21:9 Aspect Ratio Calculator

A 21:9 frame is twenty-one units wide for every nine units tall, a decimal value of about 2.333. It is noticeably wider than 16:9 and roughly a third wider again than 4:3, which is what gives ultrawide monitors their immersive, cinematic feel.

The name is a marketing simplification rather than a technical standard. Many panels sold as 21:9 are actually 64:27, which equals 2.370 — the 2560 × 1080 ultrawide size is one of them — while the widely used 3440 × 1440 size reduces to 43:18, which equals 2.389. The cinema anamorphic ratio is a separate value, 2.39:1. These figures, 2.333, 2.370 and 2.390, are close enough that they are often grouped under a single ultrawide label, but none of them is exactly 21:9.

Ultrawide maths is the same divide-and-multiply routine as any other ratio, but the numbers rarely land on a round answer, so rounding matters.

1. Divide the width by 21.
2. Multiply the result by 9 to get the height.
3. Round the height to a whole pixel — then check whether your monitor's native panel is actually 64:27, which needs a slightly different height.

Worked example

At 2560 pixels wide: 2560 ÷ 21 = 121.9, and 121.9 × 9 = 1097, so a strict 21:9 frame would be 2560 × 1097. The 2560 × 1080 panel actually sold is 64:27 instead, which is slightly shorter. At 3440 wide: 3440 ÷ 21 = 163.8 and 163.8 × 9 = 1474, so a strict 21:9 frame would be 3440 × 1474, while the 3440 × 1440 panel is 43:18. In both cases the hardware is shorter than the label suggests.

"21:9" is used loosely as a marketing label — the actual math (2.333) doesn't exactly match cinema's 2.39:1 scope ratio, even though the two are close enough that people use the terms interchangeably. Many ultrawide monitors sold as "21:9" are also not pixel-exact: a 3440 × 1440 panel actually reduces to 43:18 (about 2.389), closer to cinema scope than to a true 21:9.

For an exact 21:9 frame at 900 pixels tall: 900 ÷ 9 = 100, then 100 × 21 = 2100. That gives 2100 × 900 — a true 21:9 rectangle, useful as a reference size because most commercial ultrawide monitor resolutions are close to, but not exactly, 21:9.

Type your own numbers into the 21:9 calculator above and it will show the same division, plus the reduced ratio and the CSS to copy.

• Ultrawide gaming monitors, where the extra field of view is a competitive advantage.
• Video editing timelines, which fit more clips before scrolling.
• Cinematic letterbox framing for trailers and film-look projects.
• Music production and DAW layouts, where horizontal space is at a premium.
• Multi-window desktop work that would otherwise need two displays.
• Financial trading and data dashboards with many parallel columns.

21:9 sits at the wide end of the spectrum. The 2.39:1 calculator covers the slightly wider anamorphic cinema ratio, and the 16:9 calculator covers the common video frame you would be widening from.

• 16:9
• 9:16
• 4:3
• 1:1
• 16:10
• 3:2
• 4:5
• 2:3
• 2.39:1

This 21:9 guide is one of ten ratio calculators on the site — the common aspect ratios table lists the decimal value and the typical uses for every one of them.

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