How the calculation works
Every percentage question comes down to one of four small formulas. The calculator picks the right one for the question you choose.
X% of Y = X ÷ 100 × Y
X is what % of Y = X ÷ Y × 100
% change = (new − old) ÷ old × 100
Y increased by X% = Y × (1 + X ÷ 100)
For a decrease use 1 − X ÷ 100. The percentage change is always taken of the old value, and a negative result is a decrease.Worked examples
Percent of a number: 15% of 200.
- 15 ÷ 100 = 0.15.
- 0.15 × 200 = 30.
Percentage change: a price goes from 80 to 100.
- The difference is 100 − 80 = 20.
- 20 ÷ 80 = 0.25, which is +25%.
Add a percentage: 250 plus 21%.
- 1 + 21 ÷ 100 = 1.21.
- 250 × 1.21 = 302.50.
What this calculator does not include
- Percentage points. A rise from 4% to 5% is 1 percentage point, but a 25% increase.
- Compounding over several periods. Use the compound interest calculator for that.
- Rounding. Results show up to 4 decimals and are not rounded in the calculation.
Amounts are rounded for display only; the calculation itself is not rounded. Results are estimates for information, not financial advice. Last updated: 12 November 2026.
Frequently asked questions
How do I calculate a percentage of a number?
Divide the percentage by 100 and multiply by the number. 15% of 200 is 0.15 × 200 = 30.
How do I find what percentage one number is of another?
Divide the first number by the second and multiply by 100. 30 out of 200 is 30 ÷ 200 × 100 = 15%.
How do I calculate a percentage increase or decrease?
Subtract the old value from the new value, divide by the old value and multiply by 100. From 80 to 100 is (100 − 80) ÷ 80 × 100 = +25%. From 100 back to 80 is −20%, not −25%, because the percentage is always taken of the starting value.
How do I add a percentage to a price?
Multiply the price by 1 plus the percentage divided by 100. A price of 250 plus 21% is 250 × 1.21 = 302.50. To subtract a percentage, use 1 minus the percentage divided by 100.
Why is a 25% increase followed by a 25% decrease not back at the start?
Because the second percentage is taken of the new, larger number. 100 plus 25% is 125, and 125 minus 25% is 93.75.