The formula: diagonal pixels over diagonal inches
Resolution alone tells you nothing about sharpness. You need the physical size too. The numerator is the Pythagorean theorem giving the diagonal in pixels; dividing by the diagonal in inches gives pixels per inch. For 1920×1080 on a 24″ monitor: √(1920² + 1080²) ≈ 2203 px ÷ 24 ≈ 92 PPI.
Same resolution, different sharpness
This is the point people miss. A 1080p image is razor sharp on a phone and coarse on a big monitor, because the same pixels are packed into very different areas.
Both screens are 1920×1080. The phone crosses the retina line; the monitor does not, but it is viewed from farther away so it still looks fine.
PPI of common devices
| Device | Diagonal | Resolution | PPI |
|---|---|---|---|
| iPhone 16 Pro | 6.1″ | 2556×1179 | 460 |
| Samsung Galaxy S24 | 6.2″ | 2340×1080 | 402 |
| iPad Pro 12.9″ | 12.9″ | 2732×2048 | 264 |
| MacBook Pro 16″ | 16″ | 3456×2234 | 254 |
| 27″ 1440p monitor | 27″ | 2560×1440 | 109 |
| 55″ 4K TV | 55″ | 3840×2160 | 80 |
PPI is pixel density on a screen; DPI is dots per inch for printing. They are measured the same way but describe different things, so do not use a print DPI target to judge a display.
Common questions
How do I calculate PPI?
Find the diagonal in pixels with the Pythagorean theorem, the square root of width squared plus height squared, then divide by the screen diagonal in inches. A 1920 by 1080 screen at 24 inches works out to about 92 PPI.
What PPI counts as retina?
Apple set the retina threshold at 326 PPI for a phone held about 10 to 12 inches away. At that density a person with normal vision cannot pick out individual pixels. The threshold is lower for devices viewed farther away, like laptops and monitors.
Does higher PPI always mean a better screen?
No. Past the retina threshold the extra pixels are invisible at normal viewing distance and only cost battery and GPU power. Beyond that point, colour, brightness and contrast matter more than raw density.
