The aspect ratio is width : height reduced by their greatest common divisor. 1920 × 1080 divides by 120 to give 16:9. To resize, keep the ratio fixed: new height = new width × height ÷ width, and new width = new height × width ÷ height.
Common aspect ratios
16:9 - HD and 4K video, most monitors (1920×1080, 3840×2160)
4:3 - older TVs and monitors, many tablets (1024×768)
1:1 - square social posts (1080×1080)
9:16 - vertical video for Stories, Reels and Shorts (1080×1920)
21:9 - ultrawide monitors (3440×1440 is about 43:18)
3:2 - 35mm photos and many DSLR cameras (6000×4000)
FAQ
What is the aspect ratio of 1920x1080?
16:9.
Why does my resolution give an odd ratio?
Some sizes, like 1366×768 or 3440×1440, do not divide neatly and are marketed as the nearest common ratio (16:9 and 21:9). The decimal value shows how close they are.