Pixel Dimensions
The width and height of an image measured in pixels, which is its only fixed measure of size.
Formula / rule
Total pixels = width × height
Measured in
px
In depth
Pixel dimensions are absolute; physical size is not, because it depends entirely on the chosen output resolution. An image is 6000 by 4000 pixels regardless of whether it is destined for a phone screen or a gallery wall, and the inches only exist once a print resolution is specified.
Examples in practice
A 6000×4000 file prints at 25 inches wide at 240 ppi or at 8.3 inches at 720 ppi — the same file either way.