Aspect Ratio Calculator
Calculate width or height for any aspect ratio. Lock 16:9, 4:3, 21:9 and more, resize images and video proportionally, and read the simplified ratio.
Common ratios
Simplified ratio
16 : 9
Decimal
1.7778
Set ratio
16 : 9
What is an aspect ratio?
An aspect ratio is the proportional relationship between width and height, written W:H. A 1920×1080 frame and a 1280×720 frame are both 16:9 — the same shape at different sizes. Keeping the ratio fixed while changing one dimension is what stops images and video from looking stretched or squashed.
This tool works both ways: lock a ratio and enter a width to get the matching height (or vice-versa), and it always shows the simplified ratio (the greatest common divisor reduction) of whatever dimensions you type.
Common ratios & where they're used
| Ratio | Typical use |
|---|---|
| 16:9 | HD/4K video, monitors, YouTube, TVs |
| 9:16 | Vertical video — Reels, TikTok, Stories |
| 4:3 | Older displays, some photos, slides |
| 1:1 | Square posts, avatars, album art |
| 3:2 | DSLR photography, prints |
| 21:9 | Ultrawide monitors, cinematic |
| 2.39:1 | Anamorphic widescreen cinema |
The math
To keep ratio Rw:Rh, a new height is H = W × Rh / Rw and a new width is W = H × Rw / Rh. The simplified ratio divides both dimensions by their greatest common divisor — e.g. 1920×1080 → GCD 120 → 16:9.
Tips
- Letterboxing: when a source ratio doesn't match the target, you either add bars (letterbox/pillarbox) or crop. Match ratios first to avoid both.
- Rounding: some ratios don't divide evenly at every size — round to whole pixels and accept a sub-pixel difference rather than distorting.
- Social media: design to the exact platform spec (e.g. 1080×1920 for Stories) to avoid automatic re-cropping.