The formula
Print width (inches) = pixel width ÷ DPI. The same applies to height. A 3600×2400 file at 300 DPI is 12×8 inches. At 150 DPI it is 24×16 inches. Nothing else enters the calculation — not megapixels, not the JPEG quality slider, not the printer driver.
DPI here means pixels per inch on the paper, also called PPI. Printers lay down dots at their own native resolution (often 1440 or 2400). Those hardware dots are not the same as image pixels. For planning a file, pixels per inch is the number that matters.
Worked numbers
- 4032×3024 (12 MP phone) at 300 DPI → 13.44×10.08 in (about 34.1×25.6 cm).
- 6000×4000 (24 MP, 3:2) at 300 DPI → 20×13.33 in.
- 8192×5464 (45 MP) at 300 DPI → 27.3×18.2 in, enough for a 16×24 with crop to spare.
If you need the inverse — a paper size in, pixels out — use pixels needed. To judge whether that size is sharp, return to the print checker.
Questions people actually ask
- Does the DPI in Photoshop’s Image Size dialog change print size?
- Only if you resample. If resample is off, changing DPI just retags the file; pixel counts stay put and the print size in inches moves inversely.
- Why is my lab’s size different from this calculator?
- Labs crop to the paper’s aspect ratio. This page reports uncropped size. The checker and crop tools show the fill-crop.