Pixel density explained

Pixels per inch (PPI) is how many pixels fit along one inch of a screen. It is what makes the same resolution look crisp on a laptop and soft on a large monitor.

The formula

First find the screen’s diagonal in pixels with Pythagoras, then divide by the diagonal size in inches:

diagonal pixels = √(width² + height²)
PPI = diagonal pixels ÷ diagonal inches

A 27-inch 2560 × 1440 monitor has √(2560² + 1440²) ≈ 2937 pixels on the diagonal, so 2937 ÷ 27 ≈ 108.8 PPI.

Dot pitch

Dot pitch is the same measure turned around: the distance between the centres of neighbouring pixels. With 25.4 millimetres to the inch, dot pitch = 25.4 ÷ PPI. At 108.8 PPI that is about 0.233 mm.

Common screens

The same resolution gives very different densities at different sizes:

Sharpness depends on distance

The farther away you sit, the less density you need. A phone held 30 cm from your eyes needs several times the PPI of a monitor at arm’s length to look as sharp, and a TV across the room needs less still.

Desktop operating systems were designed around roughly 96 to 110 PPI at 100% scaling. Denser screens — a 27-inch 4K monitor is about 163 PPI — are usually run at 150% or 200% scaling, trading workspace for sharper text.

PPI is not screen size

Two screens with the same resolution show the same amount of content at the same scaling; the larger one simply draws everything bigger. Compare resolution (at your scaling) for workspace, and PPI for sharpness.

PPI and DPI

DPI — dots per inch — comes from printing, where it counts dots of ink. Screens have pixels, so PPI is the accurate term, though DPI is often used loosely for both. Mouse DPI is something else entirely: how far the pointer moves per inch of hand movement.