PPI = √(width² + height²) ÷ diagonal size in inches. Pixel pitch is the distance between pixels in millimeters (25.4 ÷ PPI). Higher PPI means sharper text and images at the same viewing distance.
Example: a 27 inch 2560×1440 monitor has about 109 PPI. A 27 inch 4K (3840×2160) monitor has about 163 PPI.
Desktop monitors at arm's length look crisp around 110 PPI and very sharp from about 140 PPI. Phones are held closer, so 400 PPI or more is common.
PPI describes screens. DPI (dots per inch) describes printers. People often use them interchangeably.
See also: Aspect Ratio · Download Time · Percentage Calculator