PPI = √(w²+h²) / diagonal. Rough geometry for flat displays.
Diagonal pixel count = √(W² + H²) for positive integer pixel width and height; PPI = diagonal pixels ÷ diagonal inches. Extremely large PPI is rejected as a sanity check.
This calculator uses plain decimal numbers. If a result looks wrong, check whether you used a comma instead of a dot (or vice versa) where the field expects one.