Aspect ratios & cropping math: what the lab cuts from your photo
Print sizes are ratios in disguise. A 4×6 is 2:3, an 8×10 is 4:5, and your camera is probably none of those exactly. When image ratio and paper ratio disagree, something gets cropped — by you, thoughtfully, or by the lab, brutally centered. The math is simple enough to do in your head, and the percentages below are exact: they are the share of your pixels that dies in that cut.
Key takeaways
- 3:2 sensors (most DSLRs/mirrorless) print 4×6/20×30/24×36 with zero crop — those sizes are 2:3.
- The classic pain: 3:2 → 8×10 or 16×20 (4:5) loses 16.7% of the frame's pixels.
- 4:3 (most phones) → 4×5 prints loses only 6.3%; → 5×7 loses 4.8%; → 4×6 loses 11.1%.
- Worst common case: 16:9 (video screenshots, some phones) → square loses 43.8% — nearly half the photo.
- Crop before ordering: cropping reduces pixels, which lowers effective PPI — run the crop result through the calculator.
The one-line method
Compare your image's width÷height with the paper's width÷height (long side to long side — you can always rotate). If the image is wider than the paper ratio, the sides get cut; if it's taller, top and bottom go. The kept fraction is simply the smaller ratio divided by the larger. Example: a 3:2 image (1.5) onto 4:5 paper (1.25) keeps 1.25 ÷ 1.5 = 83.3% of the pixel area — a 16.7% loss, all from the sides. The DPI calculator applies exactly this test and warns when your ratios differ by more than 2%.
Crop-loss table (percentage of pixels lost)
| Sensor / source ratio | → 2:3 (4×6, 20×30, 24×36) | → 5:7 | → 4:5 (8×10, 16×20) | → 7:9.1 approx (11×14) | → 1:1 (8×8, 12×12) |
|---|---|---|---|---|---|
| 3:2 (6000 × 4000) | 0% | 6.7% | 16.7% | 15.2% | 33.3% |
| 4:3 (4000 × 3000) | 11.1% | 4.8% | 6.3% | 4.5% | 25.0% |
| 16:9 (4096 × 2304) | 15.6% | 21.3% | 29.7% | 28.4% | 43.8% |
| 1:1 (square) | 33.3% | 28.6% | 20.0% | 21.4% | 0% |
Exact arithmetic: loss = 1 − (paper ratio ÷ image ratio) when the image is relatively wider, and the mirror image when it's taller. 11×14 prints as 1.273; A4 paper (1:1.414) sits between 5:7 and 3:2 — a 3:2 file loses 5.7% onto A4.
Reading the table like a photographer
The rows are cultures. The 3:2 row inherits 35-mm-film proportions, so the whole 2:3 print ladder (4×6 through 24×36) is its natural home — zero crop — while the 4:5 family (8×10, 16×20, and the 5×4 large-format tradition it echoes) charges a 16.7% toll. The 4:3 row is the phone era: remarkably efficient onto 4:5 and 5:7, mildly taxed onto 4×6. The 16:9 row is cinematic and pays for it on every still-print ratio — a square print from a widescreen screenshot keeps barely 56% of the frame. And squares are their own world: everything crops toward them, they crop toward nothing.
Percentages matter twice. First compositionally: 16.7% off the sides of a group portrait is a person. Second arithmetically: pixels lost are megapixels lost — a 24 MP 3:2 file printed 8×10 delivers from a 20 MP crop, so the effective PPI of the example above is 3000 px ÷ 10 in = 300 PPI, exactly at the classic target. Chain the two guides: pick the crop with this page, then check the result's effective PPI against the quality ladder or the calculator.
Which ratio for which print, practically
- Shooting 3:2 for an 8×10 job? Frame loose on the long edges — leave headroom you can afford to lose (16.7%).
- Phone photos to 4×6? Expect an 11.1% side-crop; keep subjects centered or pre-crop to 2:3.
- Ordering 11×14? It's a 1.273 ratio — between 5:7 and 4:5; both 3:2 (−15.2%) and 4:3 (−4.5%) survive it fine.
- Wall clusters mixing 4×6s and 8×10s force mixed crops from one ratio — decide per image which orientation flatters the crop, not the wall.
Crop it yourself
Labs center-crop by default when ratios mismatch. A centered 16.7% cut is fine for landscapes and terrible for off-center subjects. Crop deliberately in your editor at the paper's exact ratio, then upload — labs' ordering software, like mPix's, shows a preview of exactly the crop it will make; what you see at checkout is what ships.