Formula and method
PPI = √(horizontal pixels² + vertical pixels²) ÷ diagonal inches
The pixel diagonal comes from the Pythagorean theorem. Dividing it by the physical diagonal gives pixels per inch; 25.4 ÷ PPI gives pixel pitch in millimeters.
Assumptions and limits
- Resolution is the panel’s native pixel grid.
- Pixels are square.
- Advertised diagonal is accurate enough for comparison.
Worked examples
27-inch 4K display
√(3840² + 2160²) ÷ 27 = about 163.18 PPI, with a pixel pitch near 0.156 mm.
24-inch 1080p monitor
√(1920² + 1080²) ÷ 24 = about 91.79 PPI, with a pixel pitch near 0.277 mm.
Common monitor PPI values
| Size / resolution | Approx. PPI | Pixel pitch |
|---|---|---|
| 24 in / 1920x1080 | 91.79 | 0.277 mm |
| 27 in / 1920x1080 | 81.59 | 0.311 mm |
| 27 in / 2560x1440 | 108.79 | 0.233 mm |
| 32 in / 2560x1440 | 91.79 | 0.277 mm |
| 27 in / 3840x2160 | 163.18 | 0.156 mm |
| 32 in / 3840x2160 | 137.68 | 0.184 mm |
Frequently asked questions
Is higher PPI always better?
It produces smaller pixels, but readability, scaling, viewing distance, panel quality and graphics performance also matter.
Is PPI the same as resolution?
No. Resolution counts pixels. PPI combines that count with physical size to describe density.
Why can two 4K monitors have different PPI?
They have the same pixel count but different physical sizes. A smaller 4K screen packs those pixels more densely.
What is pixel pitch?
Pixel pitch is the physical distance between pixels. It equals 25.4 mm divided by PPI.