Screen Size Calculator

Why a diagonal is a strange way to sell a screen

Screens have been sold by their diagonal since televisions were round-cornered boxes, and the habit stuck. It hides the thing you usually want to know: how wide it is, and whether it fits the space. Two screens sold as 27 inches can be different widths if their shapes differ — a 16:9 panel is about 23.5 inches wide, while a 21:9 ultrawide at the same diagonal is wider and much shorter.

Worse, the diagonal is rounded. A "27-inch" monitor is usually 27 point something, or a little under, and manufacturers state the exact figure in the specifications rather than on the box. So treat the width and height here as close, not exact to the millimeter.

The other direction

Switch to Diagonal and the calculation runs backwards: give it a resolution and a pixel density and it works out how big the screen must be. That is the useful version when you are reading a specification sheet that lists 3840 × 2160 at 163 PPI and want to know what size panel that describes.

Both directions are the same arithmetic. The pixel diagonal comes from the two sides, and the physical diagonal is that divided by the density — which is exactly what the PPI Calculator does in reverse.

What it cannot do

It cannot measure anything. Every figure here is arithmetic on the numbers in the boxes, and the page never reads a physical size from your device, because no browser offers one. If you want to measure something for real, the On-Screen Ruler calibrates against a bank card first.

It also has no opinion on which shape you should want. If you are comparing shapes rather than sizes, the Aspect Ratio Calculator reduces any two numbers to their ratio.