PPI calculator
Pixel density is not printed on the box: it follows from the resolution and the diagonal. Enter both and get pixels per inch, dot pitch in millimetres, and the physical width and height of the panel.
24″ 1920 × 1080 = 92 PPI · 27″ 4K = 163 PPI · 6.1″ 2556 × 1179 = 460 PPI
Common screens
| Screen | Resolution | Diagonal | PPI | Panel size |
|---|---|---|---|---|
| 24-inch Full HD monitor | 1920 × 1080 | 24" | 92 | 53 × 30 cm |
| 27-inch QHD monitor | 2560 × 1440 | 27" | 109 | 60 × 34 cm |
| 27-inch 4K monitor | 3840 × 2160 | 27" | 163 | 60 × 34 cm |
| 32-inch 4K monitor | 3840 × 2160 | 32" | 138 | 71 × 40 cm |
| 13.6-inch MacBook Air | 2560 × 1664 | 13.6" | 225 | 29 × 19 cm |
| 14-inch laptop Full HD | 1920 × 1080 | 14" | 157 | 31 × 17 cm |
| 15.6-inch laptop Full HD | 1920 × 1080 | 15.6" | 141 | 35 × 19 cm |
| 6.1-inch iPhone 15 | 2556 × 1179 | 6.1" | 461 | 14 × 6 cm |
| 6.7-inch phone | 2796 × 1290 | 6.7" | 460 | 15 × 7 cm |
| 11-inch iPad Pro | 2420 × 1668 | 11" | 267 | 23 × 16 cm |
| 55-inch 4K TV | 3840 × 2160 | 55" | 80 | 122 × 68 cm |
| 65-inch 4K TV | 3840 × 2160 | 65" | 68 | 144 × 81 cm |
Reading the result
Dot pitch is the distance between two pixel centres (25.4 mm ÷ PPI); below about 0.2 mm individual pixels stop being visible at desk distance. The physical size assumes the diagonal is the active area, which is what manufacturers quote to within a tenth of an inch. Operating systems scale the interface on high-density screens (200 % on a 27-inch 4K, so it behaves like 1920 × 1080 with sharper text), which is why CSS pixels are not device pixels; the px to rem page deals with the CSS side.
Frequently asked questions
PPI = √(width² + height²) ÷ diagonal in inches. For 1920 × 1080 on 24 inches: √(3,686,400 + 1,166,400) = 2202.9 pixels along the diagonal, ÷ 24 = 92 PPI.
PPI (pixels per inch) describes a screen or an image’s intended size; DPI (dots per inch) describes a printer’s ink dots. Image editors use "dpi" loosely for both. A 300 dpi print needs an image of 300 pixels per printed inch.
Apple’s original threshold was 300 PPI at a 10 to 12 inch viewing distance (phones) and about 220 PPI for laptops viewed further away. Beyond those densities the eye cannot resolve individual pixels.
Only through the pixel counts. Two screens with the same diagonal and the same total pixels along the diagonal have the same PPI whatever their shape.