Aspect ratio calculator

What is an aspect ratio calculator?

An aspect ratio is a width and a height reduced to their smallest whole numbers. 1920 by 1080 divides by 120 to give 16:9, which is why that number is on every television box. The calculator does that reduction, tells you the ratio as a decimal, and works the other way too: give it one new dimension and it computes the other so the shape is preserved.

The reduction is exact — it uses whole-number arithmetic, not decimals — so the answer is the ratio rather than something close to it. That matters more than it sounds, because a surprising number of common resolutions do not reduce to the tidy name they are sold under.

It also scales. Enter a new width and you get the height that keeps the shape, along with something most calculators quietly skip: whether that height is a whole number of pixels at all.

How to use the aspect ratio calculator

1. Enter the width and height in pixels. Whole pixels only, since that is what an image has. The ratio is reported width first, so a portrait photo correctly gives a value below 1 rather than being silently flipped.
2. Read the exact ratio and its nearest name. You get the reduced ratio, the decimal, and the closest common ratio with the percentage it is off by — or the word “exactly” when it lands on one. If the resolution is one that is commonly sold under a misleading name, the tool says which.
3. Resize by filling in one new side. Enter a new width or a new height and the other is calculated. If the result is not a whole number of pixels, the page says so, rounds it, and shows what the ratio actually becomes once rounded.

“21:9” is not a ratio

Ultrawide monitors are sold as 21:9. Not one of the common ones is 21:9. The four usual resolutions reduce to three genuinely different ratios: 2560×1080 and 5120×2160 are 64:27, 3440×1440 is 43:18, and 3840×1600 is 12:5. The widest and the narrowest of them differ from each other by 1.25%, which is more than enough to matter if you are laying out a full-bleed banner.

They miss the name by 1.59%, 2.38% and 2.86% respectively. And here is the part that makes the label properly misleading rather than merely loose: every one of them is closer to the cinema ratio 2.39:1 than to 21:9. A 3440×1440 monitor is 2.39:1 to within 0.046%. It is, to a very good approximation, a cinema-shaped screen wearing a number no display actually has.

The same thing happens more quietly elsewhere. The 1366×768 panel in millions of cheap laptops is 683:384, not 16:9 — 0.05% off, which is small but is why images scaled to it never quite line up. DCI 4K's 4096×2160 is 256:135, 0.39% off the 17:9 it is quoted as.

There is a distinction worth keeping straight, because tools that flag every mismatch get it wrong. A 1440×900 screen reduces to 8:5, and 8:5 *is* 16:10 — the name is simply unreduced, not incorrect. That is a different situation from 21:9, where the name denotes a ratio the hardware does not have. This calculator only raises the alarm for the second kind.

Honest limitations

Scaling rarely lands on whole pixels, and that is a real constraint rather than a rounding detail. Across six common source resolutions and ten common target widths, 38 of the 60 combinations produce a fractional height — 63%. A 1366×768 source fails at all ten; a 1920×1080 source succeeds at all ten. There is no clever arithmetic that fixes this, because a ratio like 683:384 simply does not divide most numbers evenly.

What happens next is the part to watch. Scale 1366×768 to 1920 wide and the exact height is 1079.4729, which rounds to 1079. The result is 1920:1079 — and 1920 and 1079 share no common factor, so that is a ratio with no name at all, 0.093% from 16:9 and 0.044% from the 683:384 you started with. Rounding does not snap you back to a tidy shape; it invents a third one. The tool shows you that new ratio rather than pretending the old one survived.

The list of named ratios is a judgement call, not a standard. It holds the ones people search for, including 21:9 and 19.5:9 precisely because nothing is exactly them and the useful answer is how far off they are. A ratio that is closest to 2.39:1 is not thereby “a cinema image” — it just sits nearest that entry in a list somebody chose.

The tool reads width first and never reorients. A 1170×2532 phone screen gives 195:422, which is correct, but a phone is advertised as 19.5:9 with the long side first. Reconciling the two would need an orientation rule, and a rule that silently flips your numbers is worse than one that reports what you typed.

The engine is checked by 72 assertions: reduction against a brute-force greatest common divisor over 14,400 pairs, the whole resolution table above, the named-ratio matching, the scaling behaviour in both directions, and the 63% figure recomputed rather than remembered.

Why is it free?

It is a greatest common divisor and a multiplication, running in your browser as you type. Nothing is uploaded, there is no account, and there is no cap on how many dimensions you check.

If you need to actually resize an image rather than calculate the numbers, the image resizer on this site does that in your browser too — also without uploading the file.