How to use the percentage calculator
- Pick the question that matches what you need, like “What is X% of Y?”.
- Type the numbers into the sentence. You can use a dot or a comma for decimals.
- Read the result and the formula below it. Copy the result if you need it elsewhere.
The formulas behind each calculation
- X% of Y = X ÷ 100 × Y. For example, 20% of 150 = 0.2 × 150 = 30.
- X is what percent of Y = X ÷ Y × 100. For example, 30 is 30 ÷ 150 × 100 = 20% of 150.
- Percentage change from A to B = (B − A) ÷ A × 100. From 80 to 100: 20 ÷ 80 × 100 = +25%.
- Increase Y by X% = Y × (1 + X ÷ 100). Decrease Y by X% = Y × (1 − X ÷ 100). 200 plus 15% = 230.
Why a 50% drop isn’t undone by a 50% rise
Percentages are always relative to a starting value. If a price of 100 drops 50%, it becomes 50. A 50% rise from 50 only brings it to 75, because the second percentage is taken from a smaller number. To get back to 100 you need a 100% increase.
The same idea explains why the percentage change from A to B is not the opposite of the change from B to A: from 80 to 100 is +25%, but from 100 to 80 is −20%.
Percent vs. percentage points
When a rate goes from 10% to 12%, it rose by 2 percentage points, but by 20 percent (2 is 20% of 10). News and financial reports use “points” for the plain difference between two percentages.
Frequently asked questions
How do I calculate a percentage of a number?
Divide the percentage by 100 and multiply by the number. 15% of 80 is 0.15 × 80 = 12. Use the first calculator to do it instantly.
How do I add VAT or a tip to a price?
Use “Increase by a percentage”: 50 increased by 21% is 60.50. To remove a percentage that is already included, divide instead: 60.50 ÷ 1.21 = 50.
How do I find the original price before a discount?
Divide the discounted price by (1 − discount ÷ 100). If a product costs 80 after a 20% discount, the original price was 80 ÷ 0.8 = 100.
Can I use negative numbers or decimals?
Yes. A dot or a comma works as the decimal separator, and negative values are accepted. A comma followed by exactly three digits, like 1,000, is read as one thousand. Results show up to four decimals; a “≈” sign means the value was rounded.