How to calculate a percentage
- 1.
Pick a calculator
Choose the one that matches your question: X% of Y, X as a percent of Y, change from A to B, increase/decrease by X%, or X is P% of what.
- 2.
Enter two values
Both comma and dot work as decimal separators.
- 3.
Copy the result
The result updates live - use the copy button next to it.
Percentage formulas with examples
| Question | Formula | Example |
|---|---|---|
| What is X% of Y? | Y × X / 100 | 20% of 150 = 30 |
| X is what percent of Y? | X / Y × 100% | 30 of 150 = 20% |
| Change from A to B | (B − A) / A × 100% | 80 → 100 = +25% |
| Y increased by X% | Y × (1 + X/100) | 200 + 15% = 230 |
| Y decreased by X% | Y × (1 − X/100) | 200 − 15% = 170 |
| X is P% of what? | X × 100 / P | 30 is 20% of 150 |
“Percent” means “per hundred”, so 15% is simply 0.15. Converting the percentage to a decimal first makes every formula a single multiplication or division.
Traps: asymmetric changes and percentage points
An increase and a decrease of the same percentage do not cancel out. A price of 100 raised by 10% becomes 110; cut by 10% afterwards it is 99. A drop from 100 to 80 is −20%, but getting back from 80 to 100 needs +25%.
Percent vs percentage points. When an interest rate goes from 5% to 7%, it rose by 2 percentage points (pp) but by 40% in relative terms. The percentage-change calculator shows both values.
Working backwards. If a price after a 20% markup is 120, the original was 120 / 1.2 = 100 - not 120 − 20% = 96. Use the “X is P% of what?” calculator with P = 120.
Everyday examples
- Sale price: a jacket at 80 with 30% off costs 80 × 0.7 = 56; you save 24.
- Pay rise: a salary going from 3,200 to 3,400 is a rise of 200 / 3,200 = 6.25%.
- Test score: 42 out of 56 is 42 / 56 = 75%.
- Tip: 15% of a 64 bill is 9.60.
- Consecutive changes multiply: two 10% increases give +21% (1.1 × 1.1 = 1.21), not +20%.
When a percentage is applied repeatedly, year after year - as with interest on savings - use the compound interest calculator instead.
Frequently asked questions
How do I calculate a percentage of a number?
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Multiply the number by the percentage and divide by 100: 15% of 240 = 240 × 15 / 100 = 36.
How do I work out a percentage increase?
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Subtract the old value from the new one, divide by the old value and multiply by 100. From 80 to 100: (100 − 80) / 80 = 25%.
How do I find what percent one number is of another?
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Divide the part by the whole and multiply by 100. 45 out of 180 is 45 / 180 × 100 = 25%.
How do I find the original price before a discount?
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Divide the sale price by (1 − discount). If an item costs 80 after 20% off, it cost 80 / 0.8 = 100 before.
What is the difference between percent and percentage points?
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Percentage points are the plain difference between two percentages (5% to 7% = 2 pp); percent describes the relative change (5% to 7% = a 40% increase).
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