Aspect Ratio Calculator

Keeping proportions while you resize

An aspect ratio is the relationship between width and height, written as width:height, like 16:9 or 4:3. The calculator does two jobs: reduce a pair of dimensions to its simplest ratio, and scale one dimension to find the matching other dimension so the image or video does not stretch. Both come down to one proportion that must hold.

The one equation

If the original is W by H and you fix a new width w, the matching height is h = w * H / W. Fix the height instead and width is w = h * W / H. Everything else, from fitting a thumbnail to sizing a responsive video, is that single relationship applied in one direction or the other.

Example

Original: 1920 x 1080  -> ratio 16:9
Target width 1280:
  height = 1280 * 1080 / 1920 = 720
Result: 1280 x 720  (still 16:9)

Because 1280x720 holds the same 16:9 proportion, the resized frame looks identical, just smaller. Change the height to anything other than 720 and the picture distorts.

Pitfalls

Rounding to whole pixels can nudge the ratio off by a hair; for CSS layout, prefer the aspect-ratio property so the browser does the math and never leaves a fractional pixel. Confusing which number is width in a "ratio" is a frequent slip, since 9:16 is portrait and 16:9 is landscape. When you then shrink the file itself, hand the dimensions to the image resizer.

FAQ

How do I reduce 1920:1080 to 16:9? Divide both by their greatest common divisor (120 here). The calculator does this to show the simplest form.

Can I use the CSS aspect-ratio property instead? Yes. aspect-ratio: 16 / 9 on a box keeps the shape responsively without computing a fixed height.

What ratio for social video? 16:9 for landscape, 9:16 for vertical stories and reels, 1:1 for square feed posts.