21:9 Ultrawide Calculator
Ultrawide monitors and panoramic captures. Enter one side; get the exact other.
21:9 is the marketing label for the ultrawide monitor class. True panels are usually 64:27 (≈2.37:1) at 2560×1080 or 3440×1440 (43:18) — close enough that 21:9 is the universal shorthand. Use the solver for monitor sizing, ultrawide wallpapers and letterboxed cinematic exports.
1920 × 823
Exact ratio: 7:3 · CSS: aspect-ratio: 21 / 9 · padding-top fallback: 42.86%
Formula
height = width × 9 ÷ 21
width = height × 21 ÷ 9
// padding-top fallback: 42.86% — keeps layout stable before the image loads
aspect-ratio: 21 / 9 — modern replacement for the padding hack
All math runs locally in your browser. No upload. Type either side, the other solves live — and the URL updates so you can share an exact pair (?w=1920&rw=21&rh=9) or bookmark a preset.
Common 21:9 resolutions
| Width | Height | Use |
|---|---|---|
| 320px | 137px | |
| 640px | 274px | |
| 1280px | 549px | |
| 1920px | 823px | |
| 2560px | 1097px | |
| 3840px | 1646px | 4K |
| 5120px | 2194px | |
| 7680px | 3291px |
When to use 21:9
- Ultrawide monitors (2560×1080, 3440×1440, 5120×2160)
- Ultrawide wallpapers
- Letterboxed cinematic exports on a 16:9 timeline
Pitfalls to avoid
- 21:9 is rounded — the exact ratio of a 3440×1440 panel is 43:18 (2.388:1)
- Streaming an ultrawide game to a 16:9 audience adds letterboxing unless you crop
CSS snippet
Modern:
.frame { aspect-ratio: 21 / 9; width: 100%; overflow: hidden; }
.frame > img { width: 100%; height: 100%; object-fit: cover; }Legacy padding hack (for very old browsers):
.wrap { position: relative; width: 100%; padding-top: 42.86%; }
.wrap > * { position: absolute; inset: 0; }Pair with object-fit: contain to letterbox without cropping, or cover to fill and crop safely.
Platform reference
Frequently asked
What is true 21:9 resolution?
2560×1080, 3440×1440 and 5120×2160 are the common 21:9 family (technically 64:27 / 43:18).
Is 21:9 good for work?
Great for timelines and side-by-side windows; less ideal if you share screen to 16:9 viewers.
How wide is 21:9 vs 16:9?
At the same height, 21:9 is 33% wider — that is why 34-inch ultrawides feel like a double monitor without a bezel.
Tip: add ?w=1920 or ?h=1080 to this URL to prefill the calculator — useful for linking from a spec doc.
Other ratios
- 16:9 Aspect Ratio Calculator
- 4:3 Aspect Ratio Calculator
- 1:1 Square Calculator
- 9:16 Vertical Calculator
- 3:2 Aspect Ratio Calculator
- 16:10 Aspect Ratio Calculator
- 4:5 Portrait Calculator
- 2:1 Aspect Ratio Calculator
Look up by resolution
- 1920×1080 — 1920×1080 Full HD
- 1280×720 — 1280×720 HD
- 3840×2160 — 3840×2160 4K UHD
- 2560×1440 — 2560×1440 QHD
- 1366×768 — 1366×768 WXGA
- 3440×1440 — 3440×1440 UW-QHD
What aspect ratio is 1920×1080? → · Full FAQ →