Aspect ratio calculator

Three answers in one: the reduced aspect ratio of a width and height, the missing dimension when you know a ratio and one side, and the largest box of a given ratio that fits inside a container.

Reducing a ratio is division by the greatest common divisor, and the honest answer is sometimes useless. 1366×768 reduces to **683:384**, which is arithmetically right and is not what anyone means — that screen is sold as 16:9 and is very slightly not. Both are shown here: the exact reduction, and the common ratio it is within a percent of. The fit calculation is the one that comes up most in practice, because it is what happens when a video is letterboxed or an image is placed in a fixed frame. The largest box of ratio *w:h* inside a container is found by scaling to whichever dimension runs out first, which is the same `min()` that CSS `object-fit: contain` performs. If the numbers come out with the container’s full width and less than its height, the bars are top and bottom; the other way round and they are at the sides. A note on 16:10 versus 8:5. They are the same ratio — 2560×1600 reduces to 8:5 — but everyone says 16:10, because it sits next to 16:9 in a spec sheet. That is convention, not maths.