How the aspect ratio is calculated
The calculator divides the width and height by their greatest common divisor. 1920 and 1080 share a divisor of 120, giving 16:9. 1080 × 1350 shares 270, giving 4:5. A preview box redraws in the new shape as you type, and the decimal value, width divided by height, is shown to four places.
Some sizes don't reduce to small numbers. 1366 × 768 simplifies to 683:384, so when the simplified width would be over 100 the calculator shows the decimal form instead, here 1.78:1, which is close to 16:9.
Resizing without stretching
To scale an image without distorting it, the height must change by the same factor as the width. New height equals new width times the original height, divided by the original width. Enter a new width and the calculator does the arithmetic and rounds to whole pixels: 1920 × 1080 scaled to 1280 wide becomes 1280 × 720.
The calculation is plain arithmetic in the page, with no file to upload. To read the ratio of an existing picture, use the Image Dimensions Checker, which reports it along with the pixel size.
Common aspect ratios and where they are used
16:9 is standard for video, TVs, most monitors and YouTube thumbnails. 4:3 is the default photo shape on most phone cameras and older displays. 3:2 is typical of many dedicated cameras. 1:1 suits square social posts and profile pictures, 4:5 is a common portrait size for social feeds, 9:16 is used for stories, reels and short vertical video, and 21:9 is ultrawide. The preset buttons fill in a typical size for each.