Aspect ratio calculator

How the ratio is worked out

An aspect ratio is width divided by height, written as the smallest pair of whole numbers with the same proportion. To get there, divide both sides by their greatest common divisor (GCD), found with Euclid's algorithm: replace the larger number by the remainder of dividing it by the smaller, until the remainder is zero.

1920 mod 1080 = 840
1080 mod 840  = 240
 840 mod 240  = 120
 240 mod 120  = 0      → GCD = 120

1920 / 120 = 16,  1080 / 120 = 9   → 16:9   (decimal 1.7778)

function gcd(a, b) { while (b) [a, b] = [b, a % b]; return a; }
function ratio(w, h) { const g = gcd(w, h); return `${w / g}:${h / g}`; }

ratio(1920, 1080);  // "16:9"
ratio(1366, 768);   // "683:384"

Some sizes do not reduce to a familiar pair. 1366 × 768 gives 683:384, which is 1.7786:1, a hair wider than 16:9 (1.7778:1). The calculator shows the closest common ratio and how far off it is, so you can tell "basically 16:9" from "actually 16:10".

Finding a missing side

To resize while keeping the ratio, scale both sides by the same factor:

newHeight = newWidth  × height / width
newWidth  = newHeight × width  / height

For a 16:9 video at 1280 wide: 1280 × 9 / 16 = 720. For a 4:5 portrait post 1350 tall: 1350 × 4 / 5 = 1080. If the result has a fraction, round it, and know that the ratio is then only approximate. Video encoders using 4:2:0 chroma subsampling, the usual setting for H.264 and HEVC, need even widths and heights, so round to an even number for video.

Common aspect ratios

"21:9" is a marketing label rather than a ratio: 21 ÷ 9 is 2.333, while the monitors sold under that name are 2.370 (2560 × 1080) or 2.389 (3440 × 1440).

When the ratios do not match

If an image's ratio differs from the space it has to go into, something has to give: either empty bars (aspect fit), cropping (aspect fill) or distortion (stretch). The "Fit an image into a box" calculator on this page gives the numbers for fit and fill. The visualizer draws it and writes the code; aspect fit vs aspect fill explains the math.

Pixel aspect ratio and display aspect ratio

Everything here assumes square pixels, which is true for almost every photo, screen and web video today. Some video formats store non-square pixels: standard-definition DV, for example, stores 720 × 480 pixels and is displayed at 4:3 or 16:9 depending on a flag. In those files the storage ratio (3:2 for 720 × 480) differs from the display aspect ratio, and the player stretches the picture on playback.