Two questions, both called aspect ratio
The first is "what ratio is this?" You have a size, 1920 by 1080, and you want to know what to call it. The answer is 16:9, arrived at by dividing both numbers by everything that goes into them both.
The second is "what size do I need?" You know the ratio you want and one of the two dimensions, and you need the other. If a banner has to be 16:9 and 1280 wide, the height is 720.
It will tell you when a size is not a standard ratio
This is the useful bit. 1920 by 1081 is not 16:9. It is 1920:1081, and no amount of rounding makes it anything else. A lot of calculators quietly round to the nearest recognisable ratio and report that, which is how somebody ends up believing a slightly-wrong image is fine and then wondering why it has a hairline of background down one edge.
Here, exact means exact. If the numbers genuinely reduce to 16:9 it says 16:9. If they do not, it gives you the real ratio and names the one you were probably aiming at.
The ratios worth knowing
16:9 is widescreen: video, YouTube, TV, most monitors and nearly every presentation. 9:16 is the same thing stood up, which is Reels, Shorts, TikTok and Stories.
3:2 is what most proper cameras shoot, and it is what a 6x4 print is.4:3 is older screens, compact cameras and iPads. 1:1 is square: profile pictures and product shots. 4:5 is Instagram's tallest feed format.
1.414:1 is A-series paper, and it is the reason A4 folded in half is exactly A5. It is the only common ratio that survives being halved, which is a genuinely elegant piece of design and the reason the rest of the world uses it.
Where the mismatches bite
A phone shoots 4:3 or 3:2. Video wants 16:9. Instagram wants 1:1 or 4:5. Something has to give, and it is either bars down the sides or a crop. Knowing the ratio before you start tells you which, and how much you are about to lose.
The classic one: a 3:2 camera photo printed at 6x4 fits perfectly, and the same photo printed at 10x8, which is 5:4, loses a strip off each end. Print shops crop it for you without asking, which is how heads end up out of frame.
Whole pixels cannot always express a ratio
21:9 at a width of 3440 wants a height of 1474.28. Pixels do not do fractions, so you get 1474 and the real ratio is 1720:737.
This is why ultrawide monitors sold as "21:9" are actually 3440 by 1440, which is 43:18, or about 2.39:1 against 21:9's 2.33:1. The marketing name is a rounded approximation of a shape that does not divide neatly. Nothing is broken; the number on the box was never exact.
A ratio has no units
Pixels, millimetres, inches, metres: it makes no difference, because a ratio is a comparison rather than a measurement. 16:9 is 16:9 whether it is a phone screen or the side of a building.
That is why this tool does not ask what the numbers are. Put in whatever you have.
Common questions
What is an aspect ratio?
A comparison between the width and the height of something, with no units attached. 16:9 is 16:9 whether it is a phone screen or the side of a building, so a ratio worked out from pixels, millimetres or metres is exactly the same calculation.
How do I work out the aspect ratio of an image size?
Divide both numbers by everything that goes into them both. 1920 by 1080 reduces to 16:9 that way, because both numbers share the same largest common factor.
Why does the tool say my image is not a standard ratio?
Because it genuinely is not. 1920 by 1081 is 1920:1081, and no amount of rounding makes it 16:9. A lot of calculators quietly round to the nearest recognisable ratio and report that instead, which is how a slightly wrong image ends up looking fine until it shows a hairline of background down one edge.
What aspect ratio is a 6x4 photo print?
3:2, which is what most proper cameras shoot. The same photo printed at 10x8, which is 5:4, loses a strip off each end, because the print shop crops it to fit without asking.
Why is a monitor sold as "21:9" actually 3440 by 1440 pixels?
Because 3440 by 1440 is 43:18, or about 2.39:1, which is close to but not exactly the 21:9 ratio of 2.33:1. Pixels do not do fractions, so a shape that does not divide neatly gets a rounded marketing name instead of an exact one.
Why does my height come out as a fraction of a pixel?
Because most ratios do not divide cleanly into most widths. A 1920 wide frame at 21:9 works out at 822.857 high, and there is no such thing as part of a pixel, so something has to give. Either round and accept a ratio very slightly off, or pick a width that divides exactly: 1918 at 21:9 is 822 on the nose. Most video encoders also want even numbers on both sides, so where you are rounding anyway, round to an even pixel rather than the nearest one.