21:9 Aspect Ratio Calculator
Calculate the missing side for any aspect ratio — 16:9, 4:3, 21:9, 1:1 or custom — or simplify the ratio of two known dimensions.
Processed entirely in your browser — nothing you enter here is ever uploaded.
Result
About this tool
This page opens the aspect ratio calculator with 21:9 already chosen — type one side and the ultrawide counterpart is calculated instantly.
21:9 is the marketing name for ultrawide: monitors such as 2560 × 1080 and 3440 × 1440 (technically 64:27, close to true 21:9) and the cinematic 2.35–2.39:1 scope formats used in film. It is the go-to shape for immersive gaming, side-by-side productivity and letterboxed movie stills.
The complete tool also offers 16:9, 4:3, 1:1 and custom ratios, plus ratio simplification for two known dimensions. Everything is computed locally in your browser — your numbers never leave the page.
Example. A width of 1920 at 16:9 returns 1920 × 1080.
Limit. Displayed numbers keep at most two fraction digits, and a ratio is simplified from at most six decimal places of the typed sides.