The three percentage formulas
1. What is X% of Y?
Result = ( X ÷ 100 ) × Y
Example: a 15% tip on a $200 bill is (15 ÷ 100) × 200 = $30.
2. X is what percent of Y?
Percent = ( X ÷ Y ) × 100
Example: scoring 30 out of 120 on a test is (30 ÷ 120) × 100 = 25%.
3. Percentage increase or decrease
Change % = ( New − Old ) ÷ Old × 100
Example: rent going from $50 to $65 is (65 − 50) ÷ 50 × 100 = a 30% increase.
Everyday percentage shortcuts
- 10% — move the decimal point one place left: 10% of 84 is 8.4.
- 5% — take half of 10%: 5% of 84 is 4.2.
- 15% tip — 10% plus half again: on $60, that's 6 + 3 = $9.
- 20% discount — find 10% and double it, then subtract from the price.
- Reverse a percent — X% of Y always equals Y% of X: 8% of 25 = 25% of 8 = 2.
Frequently asked questions
How do I calculate a percentage of a number?
Divide the percentage by 100 and multiply by the number. 15% of 80 = (15 ÷ 100) × 80 = 12.
How do I calculate percentage increase or decrease?
Subtract the old value from the new, divide by the old value, multiply by 100. From 50 to 65: (65 − 50) ÷ 50 × 100 = +30%.
How do I find what percent one number is of another?
Divide the first by the second and multiply by 100. 30 out of 120 = 25%.
How do I calculate a discount?
Multiply the price by the discount percent ÷ 100 to get the amount off. A 20% discount on $45 saves $9, so you pay $36.