DPI explained: how big can you print that photo?

DPI causes more confusion than any other imaging term, mostly because it does not describe the image on its own.

An image has pixels, not DPI

A photo that is 3000 by 2000 pixels is 3000 by 2000 pixels, full stop. DPI, dots per inch, only appears when you decide how large to print it. Print those 3000 pixels across 10 inches and you get 300 DPI. Print the same file across 30 inches and you get 100 DPI. The file never changed.

This is why "save it at 300 DPI" is meaningless without a size, and why a 72 DPI tag on a web image says nothing about its quality.

The numbers that matter

• 300 DPI is the standard for photo prints and anything held in the hand.
• 240 DPI is very good and most people cannot tell it apart from 300.
• 150 DPI is fine for posters viewed from a metre away.
• 100 DPI or less suits banners and hoardings, which are seen from far off.

Working it out

Divide pixels by inches. A 3000 pixel wide photo printed at 300 DPI gives a 10 inch wide print. Print Size Calculator does this for you and shows the result in inches and centimetres at three quality levels, and DPI Calculator works in the other direction when you know the size you need.

Not sure what you have? Image Metadata and DPI Viewer reports the pixel dimensions and any DPI tag stored in the file.

Why upscaling does not help

Enlarging a small image adds pixels but no detail. The software invents them by averaging neighbours, which is why upscaled photos look soft or plasticky. If you need a large sharp print, you need a large original. The only real fix is to reshoot or rescan at a higher resolution.

For printing at an exact physical size, Resize Image in cm or inches renders at 300 DPI, which is what photo labs expect.