How to use this calculator
Choose one of the three calculation modes using the buttons at the top. "What is X% of Y?" finds a percentage of a number, useful for tips, discounts, and taxes. "X is what % of Y?" tells you what fraction one number is of another. And "% change from X to Y" calculates how much something increased or decreased in percentage terms.
Just enter your numbers and the result appears immediately, no need to press a button. The formula is shown below the result so you can verify the math or use it for your own calculations.
Common percentage formulas
| Calculation | Formula | Example |
|---|---|---|
| X% of Y | (X ÷ 100) × Y | 20% of 150 = 30 |
| X is what % of Y | (X ÷ Y) × 100 | 30 of 150 = 20% |
| % increase | ((New − Old) ÷ Old) × 100 | 80 → 100 = +25% |
| % decrease | ((Old − New) ÷ Old) × 100 | 100 → 80 = −20% |
| Reverse a % | Result ÷ (Pct ÷ 100) | 30 is 20% of ? → 150 |
Quick mental math tricks for percentages
The 10% anchor: Finding 10% is trivial, just move the decimal point one place left. 10% of 85 is 8.5. From there, build any percentage: 5% is half of that (4.25), 20% is double (17), and 15% is 10% plus 5% (12.75).
The flip trick: Percentages are commutative. 8% of 25 equals 25% of 8, which is obviously 2. Whenever one direction is hard to compute in your head, try flipping it.
The 1% method: For odd percentages, find 1% first (move the decimal two places left), then multiply. 7% of 300? 1% is 3, so 7% is 21.
Double and halve: To find 25%, halve the number twice. To find 75%, find 25% and subtract it from the whole. 25% of 120 is 30, so 75% of 120 is 90.
Frequently asked questions
How do I calculate a percentage of a number?
Divide the percentage by 100 and multiply by the number. For example, 25% of 200 = (25 ÷ 100) × 200 = 50. This calculator does it automatically, just enter the percentage and value.
How do I find what percentage one number is of another?
Divide the part by the whole and multiply by 100. For example, 50 is what percent of 200? (50 ÷ 200) × 100 = 25%. Use the "X is what % of Y?" mode for this.
How do I calculate percentage change?
Subtract the old value from the new value, divide by the absolute value of the old value, and multiply by 100. A positive result means an increase; negative means a decrease.
Why is a 50% increase followed by a 50% decrease not equal to the original?
Because the decrease is calculated from the larger number. If 100 increases by 50% to 150, then a 50% decrease from 150 is 75, not 100. Percentage changes compound relative to the current value, not the original.