Pixels to real-world size
An image in pixels has no fixed physical size until you choose a resolution. At 300 DPI a 4000×3000 photo prints at about 13.3 × 10 inches; at 72 DPI the same file would span roughly 55 × 42 inches on a screen. This calculator converts any pixel dimensions and DPI into inches, centimeters, and millimeters.
Physical size = pixels ÷ DPI
When you need it
Use it to plan a print, to check how large a scanned document will come out, or to work out the real size of a design before sending it to a printer. To then see that size at true scale, hold the result up to the actual-size ruler and measure directly on your calibrated screen.