How to calculate percentages
The four percentage calculations you will actually use, with formulas and worked examples.
By MyUtils editorialPublished: Updated:
A percentage is a fraction out of 100. Almost every everyday percentage question is one of four calculations.
1. What is X% of Y?
Multiply Y by X and divide by 100.
15% of 240 = 240 × 15 ÷ 100 = 36
2. X is what percent of Y?
Divide X by Y and multiply by 100.
45 is what percent of 250? 45 ÷ 250 × 100 = 18%
3. Percentage change
Subtract the old value from the new value, divide by the old value, and multiply by 100.
From 80 to 100: (100 − 80) ÷ 80 × 100 = +25%
From 100 to 80: (80 − 100) ÷ 100 × 100 = −20%
Notice the asymmetry: an increase of 25% is cancelled by a decrease of 20%, not 25%, because the base changes.
4. Reverse percentages
If you know the result after a percentage change, divide by the multiplier to recover the original.
A price is 120 after a 20% discount. The multiplier is 0.80, so the original price was 120 ÷ 0.80 = 150.
Common mistakes
- Using the wrong base. Percentage change is always measured against the old value.
- Adding percentages that apply to different bases. A 10% increase followed by another 10% increase is 21% overall, not 20%.
Try your own numbers with the Percentage Calculator.
Frequently asked questions
How do I calculate 15% of a number?
Multiply the number by 15 and divide by 100, or multiply by 0.15.
Why is a 25% rise not cancelled by a 25% fall?
Because the fall is applied to the larger, new value. Going from 100 to 125 and then dropping 25% gives 93.75.