What is PPI and how is it calculated?

PPI (pixels per inch) measures how densely packed a display's pixels are. It is the diagonal resolution in pixels divided by the diagonal size in inches: PPI = sqrt(width_px squared + height_px squared) / diagonal_inches. A 27-inch 2560 by 1440 monitor works out to about 109 PPI.

What is the difference between PPI and DPI?

PPI describes pixels on a screen; DPI (dots per inch) describes ink dots on a printer. They are used interchangeably in everyday speech, but technically PPI is for displays and DPI is for print. This calculator reports PPI because it works from a screen resolution and diagonal.

Dot pitch is the physical distance between the centres of two neighbouring pixels, usually in millimetres. It is the inverse of density: dot pitch = 25.4 / PPI. A higher PPI gives a smaller dot pitch, which means a sharper, finer image.

What counts as a retina display?

A display is "retina-class" when, at your normal viewing distance, your eye cannot resolve the individual pixels. The eye resolves roughly one arc-minute of detail, so the required PPI depends on distance. Apple markets phones at about 326 PPI (held ~11 inches) and laptops at about 220 PPI (viewed ~20 inches). The tool computes the threshold for the distance you enter.

Does a higher PPI always mean a better screen?

Not on its own. Beyond the retina threshold for your viewing distance, extra PPI is invisible to the eye but still costs GPU power and battery. Panel quality, colour accuracy, brightness, contrast and refresh rate matter just as much. PPI tells you about sharpness, not overall quality.

Is my data sent anywhere?

No. Every calculation runs locally in your browser using JavaScript. Nothing you type is uploaded, logged or stored on a server.

PPI Calculator — Gera Tools

A PPI (pixels-per-inch) calculator turns three simple inputs — a display’s horizontal resolution, vertical resolution and physical diagonal — into the numbers that actually describe how sharp a screen is: pixel density, dot pitch, the real active area in inches and centimetres, total megapixels, the reduced aspect ratio, and a retina sharpness check for the distance you sit from it. It is the tool to reach for when you are comparing two monitors, choosing a phone, sizing a TV for a room, or deciding whether a 4K panel is worth it at the size you want.

How it works

Pixel density is the diagonal measured in pixels divided by the diagonal measured in inches. The diagonal in pixels comes straight from Pythagoras, so the full formula is:

PPI = sqrt(width_px² + height_px²) / diagonal_inches

From that single number everything else follows. Dot pitch — the spacing between neighbouring pixel centres — is 25.4 / PPI in millimetres. The active area is each pixel dimension divided by the PPI. Megapixels is width times height divided by one million. The aspect ratio is the width and height reduced by their greatest common divisor (so 2560 by 1440 reports as 16 by 9).

The retina check is the clever part. The human eye resolves detail down to about one arc-minute. At a given distance the smallest pixel pitch you can still perceive corresponds to a required PPI of 1 / (distance_in × tan(1 arc-minute)). If your screen’s PPI is at or above that threshold, the pixels blur together and the display is effectively retina-class for you. Sit further back and the required PPI drops — which is exactly why a 55-inch 4K TV at 80 PPI looks razor sharp from the sofa, while the same 80 PPI on a desk monitor would look coarse.

Worked example

Take a 27-inch 2560 × 1440 monitor. The diagonal in pixels is sqrt(2560² + 1440²) ≈ 2937 px. Divide by 27 inches and you get about 108.8 PPI, a dot pitch of roughly 0.233 mm, an active area of about 23.5 × 13.2 inches, 3.69 megapixels and a clean 16:9 aspect ratio. At a 24-inch viewing distance the threshold is around 96 PPI, so this panel clears it — pixels are not individually resolvable at the desk. Drop the panel to a 24-inch 1080p screen and density falls to about 92 PPI, just under the threshold, which is why text on cheap 24-inch 1080p monitors can look a touch soft.

Reference / formula note

PPI = sqrt(w² + h²) / diagonal_inches
Dot pitch (mm) = 25.4 / PPI
Pixels per cm = PPI / 2.54
Megapixels = (w × h) / 1,000,000
Required PPI at distance d (in) = 1 / (d × tan(1/60°))
1 inch = 2.54 cm exactly, so a diagonal entered in cm is divided by 2.54

Common retina marketing references: phones ≈ 326 PPI, tablets ≈ 264 PPI, laptops ≈ 220 PPI, desktop monitors ≈ 218 PPI. Add a panel price to get cost per megapixel and cost per diagonal inch for a quick value comparison between two displays.

Everything is computed in your browser — no numbers are uploaded or stored.