Fill in one dimension and leave the other blank — the calculator solves for the empty field.
The Aspect Ratio Calculator solves the proportion problem behind every resize. Give it a ratio and one dimension and it returns the other — enter 16:9 and a width of 1280 and it tells you the height is 720. Give it a width and a height and it works backwards, reducing the pair to its simplest integer ratio using the greatest common divisor.
Aspect ratio is the relationship between width and height, written as two numbers separated by a colon. Getting it wrong is what produces stretched faces in a video export, letterboxed bars on a thumbnail, or a hero image that crops badly on mobile. Because the arithmetic is a single division, most people do it in their head and land on a height that is off by a pixel or two — enough for a video encoder to complain or a CSS grid to shift.
The calculator covers the four jobs that come up in practice: solve for a missing side, reduce a size to its ratio, scale a size by a percentage, and fit a size inside a bounding box without distortion. Presets cover 16:9, 4:3, 1:1, 9:16, 3:2, 21:9, 1.91:1, and 2.39:1. Every calculation runs in your browser, works offline, and shows the formula it used.
Choose a preset like 16:9 or 9:16, or type your own ratio into the two ratio fields. The preset only fills the ratio — your dimensions stay put.
Type the width you need and leave height blank, or the height and leave width blank. The calculator solves for whichever field is empty.
You get the completed width × height, the simplified ratio, the decimal ratio, and the exact formula used, plus the megapixel count for image work.
Copy the dimensions to your clipboard, or switch to Get Ratio mode to reduce an existing size like 1920 × 1080 down to 16:9.
Enter a ratio plus a width or a height and get the other dimension instantly, rounded to whole pixels so encoders and CSS do not complain
Feed it 1920 × 1080 and get 16:9 back. The reduction uses the greatest common divisor, and decimal ratios like 1.91:1 are handled by scaling before reduction
16:9, 4:3, 1:1, 9:16, 3:2, 2:3, 5:4, 21:9, 1.91:1 for Open Graph images, and 2.39:1 anamorphic — each labelled with where it is actually used
The exact arithmetic appears under the result, so you can verify the maths or reuse it in a spreadsheet or script
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