Aspect Ratio Calculator

What the ratio is, and how it is worked out

An aspect ratio is the shape of a rectangle with its size removed: 1920 × 1080 and 3840 × 2160 are both 16:9, because both sides doubled. Getting there is division. Both numbers are divided by the largest number that goes into each exactly — their greatest common divisor — which for 1920 and 1080 is 120, leaving 16 and 9.

That division is why the answer is sometimes unfamiliar. An ultrawide 3440 × 1440 screen reduces to 43:18, and a 2560 × 1080 one to 64:27. Both are sold as "21:9", which is a name rather than the arithmetic — 21:9 would be 3360 × 1440. This page shows what the numbers actually reduce to, and adds the familiar name underneath where one exists.

Where the second pair helps

Give it a width and it fills in the height that keeps the same shape, or the other way round. That is the question behind most uses of a ratio calculator: an image that must be 1200 wide and stay 16:9, a video that must fit a 1080-tall frame, a banner that has to match the shape of a screen.

Sizes can be whole numbers or decimals. A decimal answer is shown to one decimal place, because a third of a pixel is not a thing you can draw, and a size worked out from decimals is marked as rounded rather than presented as exact.

What it does not do

It does not know your screen unless you press Use my screen, which fills in the pixel numbers your browser reports — the same figures the Screen Resolution page explains. It does not know how big anything is in inches or centimeters: a ratio is a shape, not a size, which is why two screens with the same ratio can be wildly different. For that, the Screen Size Calculator turns a diagonal and a ratio into real dimensions.

And it invents no names. Where a shape has no name in common use, the page says so instead of making one up.