How to Calculate a Percentage (3 Formulas You'll Actually Use)

Percentages show up in discounts, taxes, grades, tips and interest — and almost every percentage problem is one of three formulas.

Percentages show up in discounts, taxes, grades, tips and interest — and almost every percentage problem is one of three formulas.

1. What is X% of Y?

Multiply Y by X, then divide by 100. 20% of 150 = 150 × 20 ÷ 100 = 30. Use this for discounts and tips.

2. X is what percent of Y?

Divide X by Y, then multiply by 100. 45 out of 60 = 45 ÷ 60 × 100 = 75%. Use this for scores and completion rates.

3. Percentage change from X to Y

Subtract the old value from the new one, divide by the old value, multiply by 100. From 80 to 100: (100 − 80) ÷ 80 × 100 = +25%. Use this for price changes, growth and salary raises.

The mental shortcut

Find 10% first (move the decimal one place left), then scale: 10% of 150 is 15, so 20% is 30 and 5% is 7.5. For anything messier, the calculator above does all three modes instantly.