DPI and Print Size Calculator

There is one relationship here and three ways round it: pixels equal inches times DPI. The confusion is almost never about the arithmetic and almost always about what DPI is. It is not a property an image carries in any meaningful sense — it is the ratio you choose when deciding how large to print. A file described as a 300 DPI image is only 300 DPI at one particular printed size, and the same file is 150 DPI when printed twice as large.

Which is why changing the DPI field in an image editor, without resampling, changes nothing about the pixels at all. It changes a note stored in the file about how large the image would like to be printed. Some software respects it, some ignores it, and neither adds or removes a single pixel.

300 DPI is the print standard because it is roughly where the eye stops resolving individual dots at normal reading distance. It is a convention rather than physics, and viewing distance is what actually decides it — a poster read from two metres is fine at 150, and a billboard is fine at 15. The trap is going the other way: an image printed at 100 DPI in the hand looks obviously soft, and no amount of upscaling fixes it, because enlarging cannot add detail that was never captured.

When the two axes give different figures, the image's aspect ratio does not match the print size, and the lower of the two is what limits quality. Cropping to the right shape first is usually better than stretching to fill.