21:9 Aspect Ratio - Width, Height & CSS Dimensions

21:9 expresses width-to-height as 21:9. Ultrawide cinema and gaming monitors. In CSS, write aspect-ratio: 21 / 9. The decimal width÷height value is approximately 2.3333. Lock either side in the calculator to keep that proportion when you resize frames, posters, or video embeds for this format.

Multiply width by 9/21: 2560 x (9 ÷ 21) = 1097px. Enter 2560 in the width field with ratio locked to 21:9 and the tool returns 1097px automatically. Change width anytime to regenerate height without breaking the ratio or introducing letterboxing.

21:9 ultrawide content may target 2560x1080 or 3440x1440 px on cinema-wide monitors. Confirm both dimensions before encoding so letterboxing does not appear on ultrawide players.

Use aspect-ratio: 21 / 9 on a block container. Pair it with width: 100% or a fixed max-width; the browser computes height. For embedded video, wrap the iframe in a 21:9 box to prevent cumulative layout shift while the media loads. Copy the sample dimensions above when you need a concrete pixel fallback.

Ratio-specific pages document common resolutions and use cases for 21:9 without hunting presets. The embedded calculator is pre-loaded with 21:9 so you skip manual ratio entry - helpful when sharing links with teammates or clients who always work in this format for ads, video, or social crops.

The main Aspect Ratio Calculator supports any custom ratio, simplified GCD output, and all presets. Visit /dev-tools/aspect-ratio-calculator/ for the complete tool with methodology, cross-links to other Dev Tools, and free-form width or height entry when your project is not locked to a named preset.