How the ratio is reduced
The width and height share a greatest common divisor (GCD). Dividing both by that GCD gives the simplest whole-number ratio, found with the Euclidean algorithm.
- GCD — greatest common divisor of width and height
More detail
The Euclidean algorithm
To find the GCD of two numbers, repeatedly replace the larger with the remainder of dividing it by the smaller, until the remainder is zero — whatever's left is the GCD. For 1920 and 1080 that lands on 120, so 1920÷120 : 1080÷120 = 16:9.
Common ratios. 16:9 is standard widescreen (1920×1080, 1280×720). 4:3 is old TVs and some monitors. 21:9 is ultrawide. 1:1 is square.
Frequently asked questions
What's the difference between 16:9 and 4:3?
16:9 (1920×1080, 1280×720) is modern widescreen — wider, matches most TVs and monitors. 4:3 (e.g. 1024×768) is nearly square, used by older TVs and some presentation formats. Enter either pair above to confirm which one your dimensions reduce to.
What resolution is 21:9 ultrawide?
Displays sold as 21:9 don't reduce to exactly 21:9. 2560×1080 reduces to 64:27 (GCD 40) — about 2.37:1, close to but not exactly 21:9 (2.33:1). 3440×1440 reduces to 43:18. True 21:9 simplifies to 7:3, which few real displays match.
How do I find the aspect ratio of my photo or video?
Enter its pixel width and height above (check via right-click → Properties, or your video editor's export settings). E.g. 4032×3024 (a common phone photo) reduces to 4:3, while 3840×2160 (4K) reduces to 16:9.
Why does my ratio show large numbers instead of something like 16:9?
That happens when width and height don't share a large common divisor — e.g. 1000×750 does reduce cleanly to 4:3, but odd or prime-heavy dimensions (like 1001×751) may only simplify a little. The decimal ratio column still gives you a usable comparison value.