Understanding the PPI Calculator
PPI, or pixels per inch, measures how densely a display packs its pixels — the higher the PPI, the sharper text and images look at a given viewing distance, since more pixels are used to render the same physical area. The calculator uses the standard diagonal-resolution method: it treats the screen's horizontal and vertical pixel counts as the two legs of a right triangle and finds the diagonal pixel count with the Pythagorean theorem, then divides by the diagonal screen size in inches.
The formula
For a display with a horizontal resolution of W pixels, a vertical resolution of H pixels, and a diagonal screen size of D inches:
- Diagonal resolution (pixels): √(W² + H²)
- PPI: √(W² + H²) ÷ D
- Pixel pitch (mm): 25.4 ÷ PPI — the physical width of one pixel, since one inch equals 25.4 millimeters
- Aspect ratio: W and H divided by their greatest common divisor, written as W:H
For example, a 24-inch monitor at 1920×1080 has a diagonal resolution of √(1920² + 1080²) ≈ 2203 pixels, giving a PPI of about 2203 ÷ 24 ≈ 91.8 — typical for a full-HD desktop monitor. A 6.1-inch phone screen at 1170×2532 works out to roughly 458 PPI, well into the density range marketed as "Retina" or HiDPI.
What counts as a high-density display?
There is no official cutoff, but a few reference points are widely used in the display industry: screens below about 100 PPI are common on older or larger low-resolution monitors, 150-220 PPI is typical of modern laptop and desktop panels, and phone and tablet screens above roughly 220-300 PPI are usually marketed as Retina-class or HiDPI because individual pixels become difficult to distinguish at normal viewing distance. Many current smartphones exceed 400 PPI, approaching the ~300 DPI often used as a print-quality benchmark.
Why viewing distance matters
PPI alone does not determine how sharp a screen looks in practice — viewing distance does too. A phone held close to the eyes needs a much higher PPI to look equally sharp as a TV viewed from across a room, because the same pixel subtends a smaller visual angle up close. That is why TVs can ship with far lower PPI than phones without looking noticeably pixelated.