Percentage formulas
| Question | Formula | Example |
|---|---|---|
| What is X% of Y? | X ÷ 100 × Y | 20% of 150 = 30 |
| X is what % of Y? | X ÷ Y × 100 | 30 is 20% of 150 |
| X is Y% of what? | X ÷ (Y ÷ 100) | 30 is 20% of 150 |
Everyday uses
Discounts: a $80 jacket at 25% off saves 0.25 × 80 = $20, so you pay $60.
Tips: an 18% tip on a $65 bill is 0.18 × 65 = $11.70.
Test scores: 42 correct out of 50 is 42 ÷ 50 × 100 = 84%.
Sales tax backwards: if $7.20 of tax is 8% of the price, the price was 7.20 ÷ 0.08 = $90.
Tips: an 18% tip on a $65 bill is 0.18 × 65 = $11.70.
Test scores: 42 correct out of 50 is 42 ÷ 50 × 100 = 84%.
Sales tax backwards: if $7.20 of tax is 8% of the price, the price was 7.20 ÷ 0.08 = $90.
Mental math shortcuts
- 10%: move the decimal point one place left (10% of 340 = 34).
- 5%: half of 10% (5% of 340 = 17).
- 1%: move the decimal two places (1% of 340 = 3.4).
- Swap the numbers when it’s easier: 16% of 25 = 25% of 16 = 4.
Frequently asked questions
How do I calculate a percentage of a number?
Convert the percentage to a decimal by dividing by 100, then multiply. 20% of 150 = 0.20 × 150 = 30.
How do I find what percent one number is of another?
Divide the part by the whole and multiply by 100. 30 out of 150 = 30 ÷ 150 × 100 = 20%.
How do I find the whole when I know the part and the percentage?
Divide the part by the percentage as a decimal. If 30 is 20% of a number, the number is 30 ÷ 0.20 = 150.
Is X% of Y the same as Y% of X?
Yes — multiplication works in either order. 8% of 50 equals 50% of 8, which is 4. This trick makes some mental math much easier.
How do I calculate a percentage increase?
Use the percentage change calculator: (new − old) ÷ old × 100.