DPI explained — what resolution you actually need
Why 72 dpi is a myth on screens, what 300 dpi means for printing, and how to work out the pixel dimensions you need from a physical size.
· 3 min read
DPI (dots per inch) only means something once a physical size is involved. An image on its own has pixel dimensions — 1200 × 800 — and nothing else. The dpi value is the bridge between those pixels and centimetres of paper.
pixels = physical size in inches × dpi
A 6 × 4 inch print at 300 dpi needs 1800 × 1200 pixels. The same file printed at 150 dpi covers 12 × 8 inches, at half the sharpness.
On a screen, dpi is irrelevant
This is the part that causes the most confusion. A screen displays pixels, so what matters is the pixel dimensions, not the dpi tag stored in the file. Saving an image "at 72 dpi" for the web does nothing at all — the browser ignores the value entirely.
What actually matters for the web:
| Use | Sensible width |
|---|---|
| Full-width banner | 1920 px |
| Article illustration | 1200 px |
| Thumbnail | 400–600 px |
| Logo in a header | 2× its displayed size |
The "2×" for logos and icons is the one place where screen density comes back: high-density displays render two or three physical pixels for each CSS pixel, so a logo shown at 150 px looks crisper if the file is 300 px wide.
For print, 300 dpi is the usual target
Commercial printing generally asks for 300 dpi, which is roughly where the eye stops resolving individual dots at reading distance. You can go lower as the viewing distance grows: 150 dpi is fine for a poster read from two metres, and large-format banners are often produced at 72 to 100 dpi because nobody stands close to them.
Common sizes at 300 dpi:
| Format | Pixels at 300 dpi |
|---|---|
| A4 (210 × 297 mm) | 2480 × 3508 |
| A5 | 1748 × 2480 |
| 10 × 15 cm photo | 1181 × 1772 |
| Business card (85 × 55 mm) | 1004 × 650 |
Convert PDF to JPG lets you pick the dpi directly, which is the easy way to turn a page into an image at exactly the resolution a printer asked for.
Raising the dpi does not add detail
Changing a file's dpi tag without changing its pixels only changes the size it will print at. Software that "upscales to 300 dpi" invents the extra pixels by interpolation: the file gets bigger, the picture does not get sharper, and enlarging past about 150% usually looks worse than the original.
If a printer says your file is too low-resolution, the only real fix is a larger original — reshoot, rescan at a higher setting, or use the vector version if one exists.
Scanning: what setting to choose
- 300 dpi — text documents, contracts, anything that will go through OCR.
- 600 dpi — small originals you will enlarge, or fine line art.
- 150 dpi — documents you only need to read on screen; the files stay much smaller.
A colour A4 page at 600 dpi produces roughly four times the data of the same page at 300 dpi, which is why scanned PDFs get so heavy. Resize Image brings an over-scanned image back to a sensible size.