How to calculate a percentage
A percentage is a number out of 100. To find X% of Y, divide X by 100 and multiply by Y. For example, 15% of 80 is 0.15 × 80 = 12.
To find what percent X is of Y, divide X by Y and multiply by 100. 30 out of 120 is 30 ÷ 120 × 100 = 25%.
Percentage increase and decrease
Percentage change compares a new value with an old one: (new − old) ÷ old × 100. Going from 50 to 65 is (65 − 50) ÷ 50 × 100 = a 30% increase. A negative result means a decrease.
Percentage change is not symmetric: a 50% drop followed by a 50% rise does not get you back where you started (100 → 50 → 75).
Frequently asked questions
How do I work out a percentage of a total in my head?
Find 10% by moving the decimal point one place left, then scale. 10% of 80 is 8, so 15% is 8 + 4 = 12.
What is the difference between percent and percentage points?
If an interest rate goes from 4% to 5%, it rose by 1 percentage point, but by 25 percent (1 ÷ 4).