The formula
ratio = width ÷ greatest common divisor : height ÷ greatest common divisor
Where it comes from
The aspect ratio is the proportion between width and height, reduced to the smallest whole numbers possible. It is good for two things: knowing what format you are in, and working out the missing side when you resize without distorting anything.
How to work it out by hand
- Find the greatest common divisor of the width and the height
- Divide both by it for the smallest ratio
- To scale, divide the new width by the ratio for the height
- Check the result gives a whole number of pixels
What is worth knowing
Not every resolution reduces to a clean ratio. 1366×768, very common on laptops, comes out at 683:384, which is nearly 16:9 but not exactly: hence the slight bars or the light rescaling when playing video. The formats worth knowing are 16:9 for video and television, 9:16 for phones and short video, 1:1 for social, 4:3 for the classic format and 2.39:1 for cinema. And a warning when scaling: if the new side does not land on a whole number of pixels, some program will round on its own and the ratio will shift a fraction.