Pixel density decides how sharp text and images look on a screen. Enter a resolution and a diagonal size and this calculator returns pixels per inch, dot pitch, the panel’s physical width and height, the simplified and nearest common aspect ratio, and the total pixel count — useful when choosing between a 27-inch 1440p and a 32-inch 4K monitor, or checking what “Retina” really means on a laptop. Everything is computed locally in your browser.
Working out an aspect ratio for a video or image instead? The aspect ratio calculator scales dimensions for you. Once your new monitor arrives, run the screen test to hunt for dead pixels and backlight bleed, and use the unit converter for any other inch-to-centimetre conversions.
Frequently asked questions
How is PPI calculated?
Pixels per inch is the number of pixels along the diagonal divided by the diagonal length in inches. First find the diagonal in pixels with Pythagoras — √(width² + height²) — then divide by the screen size. A 24-inch 1920×1080 monitor has a 2,202.9-pixel diagonal, so 2,202.9 ÷ 24 ≈ 91.8 PPI. The same number also tells you the physical size of each pixel: dot pitch in millimetres is 25.4 ÷ PPI, about 0.277 mm for that monitor. The calculator derives the physical width and height the same way, by dividing each pixel dimension by the PPI.
Why can’t the tool detect my screen size automatically?
Browsers deliberately never expose the physical dimensions of a display. A web page can read the resolution in CSS pixels (window.screen) and the device pixel ratio, which together give the real pixel count — that is what the Detect my screen button uses — but there is no API for inches or millimetres, partly for privacy (it would help fingerprint you) and partly because operating systems often do not know it reliably either. You still need to type the diagonal from the box, spec sheet, or a tape measure. Reset browser zoom to 100% first, because zoom changes the reported pixel ratio.
What is a good PPI for a monitor or phone?
It depends on viewing distance. Desktop monitors viewed at 50–70 cm look sharp from roughly 110 PPI (27-inch 1440p) and very crisp around 140–165 PPI (27- or 32-inch 4K); 90 PPI (24-inch 1080p) shows visible pixels on text for many people. Laptops sit closer, so 150–250 PPI is typical, and Apple’s “Retina” laptops are around 220–255. Phones are held 25–35 cm away, so 400–500 PPI is standard and extra density beyond that is hard to see. TVs are watched from metres away, so a 55-inch 4K set at 80 PPI still looks sharp from the couch.
Is PPI the same as DPI?
Not strictly. PPI (pixels per inch) describes a screen or a digital image: how many pixels fit in an inch of the display. DPI (dots per inch) originally describes printers, which lay down many tiny ink dots to form each image pixel, and mice, where it measures sensor sensitivity. In everyday speech people say “monitor DPI” when they mean PPI, and Windows calls its scaling setting DPI, so search engines treat the terms as interchangeable. For a display, the number you want is PPI, and that is what this calculator returns.
Why does my result differ slightly from the manufacturer’s spec sheet?
Marketed screen sizes are rounded, and some are measured with rounded corners or notches included. A MacBook Air sold as 13.6 inches computes to 224.5 PPI here while Apple lists 224, and an iPhone listed at 6.1 inches computes to about 461 against Apple’s 460. The difference is the true diagonal being a few hundredths of an inch different from the rounded figure. If you need precision, measure the visible active area diagonally with a ruler and type that value — the calculator accepts decimals like 13.58.