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Aspect Ratio Calculator
Aspect Ratio Calculator — Solve for Any Side
Enter any two of width, height and ratio and the third follows, reduced with a GCD rather than rounded to a famous name.
About Aspect Ratio Calculator
An aspect ratio calculator is trivial arithmetic and still worth a page, because the useful part is the part most of them skip. 1366×768 — the resolution half the laptops in the world shipped with — is not 16:9. Reduced by its greatest common divisor it is 683:384, about 0.05% wider. That is invisible on a screen and it is exactly the difference between keeping a panel's ratio and making something 16:9, so this page reports the exact reduced pair first and the nearest named ratio second, with the percentage it is out by. Enter any two of width, height and ratio and the third is computed. The computed field is read-only, so the three can never contradict one another on screen. A ratio can be written 16:9, 16/9, 2.39:1 or as a bare decimal. Rounding happens once, at the end, and the footnote says what was rounded and from what value. The fit mode scales one box into another and reports the letterbox or pillarbox bars in pixels, or, in cover mode, what gets cropped off each edge. The named ratios are stored as the integer pairs they really are: 21:9 is a marketing name that covers more than one shape — 2560×1080 is 64:27 and 3440×1440 is 43:18 — and the chip is stored as 64:27; 2.39:1 and 1.85:1 are the cinema projection standards, kept here as 239:100 and 37:20.
Questions
Why does it say 1366×768 is not 16:9?
Because it is not. Divide both sides by their greatest common divisor, which is 2, and you get 683:384. That works out to 1.7786 against 16:9's 1.7778 — about 0.05% wider, or roughly half a pixel across a 1366-pixel screen. It is invisible and it is still not the same ratio, which matters the moment you are matching a panel exactly rather than approximately. The page shows the exact reduced pair, the nearest named ratio, and the percentage between them, so you can decide which of the two you actually want.
I asked for 16:9 and the height it gave me is not 16:9.
Pixels are whole numbers and most widths do not divide cleanly. 1366 at 16:9 is 768.375 high, and there is no such thing as three-eighths of a pixel, so it rounds to 768 — and 1366×768 is 683:384, not 16:9. The footnote under the fields names the exact value it rounded from, and the banner explains that the rounding is what caused the drift. If you need an exact 16:9 box, change the width to a multiple of 16: 1360×765 and 1376×774 are both exact.
Is 21:9 a real aspect ratio?
No, it is a marketing name — and it does not even name one shape. The 2560×1080 ultrawides sold as 21:9 reduce to 64:27, which is 2.370:1, but the 3440×1440 ones reduce to 43:18, which is 2.389:1, and the 3840×1600 panels badged the same way are 12:5, or 2.4:1. Three ratios, one name. Literal 21:9 would be 2.333:1 and nothing ships at it. The preset on this page is labelled 21:9 because that is what people search for and is stored as 64:27, the commonest of the three; type 3440×1440 into the fields and the page will tell you it is 43:18, because it is.
How do I work out the black bars for a widescreen clip in a square frame?
Tick FIT INTO A FRAME in the rail, leave the mode on Contain, and enter the frame size. For a 1920×1080 clip in a 1080×1080 post, the picture scales to 1080×608 and the page reports 236 pixels of empty frame above and below, plus the share of the frame the picture covers. Switch to Cover and it reports the opposite: the picture fills the square at 1920×1080 and 420 pixels come off each side. The numbers are the ones you type into an editor; this page does not touch any video or image itself.
Does this actually resize my image or video?
No. This page is arithmetic — it takes numbers and gives back numbers, and never opens a file. There is no drop zone here because there is nothing to drop. Once you have the dimensions, Resize Image will apply them to a picture, Crop Image will cut a region to a fixed ratio, and the video compressors take a target resolution. Keeping the calculator separate is deliberate: you often want the numbers for something this site cannot do at all, like a CSS rule, a print layout or an editor's export dialog.
Is my file uploaded to a server?
No. Transmute processes everything locally in your browser using JavaScript and WebAssembly. Your files never leave your device — there is no server, no upload, no cloud processing.