Print Resolution
The density of image pixels mapped onto a printed surface, which together with the file's pixel dimensions determines the print size.
Formula / rule
Print size = pixels / ppi
Measured in
ppi
In depth
Resolution is not a property of the file but of the relationship between the file and the paper. Around 300 ppi is the conventional target for prints examined at arm's length, but large prints viewed from further away hold up perfectly at 180 or even 150 ppi — which is why billboards are printed at a fraction of that.
Examples in practice
A 6000-pixel-wide file prints 20 inches wide at 300 ppi or 33 inches wide at 180 ppi.