Aspect Ratio Calculator
Enter a width and height to get the ratio — and resize to any target without distortion.
How to use
- Enter the original width and height in pixels (or any unit).
- Read the simplified aspect ratio — common standards like 16:9 or 4:5 are named.
- Type a target width or target height to get the matching other dimension for a distortion-free resize.
Examples
Resize a Full HD frame
1920 × 1080 is 16:9. A target width of 1280 gives a height of 720 — the image scales without stretching.
Instagram portrait crop
4:5 is the tallest feed format. For a 1080-pixel-wide post, the height is 1350.
Resizing an image without distortion
A 1920×1080 image resized to a width of 1280 needs a height of 720 to keep its proportions. Getting the second number from the ratio rather than guessing prevents the subtle stretching that is obvious to viewers but easy to miss while working.
Fitting artwork to a platform requirement
Platforms specify shapes rather than sizes: 16:9 for landscape video, 9:16 for vertical, 1:1 for square feeds and 4:5 for the tallest allowed portrait post. Working out the pixel dimensions for the ratio you need before designing avoids a crop that cuts through the subject.
Planning a responsive video container
A player that keeps its shape across screen widths is defined by its ratio, not by fixed pixels. Knowing the ratio in lowest terms is what lets you express it once in CSS and let the browser compute the height, rather than maintaining a table of breakpoint-specific sizes.
FAQ
What is an aspect ratio?
The proportional relationship between width and height, written as width:height. A 1920×1080 image and a 1280×720 image are different sizes but the same 16:9 shape.
How is the missing dimension calculated?
By proportion: new height = new width × (original height ÷ original width), and the same the other way. This keeps the shape identical, so nothing looks stretched.
Why does 2560×1080 show as 64:27 and not 21:9?
21:9 is a marketing name — the true simplified ratio of most “21:9” ultrawide screens is 64:27 (≈2.37:1). The tool shows the exact ratio and notes that it is the ultrawide format.
Should the result be rounded to whole pixels?
Yes for images and video — the tool rounds targets to sensible precision, and a half-pixel difference is invisible. Keep exact values only for print or vector work.
Why do 16:9 and 1.78:1 describe the same shape?
They are two notations for one proportion. Film and photography usually normalise the second term to 1, giving 1.78:1, while screens and video use whole numbers, giving 16:9. Dividing 16 by 9 gives 1.78, so the shape is identical and only the convention differs.
What is the difference between 21:9 and the actual pixels?
Ultrawide monitors are marketed as 21:9 but usually have panels of 2560×1080 or 3440×1440, which reduce to 64:27 and 43:18. The marketing label is a rounded family name rather than the true ratio, which is why the reduced form of a real resolution rarely reads as 21:9.
Should I round to whole pixels?
Yes, for anything that will be rendered — a fractional pixel dimension is resolved by the renderer anyway, usually by rounding, and sometimes with a soft edge. Round the calculated dimension yourself so you know exactly what you are getting, and accept that one rounded side leaves the ratio off by a fraction of a percent.