How the ratio is simplified
The width and height are divided by their greatest common divisor. For 1920 by 1080 that divisor is 120, which gives 16:9. A ratio of 1.78 is the same shape expressed as a decimal, but it cannot tell you whether the source was 16:9 or something close to it.
Scaling multiplies the new width by the original height and divides by the original width. At 1280 wide, a 16:9 source becomes 720 high.
Worked example
A 1920 by 1080 image has a ratio of 16:9 and a width-to-height decimal of 1.7778. Scaled to 1280 pixels wide it becomes 720 pixels high, and scaled to 720 pixels high it becomes 1280 wide.
These figures come from the pixel counts you enter, not from a list of standard resolutions.
Why whole numbers matter
Video, screens and print all use ratios as whole numbers. A 21:9 ultrawide is not the same as 2.33:1 rounded from 2.37:1, and a 4:3 source scaled with the wrong ratio shows visible stretching.
If the simplified ratio looks unusual, such as 683:384, the source has probably been cropped by a few pixels rather than created at a standard ratio.
Limits of this calculator
It works on pixels only. It does not account for non-square pixels, anamorphic sources, letterboxing already baked into the image, or a rotation flag in the file.
It does not resample or convert an image. Use it to find the numbers, then resize in your editor.
For the physical size of a screen at a given resolution, use the PPI calculator.
Inputs for the aspect ratio calculator
Original width
Width in pixels of the image, video frame or screen you are starting from.
Original height
Height in pixels of the same source.
New width to scale to
The width you want. The matching height is calculated from the ratio.
CALCULATOR QUESTIONS
Aspect ratio calculator questions
Is 1.78 the same as 16:9?
Not exactly. 16 divided by 9 is 1.7778, so 1.78 is a rounded version and could hide a slightly different source.
Why is my ratio 683:384?
The dimensions do not share a large common divisor, usually because the image was cropped by a few pixels.
Does this resize my image?
No. It gives you the dimensions. The actual resampling happens in your editor or encoder.
What about ultrawide 21:9?
Enter the real pixel dimensions and the calculator will show the true ratio, which is often 64:27 rather than 21:9.