Free aspect ratio calculator. Enter a ratio like 16:9 plus one dimension to get the other, or reduce any pixel size (e.g. 1920x1080) to its simplest ratio.
The aspect ratio is the proportional relationship between an image or screen's width and its height, written as two numbers separated by a colon — for example 16:9. This calculator does two jobs. First, given a target ratio and one known dimension, it computes the matching dimension so your image stays perfectly proportioned. Second, it takes any pixel size such as 1920×1080 and reduces it to its simplest ratio (16:9), telling you the proportions of a file you already have.
Pick a preset (16:9, 4:3, 21:9, 1:1, 3:2, 9:16, or cinematic 2.39:1) or type a custom ratio. Then enter the width you want, and the height is filled in automatically — or enter the height to get the width. The math is simply height = width × ratioHeight ÷ ratioWidth. For a 16:9 video at 1280 px wide, the height works out to 720 px.
Type a width and height into the reducer and it divides both by their greatest common divisor (GCD) to find the simplest whole-number ratio. 1366×768 reduces to roughly 16:9, while 1024×768 reduces to exactly 4:3. The tool also shows the decimal ratio (for example 1.778:1) and a friendly name when one matches.
Keeping the correct aspect ratio when resizing prevents the stretching and squashing that happens when width and height are scaled by different amounts. All calculations run in your browser.
Select the 16:9 preset (or enter 16 and 9 as the ratio), then type your width in pixels. The height is calculated automatically using height = width x 9 / 16. For example, 1280 px wide gives 720 px tall.
Use the reducer card: enter the image's pixel width and height and the tool divides both by their greatest common divisor to give the simplest ratio. For instance, 1920x1080 reduces to 16:9 and 1024x768 reduces to 4:3.
16:9 is standard for HD video and most screens, 4:3 is used by older displays and classic photos, 1:1 is square (common on social media), 9:16 is vertical video for stories and Reels, and 21:9 is ultrawide cinematic.
If you scale width and height by different amounts, the image stretches or squashes and looks distorted. Calculating the matching dimension keeps the proportions identical so the result looks correct.