DPI, PPI, and printer dots
PPI (pixels per inch) describes the image file on paper. DPI is often used as a synonym in photography, which is how this site labels the quality bands. A printer’s hardware DPI (dots of ink per inch) is a different number. You cannot read a printer’s 1440 DPI off a JPEG and you should not try.
The 1-arc-minute rule
Normal visual acuity (Snellen 20/20) resolves detail of about one arc-minute. At a distance d inches, the corresponding spatial frequency is approximately 3438 / d pixels per inch. At 12 inches that is 287 PPI — which is why 300 DPI became the photographic default. At 48 inches it is 72 PPI. At 10 feet (120 inches) it is 29 PPI.
This is a ceiling for a viewer with typical eyesight, good lighting, and a still image. It is not a promise that a 72 DPI poster looks like a contact print. Texture, type, and high-contrast edges show softness sooner than a photographic sky. When in doubt, give posters 150 DPI and close-view products 300. The methodology page lists assumptions and limits.
Questions people actually ask
- Is 72 DPI the same as screen resolution?
- Historically Macs assumed 72 PPI and Windows 96 PPI. Modern phones exceed 400 PPI. 72 is not a screen standard anymore; it remains a rough large-format floor.
- Should I set 300 DPI in the export dialog?
- Only if the pixel dimensions already support the print size. Tagging a 1200×800 file as 300 DPI does not invent detail. Export at 1:1 pixels and let the print size determine PPI.