How PPI is worked out
PPI comes from three numbers: the horizontal resolution, the vertical resolution and the diagonal size. Count the pixels along the diagonal with Pythagoras, then divide by the diagonal in inches.
Take a 27-inch (69 cm) monitor at 2560 × 1440. It has 2937 pixels from corner to corner, and spread over 27 inches that's 109 PPI. The aspect ratio doesn't change the math. Only the pixel count and the diagonal do. That's why a 34-inch 3440 × 1440 ultrawide lands at 110 PPI, close to the 27-inch, despite its extra width.
Some makers list pixel pitch instead: the distance from the center of one pixel to the next. It's the same idea turned around. That 27-inch 1440p panel has a pitch of 0.233 mm.
When more PPI stops mattering
Higher PPI means smaller pixels, but your eye has a limit. With normal vision you can pick out detail down to about one arcminute, a sixtieth of a degree. Past a certain distance two neighboring pixels fall inside that angle and blur into one. That distance is called the Retina distance, and it works out to about 3438 divided by the PPI, in inches.
So PPI only tells half the story. A 27-inch 1440p monitor turns smooth from 32 inches (80 cm) away. A 55-inch 4K TV has fewer pixels per inch at 80 PPI, yet it turns smooth from 43 inches (109 cm). Most people sit 6.5-10 feet (2-3 m) from a TV, well past that point. A phone needs the highest density of all, because you hold it about 12 inches (30 cm) from your face.
Most people sit about 24-28 inches (60-70 cm) from a desk monitor. That's closer than the Retina distance of a 27-inch 1440p screen, so you can still see its pixels on small text if you look for them. A 27-inch 4K panel turns smooth from 21 inches (54 cm), so at a normal desk distance its pixels disappear.
PPI of common screens
Here's where popular monitor sizes land, with the distance where single pixels stop showing:
- 24-inch 1080p: 92 PPI, smooth from 37 inches (95 cm)
- 27-inch 1080p: 82 PPI, smooth from 42 inches (107 cm)
- 27-inch 1440p: 109 PPI, smooth from 32 inches (80 cm)
- 32-inch 4K: 138 PPI, smooth from 25 inches (63 cm)
- 27-inch 4K: 163 PPI, smooth from 21 inches (54 cm)
- 27-inch 5K: 218 PPI, smooth from 16 inches (40 cm)
The PPI Calculator works this out for any size and resolution, and can put two screens side by side.
What higher PPI costs you
Dense screens draw text and icons too small at 100%, so the operating system scales them up. A 27-inch 4K monitor usually runs at 150% in Windows, which leaves a desktop of 2560 × 1440. That's the space of a 1440p screen, drawn with sharper text, since the 4K panel has 163 PPI against 109. The Display Scaling Calculator shows the space you end up with at each setting.
More pixels also mean more work for your graphics card in games. 4K has 2.25 times the pixels of 1440p, so the same card renders fewer frames per second at the same settings.
PPI vs DPI
DPI means dots per inch. It comes from printing, where it counts the ink dots a printer lays down in an inch, and it's often used loosely for screens too. For a screen, PPI is the right term, since a screen is made of pixels, not dots. A mouse's DPI is something else again: how many steps it reports for each inch you move it.