PPI counts image pixels
Pixels per inch describes how tightly the pixels of an image are packed when printed. An image that is 3000 pixels wide prints 10 inches wide at 300 PPI, or 20 inches wide at 150 PPI, without changing its pixel dimensions.
For the same source image, increasing PPI decreases physical print size. Use the pixels to print size calculator to compare settings.
DPI counts printer dots
Dots per inch describes a printer’s output resolution. A printer can use multiple ink or toner dots to reproduce the colour and tone of one image pixel. A 1200 DPI printer does not mean your photograph needs to be supplied at 1200 PPI.
People sometimes use “DPI” when they mean image resolution. In an image editor’s size dialogue, the pixels-per-inch value is the relevant number for converting image dimensions to physical dimensions.
Is 300 PPI always necessary?
300 PPI is a common working setting for detailed prints viewed up close. It is a starting point rather than a guarantee. A large poster seen from further away can need less image detail per inch; a poor original image may still look poor at 300 PPI.
Follow the print provider’s specification when one is supplied. Consider the paper, printing process, viewing distance and image content, and make a test at the intended output size.
Changing a number does not create detail
Changing PPI with resampling disabled redistributes existing pixels over a different physical area. Resampling changes the number of pixels, using calculated values for any additions. It can help prepare a file for a process, but it does not recreate lost photographic information.
A useful workflow is to start with the original file, decide the physical size, check the resulting PPI, and inspect a printed sample. Avoid repeatedly enlarging and saving compressed versions.