Aspect Ratio Calculator
Find the exact aspect ratio of any size, and work out the missing width or height for a new one. Instant, precise and in your browser.
What this tool does
Type a width and a height to see the exact ratio behind them, reduced properly rather than guessed at, along with the decimal value and the common name when there is one. Then pin whichever side you already know for a new size and the other one is worked out for you, keeping the same proportions. Because whole pixels are what everything downstream needs, results are rounded, and you are told plainly when that rounding lands a hair off the true ratio instead of letting your layout drift without warning. Decimals are welcome in the inputs and still reduce exactly. It all runs in your browser, so there is nothing to upload and nothing to wait for.
What you can use it for
- Work out the height for a banner when the width is fixed by the layout.
- Check whether a screenshot is really 16:9 before dropping it into a video.
- Scale a design down for a thumbnail without squashing it.
- Find out what ratio an odd size like 1366 by 768 actually is.
- Confirm that a pair of dimensions a client sent you match the ratio they asked for.
How to use it
- Enter the width and height you are starting from.
- Read the reduced ratio, the decimal value and the orientation.
- Enter either a target width or a target height, leaving the other blank.
- The blank side is filled in for you, keeping the same proportions.
- Fill in both target sides instead if you just want to check whether they match.
Everything runs inside your browser. No file is uploaded to any server. See more tools in this field.
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Frequently asked questions
Why does 1366 by 768 not come out as 16:9?
Because it genuinely is not. It reduces to 683:384, which is very close to 16:9 but not the same. The ratio here is always the exact reduced form rather than the nearest familiar one, so you can see when a size is slightly off. That is usually the thing you actually wanted to know.
What does it mean when the result is not an exact fit?
It means the true answer was not a whole number of pixels. A 16:9 box at a width of 1000 works out to 562.5 pixels tall, so you get 563 and a note that it is rounded. The difference is invisible in most layouts, but you get told rather than left to discover a slow drift later.
Can I enter decimals?
Yes, and they still reduce exactly. Both numbers are scaled by the same power of ten until they are whole, then divided down by their greatest common divisor, so a size like 12.5 by 7.5 gives you 5:3 rather than an approximation.
How is this different from the image resizer?
This one never touches a file. It is arithmetic about dimensions, for the moment before you resize anything, when you are working out what the new size should be. The image resizer takes an actual image and rewrites its pixels. They pair up nicely: work the number out here, then go and apply it there.
What do the ratio names mean?
They are the everyday names for the ratios people recognise, like 16:9 for widescreen video or 4:3 for older screens and photos. A name only appears when the reduced ratio matches one exactly. If your size does not have a common name you simply see the numbers, which is more honest than pushing it into a category it does not belong to.