How to Calculate Screen Pixel Density (PPI)

PPI stands for pixels per inch, a measure of how densely packed the pixels on a display are. A higher PPI means smaller, tighter pixels and a sharper image, which is why a phone screen can look crisper than a much larger TV even with fewer total pixels.

PPI depends on two things: how many pixels the screen has and how large the screen is physically. The same 1920 by 1080 resolution looks razor sharp on a small laptop but noticeably coarser stretched across a large monitor, because the pixels are spread over more area.

PPI is calculated by finding the pixel count along the diagonal and dividing by the diagonal size in inches. The formula is the square root of width squared plus height squared, all divided by the diagonal inches. In other words, PPI equals sqrt(w^2 + h^2) divided by the diagonal.

For a 27-inch monitor at 2560 by 1440, the diagonal in pixels is sqrt(2560^2 + 1440^2), which is about 2938 pixels. Divide that by 27 inches and you get roughly 109 PPI. The math uses the Pythagorean theorem because screens are measured corner to corner.

The calculator does the square-root math for you, so you only need the resolution and the advertised diagonal size, both of which are easy to find in a spec sheet.

Higher PPI is not always better in isolation, because viewing distance matters. A phone held close needs high density to look smooth, while a TV watched from across a room can look perfect at a much lower PPI. Around 300 PPI at reading distance is often cited as the point where individual pixels blur together for most eyes.

Use PPI to compare candidates before buying, to judge whether text will look crisp, or to decide if a monitor needs display scaling. It is a single number that captures how the resolution and physical size work together.