Aspect ratio is the proportional relationship between width and height, written as width:height — 16:9, 4:3, 1:1, 21:9. It describes shape, not size: a phone screen and a cinema screen can share 16:9 while differing enormously in inches. To reduce a resolution to its ratio, divide both numbers by their greatest common divisor: 1920x1080 shares GCD 120, giving 16:9; 3840x2160 ('4K') shares 240, also 16:9 — which is why 4K is exactly four 1080p frames in area. Common ratios: 16:9 (HD video, most screens), 4:3 (old TV, some tablets), 21:9 (ultrawide monitors), 1:1 (social square), 9:16 (vertical video), 3:2 (DSLR photos).
The two directions of calculation: given both dimensions, reduce to the ratio; given one dimension and the ratio, find the other — height = width x (ratio height / ratio width). A 16:9 design at 1280 wide is 1280 x 9/16 = 720 tall. Mismatched ratios cause letterboxing (black bars top and bottom, e.g., 21:9 film on a 16:9 TV) or pillarboxing (bars on the sides, e.g., 4:3 content on widescreen) — or stretching, which distorts everything and should never be used.
For creators, match the delivery ratio: YouTube favors 16:9, Instagram Reels and TikTok want 9:16, and print work uses paper ratios (A-series is 1:√2). Shooting or designing in the wrong ratio means cropping away pixels you already paid to capture. This calculator reduces resolutions and finds missing dimensions from any ratio.