Pixels, inches and DPI
A digital image is a grid of pixels. When you print it, the printer spreads those pixels over the paper at a given density, measured in dots per inch (DPI) or pixels per inch (PPI). The more pixels per inch, the sharper the print, but the smaller it gets.
- Print width (in) = width in pixels ÷ DPI
- Print height (in) = height in pixels ÷ DPI
- Megapixels = width × height ÷ 1,000,000
Worked example
A 12-megapixel photo measures 4000 × 3000 pixels.
- At 300 DPI: 4000 ÷ 300 = 13.3 in wide and 3000 ÷ 300 = 10 in high, about 33.9 × 25.4 cm.
- At 150 DPI: up to 26.7 × 20 in, fine for a poster seen from a few feet away.
Which DPI to use
| Use | Recommended DPI | Why |
|---|---|---|
| Photo prints, albums, fine art | 300 | Viewed at arm’s length |
| Flyers, magazines | 300 | Text and details must be crisp |
| Posters | 150 to 200 | Viewed from 3 to 6 feet |
| Large banners, billboards | 20 to 100 | Viewed from far away |
The farther away a print is viewed, the lower the resolution can be: the eye cannot resolve fine detail at a distance.
Pixels needed for common print sizes at 300 DPI
| Print size | Pixels needed | Megapixels |
|---|---|---|
| 4 × 6 in | 1200 × 1800 | 2.2 |
| 5 × 7 in | 1500 × 2100 | 3.2 |
| 8 × 10 in | 2400 × 3000 | 7.2 |
| 11 × 14 in | 3300 × 4200 | 13.9 |
| 16 × 20 in | 4800 × 6000 | 28.8 |
Aspect ratio and cropping
Most cameras shoot at a 3:2 or 4:3 ratio, while common frames such as 8 × 10 are 5:4. Printing at a different ratio means cropping part of the image or leaving white borders. Check the crop before you order, and keep important subjects away from the edges.
Changing the DPI setting
Changing the DPI number in an editor without resampling only changes the print size, not the image itself. Upscaling adds pixels by interpolation or AI, which can help for moderate enlargements, but it cannot recover detail the camera never captured. For a big print, start from the original full-resolution file, not a copy shared through a messaging app, which is often compressed and downsized.