PPI (Pixels Per Inch) measures pixel density on a screen. Calculated as diagonal pixel count divided by diagonal inches. Higher PPI = sharper image at close viewing distance.
PPI formula: sqrt(horizontal² + vertical²) ÷ diagonal screen inches.
Example: 6.1-inch iPhone 14 Pro with 2556×1179 resolution. sqrt(2556² + 1179²) ÷ 6.1 = sqrt(8,116,317) ÷ 6.1 = 2851 ÷ 6.1 ≈ 460 PPI.
Key thresholds: 100 PPI: typical laptop 1080p 15.6-inch (text starts pixelating). 200 PPI: budget phone or large tablet (individual pixels visible). 300 PPI: Apple's "Retina" threshold (25 cm viewing distance, pixels indistinguishable). 400 PPI: flagship phones, desktop monitors at typical distance (very sharp). 500+ PPI: premium phones, VR headsets (pixel-free appearance). 1500+ PPI: VR requirement for immersion (no screen-door effect).
Context matters: a 1440p monitor viewed 60 cm away looks sharp at 100 PPI, but a phone viewed 20 cm away needs 300+ PPI for same sharpness.
Diminishing returns: jump from 200 to 300 PPI is very noticeable; jump from 400 to 500 PPI is marginal (harder to see 2 pixels become 1).
PPI vs resolution: a 4K 32-inch monitor (163 PPI) looks less sharp than a 1080p 5-inch phone (440 PPI) due to distance.
Buying implication: smartphone/tablet flagships hit ~400 PPI sweet spot (sharp, no pixel grid). Anything 250+ PPI is acceptable. Cheaper displays <200 PPI may look rough if text-heavy.