Aspect Ratio Calculator: Changing Size Without Changing Shape
An aspect ratio is just the relationship between two numbers, but the job is usually "keep this shape and make the width 1280 — what is the height?" And the more common the screen size, the less readable its exact ratio turns out to be.
Enter a width and a height and you get the ratio reduced by its greatest common divisor, plus the decimal form. Lock the ratio and changing one dimension moves the other with it.
Clicking a ratio chip keeps the width you have and recalculates the height. At 1920 wide, choosing 4:5 gives you 1920 × 2400.
When the exact ratio is unreadable
The 1366 × 768 panel common in budget laptops reduces to 683:384. Arithmetically flawless, and it tells you nothing. What you wanted to know is that it is very nearly 16:9.
So when the reduced terms get large, the nearest standard ratio is shown alongside — but only when it is within one per cent. A 1200 × 630 Facebook image is 40:21, which is not that close to anything standard, so nothing is offered. A forced label would be worse than none.
For sizes that already reduce cleanly, like 1920 × 1080 to 16:9, no approximation appears. Saying "about 16:9" about something that is exactly 16:9 would only make it look less certain.
What common sizes actually reduce to
| Size | Reduced | Where you meet it |
|---|---|---|
| 1920 × 1080 | 16:9 | Full HD, YouTube |
| 3840 × 2160 | 16:9 | 4K UHD |
| 1366 × 768 | 683:384 (about 16:9) | Budget laptop panels |
| 2560 × 1080 | 64:27 (21:9) | Ultrawide monitors |
| 1080 × 1350 | 4:5 | Instagram feed, portrait |
| 1080 × 1440 | 3:4 | Instagram profile grid |
| 1200 × 630 | 40:21 | Facebook share image |
21:9 is not 21:9
Ultrawide monitors are sold as 21:9, but the actual resolutions are 2560 × 1080 and 3440 × 1440. The first reduces to 64:27, or 2.370 in decimal. True 21:9 is 2.333. They are different numbers.
Search purely by arithmetic and the nearest standard ratio to 2.370 is 2.39:1 — the cinema scope ratio. Correct, and useless to somebody holding a monitor. So this calculator knows 64:27 under the name 21:9 as well.
Gaps like this are ordinary in marketing: the name is chosen to be memorable, the resolution is chosen to be manufacturable, and they only have to be roughly the same thing.
Pixels are whole numbers, so the fit is not always exact
Lock a ratio, change the width, and the height comes back rounded. Applying 9:16 at 1920 wide gives 3413, not 3413.33, so the real ratio is very slightly off 9:16.
Alternate between editing the two fields and that rounding accumulates a little each time. It is far too small to see, but if you need the ratio to be exact, set one axis and take the calculated value for the other rather than nudging both.
Choosing sizes that divide cleanly also works. For 16:9 there are plenty — 1920 × 1080, 1280 × 720 — where both numbers come out whole.
Common uses
- •Finding the other dimension for a target resolution in video editing.
- •Scaling an image down to an exact size without distorting it.
- •Working out the padding-top percentage for a responsive CSS box.
- •Sizing prints and frames to a chosen proportion.
- •Identifying which standard ratio an image you already have is closest to.
Extended FAQ
I clicked a ratio chip and the width did not change.
That is intended. The chip selects a shape, not a size, so it keeps your width and moves the height to match. To change the size as well, use the common sizes list below it.
Why does it show something like 683:384?
Because that is the reduction. 1366 and 768 share only a factor of two, so it cannot go further. In those cases the nearest standard ratio is shown too.
Are 16:9 and 1.78:1 the same?
Yes — 16 divided by 9 is 1.778. Video work tends to use the decimal form and display work the integer form.
How do I read the ratio of a portrait image?
Width first, exactly as written. 1080 × 1350 is 4:5; reversed it becomes 5:4, which is a different shape. The order is part of the answer.
Are my values stored?
No. Everything is calculated in your browser.