Pixel density (Pixels Per Inch, or PPI) measures how tightly packed pixels are on a screen. Higher PPI produces sharper text and images at close viewing distances.
Calculate PPI: divide display diagonal pixel count by diagonal physical size. A 6.7-inch phone with 2796×1290 (FHD+) resolution has ~460 PPI.
Apple's Retina threshold is 300 PPI for phones at 25 cm viewing distance. Above ~400 PPI, individual pixels become indistinguishable to the human eye.
Laptops typically range 100–230 PPI; phones 350–500; VR headsets target 1500+ PPI for immersion. Higher PPI consumes more battery (more pixels to drive) and CPU/GPU resources.