Aspect Ratio Calculator
Resize a width or a height without distorting the picture. Enter the size you already have, solve for the dimension you need, and get the reduced ratio, the decimal ratio, and a live proportional preview.
Dimensions
New size
Shape preview
Both rectangles are drawn to the same scale and share a corner, so you can see the shape and the size change at once.
How to use this calculator
1. Type in the size you already have
Put the source width and height into the two original fields. That pair defines the ratio everything else is locked to — a photo straight off a camera, a video timeline, a Figma frame, or a banner slot in a design spec.
2. Pick the dimension you already know
Choose whether you know the new width or the new height. Type the one you know and the other field fills itself in. If you would rather start from a standard shape, tap a ratio chip — it rescales the original pair to that exact ratio and keeps your working size.
3. Choose how to round
Whole pixels suit images and layouts. Nearest even pixel keeps video encoders happy, since chroma subsampling requires both dimensions to be divisible by two. Exact keeps the decimals when you are feeding the number into something else.
4. Grab a standard resolution
The common resolutions table lists every mainstream size that fits your ratio, flags whether it lands on the ratio exactly or was nudged to the nearest even pixel, and downloads as CSV so you can drop it into a spec sheet or an export preset list.
About aspect ratios
What is an aspect ratio?
An aspect ratio is the relationship between a rectangle's width and its height, written as width:height. It describes shape, not size: 1920×1080 and 1280×720 are different resolutions but both are 16:9, so they look identical when scaled. To turn a resolution into a ratio, divide both numbers by their greatest common divisor — 1920 and 1080 share a divisor of 120, which leaves 16:9.
Why does 21:9 reduce to 64:27 instead of 7:3?
21:9 is a marketing label rather than an exact fraction. Taken literally it would reduce to 7:3 (2.333), but no ultrawide panel is built that way. A 2560×1080 display is exactly 64:27 (2.370) and a 3440×1440 display is exactly 43:18 (2.389). This calculator reports the true reduced ratio of the numbers you enter and names the standard it is closest to.
Why do some video sizes have to use even numbers?
Most video codecs, including H.264 and HEVC, store colour at half resolution horizontally and vertically, so both dimensions must be divisible by 2 or the encoder will refuse the file or pad it. That is why 480p 16:9 content ships as 854×480 rather than the mathematically exact 853.33. Switch the rounding option to “Nearest even pixel” and every result stays encoder-safe.