How it works
A pixel has no fixed physical size — it depends on the display or print resolution, measured in dots (or pixels) per inch. Dividing the pixel count by the DPI gives the size in inches.
inches = pixels ÷ DPI
cm = inches × 2.54 · points = inches × 72
cm = inches × 2.54 · points = inches × 72
Worked example
A 1920-pixel width at 96 DPI is 1920 ÷ 96 = 20 inches on screen, but only 6.4 inches at 300 DPI for print.