How this calculation works
The greatest common divisor reduces an integer width-height pair to its simplest ratio. Resizing preserves that proportion, while the actual pixel height may need an explicit rounding choice in an export.
Ratio = width/gcd : height/gcd; new height = new width × original height ÷ original widthA worked example
1,920 by 1,080 simplifies to 16:9. At width 1,280, the matching height is 720 pixels.
Good to know
An estimate under the stated inputs and model. Displayed decimals are rounded; verify assumptions before relying on the result.
An aspect ratio tells you shape, not resolution
1,920 × 1,080 and 1,280 × 720 both simplify to 16:9. The first has more pixels but the same width-to-height proportion. A ratio therefore does not tell you the total detail or file size.
Swapping width and height changes 16:9 into 9:16. That describes a different orientation rather than an error in simplification.
A mathematically matching dimension may not be a whole pixel
The resized height is computed from the requested width without automatic cropping. Some widths produce fractional heights, which a real pixel grid cannot represent exactly. Choose a suitable export size or an explicit rounding/cropping policy.
The tool assumes square pixels and ordinary integer image dimensions. Pixel aspect ratio, display aspect ratio and an encoded frame with non-square pixels can require additional information.
A few questions, answered
Does the tool crop my image?
No. It returns ratios and matching dimensions without opening or modifying an image.
Why is the ratio reduced with a GCD?
Dividing both integer dimensions by their greatest common divisor gives the simplest equivalent width-to-height pair.