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Percentage Calculator
Sign up to favourite this calculatorPick the question you are asking, enter your two numbers, and read the answer with the working shown. Five percentage sums, one calculator.
15% of 200
Working
15 ÷ 100 × 200 = 30A% as a decimal
0.15B plus 15%
230B minus 15%
170Save to My calculations
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Assumptions and limits
- An estimate from the numbers you entered, for guidance only. It is not financial, tax or medical advice.
- Calculator version 1, last updated 25 October 2026.
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How to use this calculator
- Choose your question from the dropdown. Each one is written out with
AandBin it, so you can see which number goes where. - Enter
Aas the first number andBas the second. For "What is A% of B?" that means the percentage first and the amount second. - Read the answer, then check the question line under the inputs and the working shown with the result. They repeat your sum in words and in figures, which is the quickest way to catch two numbers entered the wrong way round.
Nothing is submitted anywhere. The calculation runs in the page as you type, and the address bar carries your figures, so a result can be shared or bookmarked.
What do I need to enter?
- Question — one of five: What is A% of B?, A is what percent of B?, A is B% of what?, A increased by B%, or A decreased by B%.
- First number (A) and Second number (B) — any number from −1,000,000,000 to 1,000,000,000. Negatives and percentages above 100 are allowed.
What do the results show?
- The answer — the main figure, labelled with your own numbers, such as "15% of 200".
- Working — the sum written out with your figures.
- Supporting figures — three extras that depend on the question: for example the decimal form, the fraction, the amount of an increase or decrease, or a check that reverses the sum.
- Not defined — shown when B is zero in the two questions that divide by it.
How percentages are calculated
A percentage is a fraction of 100 — per cent means "per hundred". Every percentage sum is therefore the same two steps in a different order: turn the percentage into a decimal by dividing by 100, then multiply or divide by it.
Result = Percentage ÷ 100 × Number
- Percentage — the rate you are applying, e.g. 15
- Number — the amount you are applying it to, e.g. 200
- Result — the answer, e.g. 30
Reversing the sum answers the other questions. Dividing the part by the whole gives the rate instead of the amount, and dividing the amount by the rate gives the whole back.
| Question | Formula | Example |
|---|---|---|
| What is A% of B? | A ÷ 100 × B | 15% of 200 = 30 |
| A is what percent of B? | A ÷ B × 100 | 1,245 out of 4,000 = 31.125% |
| A is B% of what? | A ÷ (B ÷ 100) | 10 is 5% of 200 |
| A increased by B% | A × (1 + B ÷ 100) | 5 increased by 30% = 6.50 |
| A decreased by B% | A × (1 − B ÷ 100) | 80 decreased by 25% = 60 |
The last two rows are the ones worth reading twice. Adding 30% to 5 gives 6.50 because the multiplier is 1.3, and taking 30% back off 6.50 gives 4.55 rather than 5 — the second percentage is taken from a larger number, so the two do not cancel out.
A worked example
A £4,000 monthly budget includes £1,245 of rent. To find what share the rent takes, choose "A is what percent of B?", enter 1,245 and 4,000, and the calculator divides one by the other:1,245 ÷ 4,000 = 0.31125, then multiplies by 100 to give 31.125%.
Working the same figures the other way is the third question. If you know rent is 31.125% of the budget and comes to £1,245, then 1,245 ÷ 0.31125 = 4,000 recovers the budget. Any two of the three numbers give the third.
Prefer a single-purpose calculator?
Each of the five questions above also has its own dedicated page, if you would rather skip the dropdown: the Percentage of a Number Calculator, the Percentage Change Calculator, and the Percentage Increase and Percentage Decrease calculators.
Frequently asked questions
How do I calculate a percentage of a number?
Divide the percentage by 100, then multiply by the number. For 15% of 200: 15 ÷ 100 gives 0.15, and 0.15 × 200 gives 30. Dividing by 100 is what turns a percentage into the decimal you can multiply with.
How do I work out what percentage one number is of another?
Divide the part by the whole, then multiply by 100. If you scored 18 out of 25, that is 18 ÷ 25 = 0.72, and 0.72 × 100 = 72%. Choose "A is what percent of B?" and the calculator shows the same working.
How do I add a percentage to a number?
Multiply by 1 plus the percentage as a decimal. Adding 20% means multiplying by 1.2, so 150 becomes 180. Taking 20% off means multiplying by 0.8, which gives 120. Those two are not reversible: 150 plus 20% then minus 20% comes back to 144, not 150.
What is the difference between a percentage and a percentage point?
A percentage point is the gap between two percentages. A rate moving from 10% to 12% has risen by 2 percentage points, but that is a 20% increase in relative terms, because 2 is a fifth of 10. Financial and polling copy uses percentage points to keep the two apart.
Can I use negative numbers and percentages above 100?
Yes. Above 100% simply means more than the whole, so 125% of 400 is 500. Negatives work as typed: −15% of 200 is −30. Only two sums have no answer — asking what percentage a number is of zero, and asking what number something is 0% of.
Why does the calculator say "Not defined"?
Because the sum would divide by zero. Nothing can be expressed as a percentage of zero, and every number is 0% of every other number, so neither question has a single answer. Change the second number and the result reappears.
Last updated 25 October 2026.