Aspect Ratio Calc is a free online tool that fits any dimensions to any aspect ratio — type a width or a height and get the matching dimension at 16:9, 4:3, 1:1, 21:9, 9:16, 2.39:1 or a custom ratio, with a live preview box and common resolutions to load in one click. No account, nothing leaves the page.

Width

Height

Custom ratio — width

Custom ratio — height

Type in either field — the other updates instantly. Press Enter in a dimension field to copy the result. Your ratio and dimensions are remembered on this device.

Common resolutions ·

Width	Height	Name

Click any row to load that resolution into the calculator. Standard sizes are the marketed resolutions — a few sit a pixel or two off the exact ratio, and the readout above says so when they do.

How it works

The formula — height = width × ratio height ÷ ratio width, and width = height × ratio width ÷ ratio height. At 16:9, a width of 1280 gives a height of 720.
Simplifying a ratio — the calculator divides both dimensions by their greatest common divisor: 1920 × 1080 simplifies to 16:9. If the dimensions do not match a named ratio, the nearest one is shown instead.
Rounding — results are rounded to the nearest whole pixel, so 1920 wide at 21:9 is 823 tall, and 2.39:1 is stored as 239:100 so the maths stays exact.