Aspect Ratio Calculator - Width, Height & Social Media Sizes

An aspect ratio is the proportional relationship between an image or video width and height, expressed as two numbers separated by a colon, like 16:9 or 4:5. Getting it right matters for every visual medium: a YouTube thumbnail that is not 16:9 may be cropped or letterboxed, a TikTok video that is not 9:16 can show black bars, and an OG image that is not close to 1.91:1 can look wrong when your link is shared on Slack or LinkedIn.

Under the hood, the calculator simplifies whatever width and height you enter using the same math you would do by hand, just automated and rounding-safe. It scales both numbers by 1000 to preserve up to three decimal places (so a ratio like 1.91:1 is not truncated to a whole number), then finds the greatest common divisor of the two scaled integers with a recursive Euclidean algorithm and divides both sides by it. That is why 1920x1080 reduces to 16:9 and 1080x1350 reduces to 4:5 instantly as you type, without you needing to know the divisor is 120 or 270. A common hand-calculation mistake is assuming two images with the "same" ratio are interchangeable — 1920x1080 and 3840x2160 are both exactly 16:9, but a 1280x719 image is not, even though it looks close, so the tool also computes a decimal ratio (width divided by height, to four places) and matches it against nine standard presets to report the closest one along with how far off it is.

Beyond ratio math, the calculator scales dimensions bidirectionally — change either the new width or new height and the other is recalculated instantly so you never distort an image by typing both values by hand — and lets you upload an image to read its real pixel dimensions directly in the browser via the File and Image APIs, with nothing uploaded to a server. Once an image is loaded you can preview exactly how it will be cropped into six common frames (YouTube, Instagram post, Story/Reel, square, OG image, and Pinterest) using CSS aspect-ratio boxes, so you can see cropping problems before you export from a design tool. A Social Sizes reference tab covers exact pixel dimensions for 14 platforms, from YouTube channel art to LinkedIn covers, so you can jump straight from a platform name to the correct width, height, and simplified ratio.