How to use it
Find the line that matches your question and type the two numbers into it. Each line answers on its own and shows the formula with your numbers filled in. Decimals and thousands separators are accepted — 1,250.5 and 12.5 both work.
- What is X% of Y? — tips, tax, a share of a budget.
- X is what percent of Y? — a test score, a completion rate, market share.
- Percentage change — price rises, growth, changes between two measurements.
- Increase or decrease by a percentage — a raise, a discount, adding sales tax. Type a negative percentage to decrease.
- Reverse percentage — the price before VAT or before a markup, when you only know the final amount.
The formulas
A percentage is a fraction of 100, so every calculation here comes down to dividing or multiplying by 100 at the right moment.
Percent of a number
X% of Y is X ÷ 100 × Y. Percentages are symmetric in a way that is handy for mental arithmetic: 8% of 50 is the same as 50% of 8, which is 4.
What percent one number is of another
Divide the part by the whole and multiply by 100. The "whole" must be the reference amount: 30 is 25% of 120, but 120 is 400% of 30.
Percentage change
The change from an old value A to a new value B is (B − A) ÷ A × 100. The old value is the reference, which is why going up and coming back down are not symmetric: a rise from 50 to 75 is +50%, but a fall from 75 back to 50 is −33.3%. When the old value is negative, the calculator divides by its absolute value so that an improvement from −10 to −5 comes out as a positive change. A change from zero has no percentage — any increase from nothing is infinitely large — so the calculator says so instead of printing a meaningless number.
Reverse percentages
The most common percentage mistake is undoing an increase by subtracting the same percentage. If a price including 15% tax is 230, the price before tax is not 230 minus 15% (195.50); it is 230 ÷ 1.15 = 200. The last line of the calculator does this division for you.
Percentage points
When a rate moves from 4% to 5%, it has risen by one percentage point, but by 25% in relative terms. News reports often mix the two up. Use the percentage change line with 4 and 5 to get the relative change; the difference in points is simply 5 − 4.
Examples
| Question | Answer | Working |
|---|---|---|
| What is 15% of 80? | 12 | 15 ÷ 100 × 80 |
| 30 is what percent of 120? | 25% | 30 ÷ 120 × 100 |
| Change from 50 to 75 | +50% | (75 − 50) ÷ 50 × 100 |
| Change from 80 to 60 | −25% | (60 − 80) ÷ 80 × 100 |
| 200 increased by 15% | 230 | 200 × 1.15 |
| 200 decreased by 15% | 170 | 200 × 0.85 |
| 230 after adding 15% — original? | 200 | 230 ÷ 1.15 |
| Change from 3 to 7 | +133.333333333% | (7 − 3) ÷ 3 × 100 |
Results are shown to twelve significant digits. That hides the rounding noise of binary arithmetic — 0.1 × 3 is shown as 0.3 rather than 0.30000000000000004 — while keeping recurring decimals like 133.333… visible.
Limits
- Successive percentages multiply. Two 10% discounts are not a 20% discount: 100 × 0.9 × 0.9 = 81, a 19% discount. Apply them one at a time using "Use a minus sign to decrease" twice.
- Percentage change from zero is undefined, and very small old values make percentage changes huge and misleading.
- Rounding for money is up to you. The calculator shows the exact result; round to cents as your invoice or tax rules require.
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.
How do I calculate percentage increase?
Subtract the old value from the new one, divide by the old value and multiply by 100. From 50 to 75 is (75 − 50) ÷ 50 × 100 = 50%.
How do I remove a percentage that was added?
Divide by 1 plus the percentage as a decimal. A price of 230 including 15% tax was 230 ÷ 1.15 = 200 before tax.
What is the difference between percent and percentage points?
Percentage points are the plain difference between two percentages (4% to 5% is one point). Percent change is relative to the starting value (4% to 5% is a 25% increase).
Is anything I type stored?
No. Calculations run in your browser and nothing is saved.