A proportion is how much of something relative to the whole, usually expressed as a percent. To calculate a proportion, divide how much of the thing in question by the whole to get a decimal, and multiply the decimal by 100 to get a percent.

For example, a class of 8 students has 4 boys. To calculate the proportion of boys in the class, do the following:

boys     = 4
students = 8
decimal  = 4 ÷ 8     = 0.5
percent  = 0.5 × 100 = 50

When calculating proportions, you don't necessarily need to divide the exact numbers. If the example above was modified so that a university of 800 students has 400 boys, you could still divide 4 by 8 to get the same proportion.

Sometimes the whole amount is not explicitly given, so to calculate a proportion, first add all of the parts of the whole to get the whole amount. For example, a class has 4 boys and 12 girls. To calculate the proportion of boys in the class, do the following:

boys              =  4
girls             = 12
students = 4 + 12 = 16
decimal  = 4 ÷ 16     = 0.25
percent  = 0.25 × 100 = 25