PPI calculator

What PPI means for sharpness

PPI, pixels per inch, counts the pixels along one inch of screen. The more pixels share an inch, the smaller each one is: about 0.28 mm across on a 24-inch 1080p monitor, and 0.16 mm on a 27-inch 4K one.

RTINGS puts a 27-inch 4K monitor at 163 PPI and a 65-inch 4K TV at 68. From a distance the gap fades, RTINGS notes, but at a desk text looks much sharper on the denser screen. On OLED monitors, whose subpixels aren't laid out in plain stripes, RTINGS' text clarity tests found that more density also shrinks the color fringes around text, which you can check on the monitor test's text pattern.

Density also decides how much scaling a screen needs. Microsoft's guide for Windows developers sets 100% at 96 pixels per inch, so on a 163 PPI screen, text at 100% comes out at 59% of its intended size. At 175%, which Windows treats as 168 DPI, it comes out close to its intended size.

The smaller the pixel, the harder a single dead one is to spot from your seat, which is why the dead pixel test has a magnifier.

PPI at common sizes

1080p is 1920 × 1080, 1440p is 2560 × 1440 and 4K is 3840 × 2160 pixels, all 16:9. Each figure uses the calculator's formula.

How the PPI calculator works

Screens are sold by their diagonal, so that's the size the calculator asks for. Pythagoras gives the diagonal in pixels, and dividing that by the diagonal in inches gives the density:

PPI = √(width² + height²) ÷ diagonal

For 2560 × 1440 at 27 inches, √(2560² + 1440²) is 2,937.2 pixels from corner to corner, and 2,937.2 ÷ 27 = 108.79 PPI.

• Pixel pitch 25.4 ÷ PPI, in millimeters: the distance from the center of one pixel to the center of the next.
• Width and height Pixels ÷ PPI gives inches, and inches × 2.54 gives centimeters. A diagonal typed in centimeters is divided by 2.54 first.
• Aspect ratio Width and height divided by their largest common factor. 1920 × 1200 reduces to 8:5 and shows as 16:10; ratios with large terms show as a decimal, such as 2.39:1 for 3440 × 1440.

Apple lists the Studio Display, 5120 × 2880 at 27 inches, at 218 PPI, and the formula gives 217.6. The 13.6-inch MacBook Air, 2560 × 1664, is listed at 224 and computes to 224.5. The math runs on your device, and nothing you type is sent anywhere.