PPI / DPI Calculator

What pixel density is

Pixels per inch is how tightly the pixels are packed: the number of pixels along the diagonal, divided by how many inches that diagonal is. A 27-inch screen at 1920 × 1080 works out at about 82 PPI; the same 27 inches at 3840 × 2160 is about 163. The pixels have not changed size because of anything the screen does differently — there are simply four times as many of them in the same space.

Higher density means text edges you cannot pick out at normal distance. It also means the operating system usually scales everything up, which is why a high-density screen shows a web page at the same apparent size as a low-density one.

Where the numbers come from

1. 1

   The pixel count is prefilled from what your browser reports, and you can change it — useful when you are working out a screen you are not sitting at.

2. 2

   The diagonal is yours. No web page can measure a screen, so type the size it was sold as: 24, 27, 15.6 for a laptop.

3. 3

   The answer is arithmetic on those two things, and nothing else. Change either and the result follows immediately.

What to do with it

The dot pitch — the width of one pixel in millimeters — is the same fact stated the other way round, and it is the figure display manufacturers publish. Comparing two screens on dot pitch is comparing how fine each one can draw.

The panel's width and height come from the diagonal and the shape of the pixel grid, which is how a "27-inch" screen turns into something you could measure with a tape. If the answer looks wrong, the diagonal is usually the culprit: manufacturers round it, and a screen sold as 27 inches often measures a little under.

For the reverse calculation — a diagonal from a resolution and a density — use the Screen Size Calculator. To check the numbers your browser reports, see Screen Resolution; to measure something on the screen for real, the On-Screen Ruler calibrates against a bank card.