Calculating pixels per inch

Use the Pythagorean theorem to find diagonal pixel count: the square root of horizontal pixels squared plus vertical pixels squared. Divide that value by the physical diagonal in inches to obtain pixels per inch.

Pixel pitch

Pixel pitch is the approximate distance from one pixel center to the next. Dividing 25.4 millimeters per inch by PPI gives a pitch in millimeters when pixels are square and evenly spaced.

What PPI does not describe

Perceived sharpness also depends on viewing distance, eyesight, scaling, subpixel layout, panel technology, content, and rendering. Advertised diagonal size may not equal the exact active image area.

Compare displays with a complete example

A 1,920 × 1,080 panel has about 2,202.9 pixels along its diagonal. At 24 inches, that is about 91.8 PPI and a nominal pitch near 0.277 mm. The same resolution at 15.6 inches is about 141.2 PPI, so interface elements drawn at one physical pixel appear smaller unless the operating system scales them. Re-entering the same resolution with a different diagonal isolates the effect of screen size.

Measure the right dimensions

Use the panel’s native pixel resolution and physical diagonal of the active display area. Do not enter a screenshot’s dimensions with the outside size of an unrelated screen, and do not substitute a scaled desktop resolution when the panel has more physical pixels. PPI can help compare likely text size and pixel pitch, but it cannot predict contrast, color accuracy, response time, subpixel rendering, or whether a chosen scaling setting will feel comfortable at your viewing distance.

The result assumes the entered resolution maps one-to-one to a rectangular active area with square pixels. It does not measure print DPI, device scaling, or image quality.