How do you calculate a percentage?
To calculate a percentage, divide the part by the whole and multiply by 100. For example, 15 out of 60 is 15 ÷ 60 = 0.25, and 0.25 × 100 = 25 percent.
To find a percentage of a number, turn the percentage into a decimal and multiply. For 20 percent of 80, calculate 0.20 × 80 = 16. A quick trick for 10 percent is to move the decimal point one place to the left: 10 percent of 250 is 25.
To find a percentage change, subtract the old value from the new one, divide by the old value and multiply by 100. If a price rises from 50 to 60, the change is 10 ÷ 50 × 100 = 20 percent.
Useful shortcuts
Percentages can be swapped: 8 percent of 50 is the same as 50 percent of 8, which is 4. To add a percentage, multiply by 1 plus the decimal, so adding 15 percent to 40 is 40 × 1.15 = 46.
Did You Know?
Reversing a percentage change does not return to the start. A 50 percent fall followed by a 50 percent rise leaves you with 75 percent of the original amount.