Calculate pixel density (PPI) from a screen's resolution and diagonal size.
Calculator verified • Last updated: August 2026
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Higher PPI means finer detail per inch — useful for comparing sharpness across different screens and resolutions.
PPI (pixels per inch) measures pixel density: how tightly packed a screen's pixels are relative to its physical diagonal size. It's found by first calculating the diagonal resolution in pixels — using the Pythagorean theorem on the width and height in pixels — then dividing that by the diagonal size in inches.
$$\text{PPI} = \frac{\sqrt{W^2 + H^2}}{D}$$
W: horizontal resolution, in pixels.
H: vertical resolution, in pixels.
D: diagonal screen size, in inches.
A 24-inch monitor at 1920×1080 resolution: diagonal pixels = square root of (1920 squared plus 1080 squared) = square root of 4,852,800 ≈ 2,202.9 pixels. Dividing by the 24-inch diagonal gives about 91.8 PPI — a fairly typical density for a mid-size desktop monitor, noticeably lower than a modern phone screen, which packs far more pixels into a much smaller diagonal.
Two screens can share the exact same resolution — say, 1920×1080 — yet look completely different in sharpness depending on their physical size. A 55-inch 1080p TV has a much lower PPI (and visibly larger, softer-looking pixels up close) than a 24-inch 1080p monitor, because the same pixel count is stretched across far more physical area. This is why "resolution" alone is an incomplete way to judge how sharp a display will actually look — PPI (or its close cousin, PPD — pixels per degree, which also factors in viewing distance) is the more complete picture.
PPI and DPI are often used interchangeably but technically describe different things: PPI (pixels per inch) describes a digital display's pixel density, while DPI (dots per inch) technically refers to a printer's physical ink dot density. In everyday conversation the terms get mixed constantly, but on a calculator specifically about screens, PPI is the technically correct term.
Using the screen's width or height alone instead of the true diagonal resolution is the most common error — PPI is defined against the diagonal, not a single dimension, because diagonal size (the number manufacturers advertise) is measured corner to corner. Confusing a device's marketed resolution with its actual rendering resolution is another — some displays render at a lower native resolution and upscale, which this calculator can't detect from specs alone.
Pixel Density — how many pixels are packed into a given area of a screen, most commonly expressed as PPI.
Retina Display — Apple's marketing term for a PPI high enough that individual pixels aren't distinguishable at that device's typical viewing distance.
PPD (Pixels Per Degree) — a more complete sharpness measure than PPI alone, factoring in viewing distance as well as pixel density.
PPI (pixels per inch) is a measure of pixel density — how many pixels are packed into each inch of the screen's diagonal. Higher PPI generally means a sharper, more detailed image at normal viewing distance.
Apple popularized "Retina" to mean a PPI high enough that individual pixels aren't distinguishable at typical viewing distance for that device — roughly 300 PPI for phones held close, and lower (around 220-260 PPI) for laptops and tablets viewed further away. There's no single universal cutoff; it depends on viewing distance.
Only up to the point where individual pixels are no longer distinguishable at normal viewing distance — beyond that, further PPI increases have diminishing visible benefit but still cost more in processing power and battery life to drive the extra pixels.