Aspect Ratio Calculator

Resize a video without stretching it, scale a banner to fit a slot, or work out what height a 1200-pixel-wide 16:9 image needs — every one of those is an aspect ratio problem, and this calculator solves it in one step. Pick the ratio, enter the side you know, and the missing dimension is computed exactly.

The presets cover the shapes you meet daily: 16:9 for HD and 4K video, 4:3 for classic screens and photos, 21:9 for ultrawide monitors and cinematic footage, and 1:1 for square social-media posts. If your project uses something else — 3:2 photography, 9:16 vertical video, a bespoke banner — switch to the custom ratio and type any width:height pair, decimals included.

It also works in reverse: enter both dimensions of an existing image or screen and the tool reduces them to the smallest whole-number ratio using the greatest common divisor, alongside the decimal form designers quote (like 1.78:1). That is the quickest way to identify what ratio a file actually is before you crop or letterbox it.

Like every tool on this site, the calculator runs entirely in your browser: no upload, no account, no waiting on a server. Open it on a phone on set or a workstation in the studio and get the same instant answer.