Pixels, Inches, Millimetres and DPI
The one conversion table that settles the dpi argument.
The only formula you need
pixels = inches × dpi, and millimetres = inches × 25.4. Everything else in this topic is consequence. From those two, a 4 × 6 inch print at 300 dpi needs 4 × 300 = 1200 by 6 × 300 = 1800 pixels. An A4 sheet is 8.27 × 11.69 inches, so at 300 dpi it needs 2480 × 3508 pixels.
Common print sizes, resolved to pixels
| Print size | At 300 dpi | At 150 dpi | Typical use |
|---|---|---|---|
| 4 × 6 in (102 × 152 mm) | 1200 × 1800 | 600 × 900 | Standard photo print |
| 5 × 7 in (127 × 178 mm) | 1500 × 2100 | 750 × 1050 | Framed photo |
| 8 × 10 in (203 × 254 mm) | 2400 × 3000 | 1200 × 1500 | Portrait print |
| A4 (210 × 297 mm) | 2480 × 3508 | 1240 × 1754 | Document, poster |
| A3 (297 × 420 mm) | 3508 × 4961 | 1754 × 2480 | Large poster |
| Business card (3.5 × 2 in) | 1050 × 600 | 525 × 300 | Print at 300 dpi is standard |
What dpi should you actually target
- 300 dpi — the commercial printing standard. Use it unless a lab or specification tells you otherwise.
- 150 dpi — acceptable for a poster viewed from a distance, or a large banner where the viewing distance compensates for the lower density.
- 600 dpi — required for line art, small text and anything with fine detail, where 300 dpi would alias.
- 72 or 96 dpi — screen conventions with no bearing on print. These numbers are historical artefacts of CRT and early display densities; a JPEG carrying '72 dpi' in its header can still print beautifully if it has enough pixels.
A file's dpi tag changes nothing about its pixels. Setting an image's dpi from 72 to 300 in an editor does not add detail — it changes the physical size the pixels are mapped onto. If a printer asks for '300 dpi at 8 × 10 inches', the requirement is 2400 × 3000 pixels, and that is the number to work with.
Working backwards from a file you already have
Any existing image can be evaluated: divide its pixel dimensions by the print size. A 1920 × 1080 photo printed at 8 × 10 inches gives 1920 ÷ 8 = 240 and 1080 ÷ 10 = 108 dpi. It is limited by the shorter axis, so the effective resolution is 108 dpi — acceptable for a poster seen from a metre away, marginal for a hand-held print. The same file at 4 × 6 inches yields 1920 ÷ 4 = 480 and 1080 ÷ 6 = 180, so 180 dpi, which is fine.
Interpolation does not create resolution
Upscaling software produces new pixels by interpolation and can look smoother, but it cannot recover detail the lens and sensor never captured. The practical rule: the pixel count is the hard ceiling on detail, and printing beyond it trades sharpness for size. For anything where sharpness is judged — product photography, architectural prints, gallery work — get the pixels right in the camera rather than in a converter.
How to use it
- Enter any two of pixel dimensions, print size, or dpi.
- Read the third value.
- Check the resulting dpi against the target for your print method.
- Use the pixel figure as the export size in your editor.
Worth knowing
- pixels = inches × dpi; millimetres = inches × 25.4.
- A4 at 300 dpi is 2480 × 3508 pixels.
- Effective print resolution is limited by the shorter axis of the image.
- The dpi value stored in a file header has no effect on the pixel data.
Limitations
- This is arithmetic on your inputs — it cannot judge the sharpness of a file.
- Printers resample internally, so actual output depends on the RIP or driver.
- Interpolation settings in an editor affect perceived quality and are outside this calculation.
- Non-standard print sizes vary by lab and by paper stock.