mm (413 × 626 px) by some. Always follow the numbers on the page you're uploading to.
Print Table (300 DPI)
| Pixels | |
|---|---|
| 4 × 6 in (10 × 15 cm) | 1200 × 1800 |
| 5 × 7 in (13 × 18 cm) | 1500 × 2100 |
| 8 × 10 in (20 × 25 cm) | 2400 × 3000 |
| A5 (148 × 210 mm) | 1748 × 2480 |
| A4 (210 × 297 mm) | 2480 × 3508 |
| A3 (297 × 420 mm) | 3508 × 4961 |
Applying It
Once you know the target in pixels, a resize tool gets you there in seconds — enter the width and height, keeping the aspect ratio locked if the ratio already matches, or cropping to the ratio first if it doesn't. Two cautions: never *upscale* to reach a print target (you'd be inventing pixels — the print will look soft), and check the file-size limit separately, since dimensions and kilobytes are independent constraints.