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.