Printing is where resolution stops being abstract: the paper size is fixed, so fewer pixels simply means each pixel prints bigger and blurrier.
The Requirement Table (300 DPI)
| Print size | Pixels needed | Megapixels |
|---|---|---|
| 4 × 6 in / 10 × 15 cm | 1200 × 1800 | 2.2 MP |
| 5 × 7 in / 13 × 18 cm | 1500 × 2100 | 3.2 MP |
| 6 × 8 in / 15 × 20 cm | 1800 × 2400 | 4.3 MP |
| 8 × 10 in / 20 × 25 cm | 2400 × 3000 | 7.2 MP |
| A5 (148 × 210 mm) | 1748 × 2480 | 4.3 MP |
| A4 (210 × 297 mm) | 2480 × 3508 | 8.7 MP |
| A3 (297 × 420 mm) | 3508 × 4961 | 17.4 MP |
The formula behind every row: pixels = inches × DPI (and inches = mm ÷ 25.4).
Good news for most people: any modern phone camera (12 MP and up) comfortably covers everything through A4. The photos that fail are the ones already shrunk — a WhatsApp-received image, a screenshot, a web download.
How Low Can You Go?
300 DPI is the quality standard, but viewing distance changes what's acceptable:
- 200 DPI — hard to distinguish from 300 in a held print; a perfectly reasonable floor for a 4 × 6.
- 150 DPI — visibly softer up close, fine for a poster on a wall.
- Below 100 DPI — obviously blurry at any normal distance.
So a 1000 × 1500 px photo prints beautifully at 4 × 6 (250 DPI) and poorly at A4 (~120 DPI).
The Step Everyone Forgets: Crop to the Print Ratio
Print sizes have their own aspect ratios, and they don't match your camera:
- Phone/mirrorless photos are usually 4:3 or 3:2.
- 4 × 6 prints are 3:2, 5 × 7 is 7:5, 8 × 10 is 5:4, A-series is roughly 1.41:1.
If you hand a 4:3 photo to a 4 × 6 print, the lab crops the difference — usually from the top and bottom, which is exactly where heads get clipped. Crop to the print's ratio yourself so you decide what leaves the frame, then resize to the pixel target.
Practical Order of Operations
- Check the lab's required ratio and pixels for your chosen size.
- Crop to that aspect ratio (fixed-ratio crop, keeping faces off the edges).
- Resize to the pixel target — downscale only; never upscale a small photo to fake resolution, since the print just magnifies the softness.
- Export JPEG at quality 90+ (printing is not the place to save kilobytes) and keep sRGB unless the lab asks otherwise.
Do those four and what comes back from the lab looks like what you saw on screen.