Write down the measurements of each side of a figure in relation to a perimeter. For example, you might have a rectangle with a perimeter of 12 inches. Two sides might be 4 inches long each, and the other two sides could be 2 inches long each.
Write the first number that you plan to compare in the ratio.
Write a colon, a fraction slash or the word "to."
Write the second value in the comparison. For example, if you were comparing a 4-inch side of the rectangle to a 2-inch side, you could write "4:2," "4/2" or "4 to 2."
Simplify the numbers if necessary. Do this by dividing the measurements by the largest number that will fit into both of them evenly. In this example you would divide 4 and 2 by 2 to get 2:1, 2/1 or 2 to 1.