What PPI measures and why it matters
Pixels per inch (PPI) describes how many pixels are packed into each inch of a display. A higher PPI means smaller individual pixels, which makes text crisper and images smoother. At typical viewing distances, the eye cannot distinguish individual pixels above about 300 PPI, which is why Apple coined the term "Retina" display for screens above approximately 220–300 PPI depending on the device type and expected viewing distance.
PPI is also useful for comparing screens with different sizes and resolutions. A 27-inch 1440p monitor (about 109 PPI) and a 6-inch 1080p phone screen (about 367 PPI) have the same number of total pixels, but the phone screen looks far sharper because those pixels are packed into a much smaller area.
Diagonal resolution = √(Width² + Height²)
Total pixels across the diagonal, found by applying the Pythagorean theorem to the horizontal and vertical pixel counts.
PPI = Diagonal resolution / Diagonal size (inches)
Dividing the diagonal pixel count by the physical diagonal in inches gives pixels per inch.
Dot pitch (mm) = 25.4 / PPI
Physical distance between pixel centres. Lower dot pitch means pixels are closer together and the image appears finer.