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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.

Fill in one side to calculate the other, or fill in both to simplify their ratio.

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Reading dimensions
Applying the ratio
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Result

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.

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Executed in Browser

Your files and inputs never leave this device. Processing runs completely offline. 100% Secure.

Accepted Formats & Rules

  • Common presets 16:9, 4:3, 21:9 and 1:1 plus a fully custom width:height ratio.
  • Computes width from height or height from width, with decimal input supported.
  • Simplifies any pair of dimensions via greatest common divisor, with the decimal ratio shown too.

Key Benefits

✓ Enter the one side you know — the other is computed exactly for the chosen ratio.
✓ Type both sides instead and the tool simplifies them to their smallest whole-number ratio.
✓ Runs entirely in your browser: no upload, no sign-up, results in a click.

Use Cases

  • • Work out the exact height a 16:9 video or thumbnail needs at a given width before exporting.
  • • Scale a banner, photo or screen recording to a new size without stretching or distortion.
  • • Enter two existing dimensions to identify their simplified aspect ratio before cropping.