Aspect Ratio Calculator
Work out missing width or height for a target aspect ratio, and identify the ratio of any dimensions you have — for video, print, screens and social media.
Most ratios do not give whole pixels
A ratio only produces a whole number of pixels when the height is a multiple of its reduced denominator. Counting heights from 1 to 2160 that give an integer width:
| Ratio | Heights that work | Which is |
|---|---|---|
| 3:2 | 1,080 of 2,160 | every 2 pixels |
| 4:3 | 720 of 2,160 | every 3 pixels |
| 21:9 | 720 of 2,160 | every 3 pixels |
| 16:9 | 240 of 2,160 | every 9 pixels |
| 1.85:1 | 108 of 2,160 | every 20 pixels |
| 2.39:1 | 21 of 2,160 | every 100 pixels |
| golden | 4 of 2,160 | every 500 pixels |
Cinemascope at 2.39:1 lines up on only 21 heights in the whole range, which is why letterboxed video is full of off-by-one black bars and why encoders round to the nearest even number rather than the exact ratio. The golden ratio manages four.
16:9 works at every size everyone uses — 720, 1080, 1440 and 2160 are all multiples of 9. That is not a coincidence; those resolutions were picked partly because they divide cleanly.
And “21:9” monitors are not 21:9
True 21:9 is 2.333:1. No mainstream monitor is that shape:
| Sold as 21:9 | Really is | Which is |
|---|---|---|
| 2560 × 1080 | 64:27 | 2.370:1 |
| 3440 × 1440 | 43:18 | 2.389:1 |
| 3840 × 1600 | 12:5 | 2.400:1 |
64:27 is a genuine standard from the ultra-wide television specifications, and it is 4:3 cubed — (4/3)³ = 64/27 exactly, equivalently 16:9 multiplied by 4:3. Meanwhile 3440 × 1440 sits closer to the cinema 2.39:1 than to anything with a 21 in it.
This matters if you are computing dimensions rather than reading a box: ask for 21:9 at 1440 tall and you get 3360 wide, which is 80 pixels narrower than the monitor you were aiming at.
How to use
- Enter the dimensions you know.
- Set the target ratio, or read the ratio of what you entered.
- The missing dimension is calculated for you.
- Round carefully — video encoders often require even numbers.
Frequently asked questions
What is an aspect ratio?
The proportion of width to height, written as two numbers separated by a colon. 16:9 means sixteen units across for every nine down. It describes shape independently of size, so a 16:9 image is the same shape at any resolution.
What is 16:9 in pixels?
Any pair in that proportion: 1920 by 1080, 1280 by 720, 3840 by 2160. Multiplying both sides of the ratio by the same number preserves the shape, which is why these standard resolutions all look identical in proportion.
Why is cinema wider than television?
Deliberate differentiation. When television threatened cinema attendance in the 1950s, studios moved to wider formats that television could not reproduce. Those ratios — 1.85:1 and 2.39:1 — stuck, which is why films still show black bars on a 16:9 screen.
What is 21:9 actually?
Usually 64:27, or about 2.37:1 — the name is marketing rounding rather than the true ratio. Monitors sold as 21:9 commonly run 2560 by 1080 or 3440 by 1440, neither of which is exactly 21 divided by 9.
How do I resize without distorting?
Scale both dimensions by the same factor. If you know the target width, the height is the original height times the new width divided by the original width. Changing one dimension alone stretches the image, which is immediately obvious on faces and circles.
Why do video encoders want even numbers?
Because most video compression works on blocks of pixels and subsamples colour information at half resolution horizontally and vertically. An odd dimension leaves a partial block, which encoders handle by padding or by refusing the input entirely.
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