How to use the percentage calculator
The calculator has three parts, one for each common percentage question. Fill in the two boxes of the one you need and the answer appears under it as you type:
- What is X% of Y? For example, a 15% tip on an 80 bill, or 20% of a class of 35 students.
- X is what percent of Y? For example, 18 correct answers out of 24 questions.
- X is Y% of what number? For example, you know 30 people is 15% of a group and want the size of the whole group.
The results panel shows the working, step by step, for the part you used last. Copy result copies a one-line summary with the working. Try the examples fills in all three parts with the examples on this page.
For a change between two numbers, such as a price going from 64 to 80, use the percentage increase calculator instead. It also adds or takes off a percentage and finds the percentage difference.
The three percentage formulas
"Percent" means "per hundred", so 15% is 15 out of every 100, or 0.15 as a decimal. Every percentage question uses the same relationship:
part = percentage ÷ 100 × whole
Each part of the calculator solves it for a different missing piece:
| Question | Formula | Example | Answer |
|---|---|---|---|
| What is X% of Y? | X ÷ 100 × Y | 15% of 80: 0.15 × 80 | 12 |
| X is what percent of Y? | X ÷ Y × 100 | 18 of 24: 0.75 × 100 | 75% |
| X is P% of what? | X ÷ (P ÷ 100) | 30 is 15% of: 30 ÷ 0.15 | 200 |
You can check any answer by working it back the other way. 15% of 200 is 0.15 × 200 = 30, which matches the third example.
Worked examples
What is 15% of 80?
- Turn the percentage into a decimal: 15 ÷ 100 = 0.15.
- Multiply by the number: 0.15 × 80 = 12.
A quick way to do this in your head: 10% of 80 is 8, 5% is half of that, 4, so 15% is 8 + 4 = 12.
18 is what percent of 24?
- Divide the part by the whole: 18 ÷ 24 = 0.75.
- Multiply by 100: 0.75 × 100 = 75%.
This is how a test score out of 24 becomes a percentage. The test grade calculator does the same for a whole class, with letter grades.
30 is 15% of what number?
- Turn the percentage into a decimal: 15 ÷ 100 = 0.15.
- Divide the number by it: 30 ÷ 0.15 = 200.
The same works for a sale price. If an item costs 60 after 25% off, you paid 75% of the original price, so the original was 60 ÷ 0.75 = 80.
Decimals, zeros and negative numbers
- Decimals work in every box. 2.5% of 1,200 is 0.025 × 1,200 = 30. Answers are shown to up to four decimal places.
- Percentages over 100 are fine. 150% of 40 is 60, more than the number you started with. 45 out of 40 is 112.5%.
- Zero. 0% of anything is 0, and 0 is 0% of any number other than 0. But "X is what percent of 0?" and "X is 0% of what number?" have no answer, because both mean dividing by zero. The calculator says so instead of showing a number.
- Negative numbers follow the usual sign rules. −20 is −25% of 80, and 10% of −50 is −5. A negative percentage is rare in everyday use, so check that a minus sign is what you meant.
Percentages in Excel or Google Sheets
Microsoft's Excel help page on calculating percentages gives a formula for each of the three questions:
- A percentage of a total: 42 correct answers out of 50 is
=42/50, which gives 0.84. Format the cell as a percentage to show 84%. - An amount from a total and a percentage: to "find 8.9% of 800", Microsoft uses
=800*0.089, which gives 71.2. - A total from an amount and a percentage: to "find 75% of which number equals 15", it uses
=15/0.75, which gives 20.
Notice that the spreadsheet formulas use the percentage as a decimal (0.089 for 8.9%), which is the "divide by 100" step this calculator shows in its working.
Percent and percentage points
When two percentages are compared, the gap between them is measured in percentage points. If a pass rate goes from 40% to 50%, it has risen by 10 percentage points, but by 25% in relative terms, because 10 is 25% of 40. The percentage increase calculator explains this in more detail.
Questions and answers
How do I work out a percentage of a number?
Divide the percentage by 100 and multiply by the number. 15% of 80 is 15 / 100 x 80 = 12.
How do I find what percent one number is of another?
Divide the first number by the second and multiply by 100. 18 out of 24 is 18 / 24 x 100 = 75%.
How do I find the whole number from a percentage?
Divide the part by the percentage written as a decimal. If 30 is 15% of a number, the number is 30 / 0.15 = 200.
Is X% of Y the same as Y% of X?
Yes. Both are X x Y / 100, so 8% of 50 and 50% of 8 are both 4. Sometimes one is much easier to work out in your head.
Can a percentage be more than 100?
Yes. 45 out of 40 is 112.5%, and 150% of 40 is 60. A percentage above 100 just means the part is bigger than the whole you compare it with.
How do I calculate a percentage increase?
Subtract the old value from the new one, divide by the old value and multiply by 100. The percentage increase calculator on this site does this and shows the working.
Sources
Checked on 22 October 2026. If a rule has changed, please tell us.