Tech calculator
Screen PPI Calculator
Measure screen pixel density from its native resolution and diagonal.
Pixel density
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How the calculator finds pixel density
Enter the horizontal and vertical pixel counts and the diagonal screen size in inches. The calculator treats the pixel grid as a rectangle and applies the Pythagorean theorem to find its diagonal length in pixels, then divides that by the physical diagonal to get pixels per inch. It also reports pixel pitch — the physical width of a single pixel — as 25.4 ÷ PPI millimeters, since there are 25.4 mm in an inch.
A worked example
A 1920×1080 monitor with a 24-inch diagonal: √(1920² + 1080²) = √(3,686,400 + 1,166,400) = √4,852,800 ≈ 2202.91 pixels along the diagonal. Divide by 24 inches and you get about 91.79 PPI. Its pixel pitch is 25.4 ÷ 91.79 ≈ 0.277 mm — each pixel is roughly a quarter of a millimeter wide.
A modern phone panel packs the same math into a much smaller frame: 1080×2400 pixels on a 6.5-inch diagonal gives √(1080² + 2400²) = √6,926,400 ≈ 2631.81, and 2631.81 ÷ 6.5 ≈ 404.9 PPI — more than four times denser than the monitor above, despite having fewer total pixels (2,592,000 versus the monitor's 2,073,600).
Why more PPI eventually stops mattering
The human eye has a limited angular resolution — commonly cited as around 1 arcminute for someone with normal vision. That limit, combined with viewing distance, caps how much pixel density is actually visible: at a distance of d inches, the finest pitch a typical eye can resolve corresponds to roughly 3438 ÷ d PPI. Hold a phone at a typical 10 inches from your face and the ceiling is about 3438 ÷ 10 ≈ 344 PPI; at 12 inches it drops to about 286 PPI.
That means a 404.9 PPI phone screen already exceeds what an average eye can distinguish at normal reading distance — held at 10 inches, that same phone at 800 PPI would look no sharper. It's also why TVs get away with far lower density: viewed from 8 feet (96 inches), the resolvable ceiling falls to about 3438 ÷ 96 ≈ 35.8 PPI, well under the roughly 67.8 PPI of a 65-inch 4K TV (√(3840² + 2160²) ÷ 65), so the picture still reads as pixel-free from the couch.
Situations this calculator is built for
- Comparing two phone specs before buying — a 1170×2532 panel on a 6.1-inch screen works out to about 457.3 PPI, denser than the 404.9 PPI example above despite the smaller diagonal
- Checking a monitor's sharpness for design or photo work — a 2560×1440 (1440p) panel on a 27-inch diagonal comes out to about 108.8 PPI, a figure often quoted for that exact panel size
- Deciding whether an OS display-scaling setting needs adjusting, since panels above roughly 150 PPI often need more than 100% scaling to keep on-screen text a comfortable physical size
- Working out an LED video wall's pixel pitch in millimeters for signage planning, where a wall spec'd at 6.35 PPI has a pitch of exactly 25.4 ÷ 6.35 = 4 mm between pixel centers
- Judging whether upgrading from one screen to another will be visible at your actual viewing distance, using the roughly 3438 ÷ distance-in-inches resolvable-PPI estimate
- Sanity-checking a manufacturer's advertised PPI figure by recomputing it from the resolution and diagonal size on the spec sheet
Frequently asked questions
Is there an official PPI number that counts as "retina"?
There's no fixed threshold — it depends on viewing distance. Using the eye's roughly 1-arcminute angular resolution limit, the highest PPI a typical eye can resolve at distance d inches is about 3438 ÷ d. That works out to roughly 344 PPI at a 10-inch phone-reading distance and about 36 PPI at an 8-foot TV-viewing distance.
Why does pixel pitch matter separately from PPI?
Pixel pitch (25.4 ÷ PPI millimeters) gives the physical size of one pixel, which is more intuitive than a PPI number for things like LED signage. A wall with a 4 mm pitch works out to 25.4 ÷ 4 = 6.35 PPI — useful for planning where a viewer stands, since it's viewed from many feet away.
Does a higher PPI always mean a better display?
No. PPI only measures how tightly pixels are packed, not color accuracy, contrast ratio, brightness, or panel technology like OLED versus LCD. Two screens with identical PPI can look very different once color and contrast are taken into account.
What's the difference between PPI and DPI?
PPI describes a digital screen's pixel density; DPI describes a printer's ink-dot density on paper. They get used interchangeably but measure different physical outputs — 300 DPI became the standard for photo-quality prints partly because prints are typically viewed 12-14 inches away, which by the same arcminute math caps resolvable detail at roughly 246-286 PPI-equivalent, in the same range.
Why is a small phone screen denser than a big monitor with more pixels?
PPI depends on pixel count relative to physical size, not total pixel count alone. A 1080×2400 phone panel has 2,592,000 pixels packed into a 6.5-inch diagonal for 404.9 PPI, while a 2560×1440 monitor has 3,686,400 pixels — more pixels overall — spread across a 27-inch diagonal, giving only about 108.8 PPI.
Should I enter the diagonal size in centimeters?
No, use inches. Display sizes are conventionally advertised in inches, and the pixel-pitch calculation uses 25.4 mm per inch as a fixed constant. To convert a centimeter measurement first, divide by 2.54 to get inches.