Monitor PPI Calculator

How Monitor PPI Is Calculated

PPI (Pixels Per Inch) measures the pixel density of a display: how many pixels fit into one inch of screen space.
Higher PPI means sharper images and crisper text, but the benefit vanishes past a certain viewing distance, and that distance is doing most of the work.

PPI formula:

PPI = √(Horizontal pixels² + Vertical pixels²) ÷ Screen diagonal (inches)

The diagonal pixel count is calculated using the Pythagorean theorem across the resolution.

Worked example: 27-inch 4K monitor (3840 × 2160):

Diagonal pixels = √(3840² + 2160²) = √(14,745,600 + 4,665,600) = √19,411,200 = 4,405.8 pixels

PPI = 4,405.8 ÷ 27 = 163.2 PPI

Common monitor PPI values:

The 1 arcminute rule, which is what this calculator uses:

Critical PPI = 3,438 ÷ Viewing distance (inches)

20/20 vision resolves detail about one arcminute apart. Convert that angle to a pixel pitch and you get the constant 3,438. Below the critical PPI you can see pixels; at or above it you cannot.

At 24 inches the threshold is 143 PPI, so a 27-inch 4K monitor at 163 PPI is comfortably past it. Every pixel beyond that costs graphics performance and buys nothing your eye can collect.

A note on “Retina.” Apple’s marketing thresholds are looser than the arcminute rule and are quoted per product rather than per distance: roughly 300 PPI for phones and 220 for tablets, which line up reasonably with the 287 and 229 above. Their desktop and TV claims do not line up, because those assume a viewer sitting further back than the numbers usually printed alongside them. Where the two disagree, the table above is the physics and Apple’s figure is the brand.