Aspect Ratio Calculator

Resizing an image to fit a different aspect ratio without distorting it requires arithmetic you don't want to do in your head while a video editor is open.

Feed it any two of width, height, or ratio and it gives you the third. You can lock the aspect ratio and scale dimensions, or check whether two arbitrary numbers reduce to a common ratio like 16:9 or 4:3.

The practical use is figuring out target heights for a given width when cropping for social platforms, exporting video frames, or designing responsive containers. It also flags common ratios — 16:9, 21:9, 1:1, 9:16, 4:5 — when your custom numbers happen to match one, which saves a lot of guessing about whether 1920×817 is meant to be 21:9 (it's not — it's 2.35:1).

The arithmetic is just division — height divided by width, or expressed as a reduced fraction. The reason it gets confusing is that the same ratio can be expressed in several ways. 16:9, 1.78:1, 1920×1080, 1280×720 are all the same shape. The calculator reduces any pair of dimensions to lowest terms using the greatest common divisor, then matches the result against a lookup of common ratios so you can see whether your custom numbers are actually a known one in disguise.

Worked example: you have a 2436×1125 screenshot from an iPhone X and want to know what aspect ratio that is. GCD of 2436 and 1125 is 3, which reduces to 812:375. Multiply both by 8 in your head and it lands near 19.5:9 — Apple's chosen ratio for that era of phones. Knowing this matters when you're building a responsive layout, because 19.5:9 doesn't fit the grid most CSS frameworks assume.

The pain this exists to solve: cropping for social. You shoot a video at 16:9, then need it at 9:16 for TikTok and 1:1 for Instagram feed. Each crop loses content. The calculator tells you exactly how much. A 1920×1080 frame cropped to 9:16 keeps a strip 608 pixels wide in the middle — anything outside that is gone. If your subject doesn't sit center-frame, the crop will cut their face off. Knowing this before you shoot beats reshooting.

Where it can't help: ratios that are technically correct but practically useless. Some printer drivers and scanner outputs spit out dimensions like 2480×3508 (which is exactly A4 at 300dpi) and the reduced ratio is 620:877 — a real number that means nothing to anyone. The tool will compute it; you'll still want to know it as 'A4 at 300dpi' for any conversation with another human. Common ratios are more useful than mathematically reduced ones in most contexts.