Percentage Calculator
Four everyday percentage sums in one place. Type your numbers and each answer appears with the formula that produced it, so you can check the working or reuse it in a spreadsheet.
What is X% of Y?
X is what percent of Y?
Percent change from X to Y
Increase or decrease Y by X%
All calculations happen in your browser. Nothing you type is sent to our server.
How to use the Percentage Calculator
- Find the box that matches your question, for example "What is X% of Y?" for a discount or "Percent change" for a price rise.
- Type the two numbers. Decimals and negative numbers are fine; leave out the % sign.
- Read the answer and the formula beneath it. The working uses your own numbers so you can follow each step.
- Use Clear all to start again. Each box works on its own, so you can keep several answers on screen at once.
The four formulas
- X% of Y = X ÷ 100 × Y. 15% of 80 is 0.15 × 80 = 12.
- X as a percent of Y = X ÷ Y × 100. 18 out of 24 is 0.75, which is 75%.
- Percent change = (new − old) ÷ |old| × 100. Going from 40 to 46 is a change of 6 on a base of 40, so +15%. The base is always the starting value, which is why a change from 46 back to 40 is only about −13%.
- Increase or decrease by X% = Y × (1 ± X ÷ 100). Adding 12% to 250 is 250 × 1.12 = 280; taking 12% off is 250 × 0.88 = 220.
When the starting value is zero, a percent change cannot be calculated: any increase from nothing would be infinitely large in percentage terms. The calculator says so and shows the plain difference instead. For a negative starting value it divides by the absolute value, so a move from −50 to −25 shows as an increase of 50%. That is the usual convention, but in finance a percent change on a negative number is often better described in words.
Common traps
Up and down by the same percent does not cancel out. Raise 100 by 20% and you get 120; cut 120 by 20% and you get 96, not 100. The second percentage is taken from a bigger base.
Undoing a percentage. If a price already includes 27% tax and you want the price before tax, divide by 1.27 rather than subtracting 27%. For a total of 127, that gives 100; subtracting 27% would give 92.71.
Percent versus percentage points. If an interest rate goes from 4% to 5%, it has risen by one percentage point, but by 25% in relative terms. Use the percent-change box for the relative figure, and simple subtraction for points.
Frequently asked questions
How do I work out a discount?
Use "Increase or decrease Y by X%". Enter the original price as Y and the discount as X; the second line is the sale price and the hint shows how much you save. For example, 30% off 59.90 is 41.93, a saving of 17.97.
Why does percent change from 0 say "not defined"?
The formula divides by the starting value. Dividing by zero has no answer, so there is no meaningful percentage. The calculator shows the absolute change instead, which is the honest way to describe growth from zero.
How many decimal places are shown?
Up to six, with trailing zeros removed. Tiny floating-point errors that computers produce, such as 0.30000000000000004, are rounded away so you see 0.3.
Can I use percentages above 100?
Yes. 250% of 40 is 100, and increasing 40 by 150% gives 100. Decreasing a number by more than 100% gives a negative result, which the calculator shows rather than hides.
What is the difference between "X is what percent of Y" and percent change?
The first compares a part with a whole: 18 is 75% of 24. Percent change compares a new value with an old one: going from 18 to 24 is a 33.3% increase. Same numbers, different questions.