Understanding aspect ratios can be intimidating, but it’s really quite easy if you can remember how to factor fractions. Factor your width and height to the lowest common denominator and you will have your aspect ratio.
For example, if you have a painting that is 18″x24″, then you factor 18/24 to its lowestcommon denominator to understand the ratio. Dividing 18/24 by 6, you get 3/4, or a 3:4 aspect ratio. If you have a 20″x30″ painting, or 20/30 = 10, which equals 2/3, this gives you have a 2:3 aspect ratio.
Now for example, if you want to make an 8″x10″ print from a 20″x30″ painting, you will realize quickly that you can’t do it. You can have an 8″x12″, but not an 8″x10″. There are many other examples, but this should suffice in understanding the concept. The calculator will do the rest for you.