The three percentage formulas
- X% of Y = X ÷ 100 × Y. Example: 15% of 80 = 12.
- A is what % of B = A ÷ B × 100. Example: 12 is 25% of 48.
- Percent change = (new − old) ÷ old × 100. Example: 50 → 65 is +30%.
Discounts and stacked discounts
A 25% discount on 80 costs 60. Two discounts do not add up: 20% off and then another 10% off is 28% off in total, not 30%. Use the Extra % off box to see the real total.
Percent vs percentage points
If an interest rate goes from 4% to 5%, it rose by 1 percentage point, which is a 25% increase. The % change tab gives the second number.
FAQ
How do I calculate a percentage of a number?
Divide the percent by 100 and multiply by the number. 15% of 80 is 0.15 × 80 = 12.
How do I calculate percent increase?
Subtract the old value from the new value, divide by the old value and multiply by 100.
How do I work out a discount?
Multiply the price by (1 − discount ÷ 100). 25% off 80 is 80 × 0.75 = 60.