For example, if you have a painting that is 18×24 inches, then you factor 18/24 to its lowest common 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.