PPI
Pixels per inch: the number of image pixels mapped to each inch of output.
Formula / rule
ppi = pixels / inches
Measured in
ppi
In depth
PPI describes the image, not the device, and it is the figure that actually governs how much detail a print can show. It is routinely confused with DPI, which describes ink droplets — a printer laying 2,880 droplets per inch may be rendering an image at only 300 ppi, and the two numbers are not comparable.
Examples in practice
Setting an export to 240 ppi for a 20-inch print means the file must be 4,800 pixels on that edge.