The formula
Percent change = (new − old) ÷ old × 100. Positive means an increase, negative a decrease.
- 80 → 100: (100 − 80) ÷ 80 × 100 = +25%.
- 100 → 80: (80 − 100) ÷ 100 × 100 = −20%.
Why −20% then +20% doesn't get you back
The second percentage is taken from a smaller number: 100 − 20% = 80, then 80 + 20% = 96. To return to 100 you need +25%.
Adding or removing a percentage
- Add 15%: multiply by 1.15 (200 → 230).
- Take 25% off: multiply by 0.75 (200 → 150).
- Remove a percentage that was already added (e.g. tax): divide by 1.15, don't subtract 15%.
Percent vs percentage points
If an interest rate goes from 4% to 5%, that's a rise of 1 percentage point but a 25% increase. News reports often mix these up.
Do it instantly
The percentage calculator covers all four cases — X% of Y, X is what % of Y, percent change, and add/subtract a percentage — and shows the formula. For shopping, the discount calculator shows what you save.
Open the percentage calculatorOpen the tool
Frequently asked questions
How do I calculate a percentage of a number?
Multiply by the percentage and divide by 100: 15% of 200 = 15 × 200 ÷ 100 = 30.
How do I find the original price before a discount?
Divide the sale price by (1 − discount): 150 after 25% off → 150 ÷ 0.75 = 200.
Can percent change be more than 100%?
Yes — going from 50 to 150 is a 200% increase.