How to calculate a percentage
- Find the question that matches yours, for example Percentage increase or decrease.
- Type the two numbers. The answer appears as you type.
- Check the formula under the result, and click Copy to use the number elsewhere.
The six formulas
- X% of Y = Y × X ÷ 100. 15% of 80 = 12.
- X is what % of Y = X ÷ Y × 100. 45 out of 60 = 75%.
- Percent change = (new − old) ÷ old × 100. 80 → 92 = +15%.
- Add or subtract % = value × (1 ± % ÷ 100). 59.99 + 20% = 71.99 (rounded).
- Price before discount = price ÷ (1 − discount ÷ 100). Paid 68 after 15% off → originally 80.
- Percentage points = new rate − old rate. 4% → 5% is +1 percentage point, but +25% in relative terms.
Common mistakes we avoid
A discount followed by an increase of the same percentage doesn’t get you back where you started: 100 − 20% = 80, and 80 + 20% = 96. To undo a 20% discount you need +25%. That is exactly what Price before discount computes.
Two discounts don’t add up either. 30% off and then another 20% off is 100 × 0.7 × 0.8 = 56, a total of 44% off, not 50%.
When to use it
Working out a sale price or a tip. Checking whether a price increase or a salary raise is as big as it sounds. Calculating VAT or sales tax on top of a net price. Reading news about interest rates, where “up 1 point” and “up 25%” can describe the same change.
Converting units at the same time? Use Unit Converter. For durations and deadlines, try Date, Age & D-day Calculator.
Limits
- Results are rounded to 4 decimals (2 for percentages). Copy uses up to 6 decimals.
- Percent change from 0 is undefined, so the tool shows a message instead of a number.
- For tax, check your local rate; rules on rounding receipts vary by country.
Frequently asked questions
How do I calculate a percentage of a number?
Multiply the number by the percentage and divide by 100. 15% of 80 is 80 × 15 ÷ 100 = 12.
How do I find the original price before a discount?
Divide the price you paid by (1 − discount). After 25% off, a price of 60 was originally 60 ÷ 0.75 = 80.
What’s the difference between percent and percentage points?
Percentage points are the plain difference between two percentages (4% → 5% = 1 point). Percent change compares that difference with the starting value (1 ÷ 4 = 25%).
Why isn’t +20% then −20% zero?
The second 20% is taken from a different base. 100 + 20% = 120, and 120 − 20% = 96.