CalcCommonsHow to use
Enter the original width and height to reduce the ratio. To find a resized dimension, select a new width or height and enter its target pixel count. The other dimension is calculated proportionally.
For related tasks, try the Percentage Calculator.
Formula & how it works
Reduced ratio = width / gcd(width, height) : height / gcd(width, height). New height = new width × original height / original width. New width = new height × original width / original height.
Worked examples
Example 1
1920 × 1080 pixels share a greatest common divisor of 120, so the reduced ratio is 16:9. At a new width of 1280 pixels, the matching height is 720 pixels.
Example 2
A portrait image of 1200 × 1800 pixels reduces to 2:3. At a target height of 900 pixels, its proportional width is 600 pixels, a 50% scale.
Assumptions & practical notes
Dimensions must be positive whole pixels. This tool preserves the ratio mathematically and rounds final dimensions to whole pixels. Very small outputs can have noticeable rounding differences.
An aspect ratio describes shape, not pixel count, print resolution, file size or visual quality. 1920×1080 and 3840×2160 share 16:9 but have different pixel counts.
No image file is opened or edited here. Apply the calculated dimensions in your preferred image editor to resize a file.
See our calculation methodology for unit definitions and testing practices.
References & measurement guidance
References explain the underlying mathematics. The examples and interactive calculations on this page are specific to CalcCommons.
Frequently asked questions
Can I enter a ratio instead of image dimensions?
Yes. Enter 16 and 9 as the original width and height to calculate proportional dimensions for a 16:9 target.
Does changing aspect ratio crop my image?
This tool calculates dimensions at the original ratio, so it does not crop. Producing a different ratio requires cropping, padding or distortion in an image editor.
What is the difference between ratio and resolution?
Ratio is the relationship of width to height. Pixel dimensions describe the number of pixels along each edge.
Why is the resulting ratio slightly different?
Whole-pixel rounding can alter the exact ratio when the target dimension is not a multiple of the reduced ratio. Choose a suitable multiple when exact integer proportions matter.