Which percentage calculation do you need?
| Your question | Use | Formula |
|---|---|---|
| What is 15% of 240? | % of a number | 15 ÷ 100 × 240 = 36 |
| 36 is what percent of 240? | X is what % | 36 ÷ 240 × 100 = 15% |
| Sales went from 80 to 100. By what %? | % change | (100 − 80) ÷ 80 × 100 = 25% |
| How different are 120 and 150? | % difference | 30 ÷ 135 × 100 = 22.22% |
| Raise a 250 price by 12% | Increase by % | 250 × 1.12 = 280 |
| Cut a 250 budget by 12% | Decrease by % | 250 × 0.88 = 220 |
The percentage formula
Percentage = Part ÷ Whole × 100
Part = Percentage ÷ 100 × Whole
A percentage is a fraction out of 100. Dividing the part by the whole gives a decimal; multiplying by 100 turns it into a percentage. Every calculation on this page is a rearrangement of that one idea.
Percentage change formula
Percentage change = (New value − Original value) ÷ Original value × 100
Example: your online shop made 80 sales in May and 100 in June.
(100 − 80) ÷ 80 × 100 = 25% increase. If sales fell back from 100 to 80, that's (80 − 100) ÷ 100 × 100 = 20% decrease. The percentages differ because the starting value differs.
Percentage difference vs percentage change
Use percentage change when one value clearly comes first: an old price and a new price, last year and this year. Use percentage difference when you're comparing two things side by side, like quotes from two suppliers. Percentage difference divides by the average of the two values, so the answer is the same whichever order you enter them.
Percentage difference = |A − B| ÷ ((|A| + |B|) ÷ 2) × 100
Why +10% then −10% doesn't get you back
Raise 100 by 10% and you get 110. Cut 110 by 10% and you get 99, not 100, because the second 10% is taken from a bigger number. The same trap applies to discounts, price rises and investment returns. To undo a 10% increase you need a decrease of about 9.09% (10 ÷ 110).
Frequently asked questions
How do I calculate a percentage of a number in my head?
Find 10% by moving the decimal point one place left, then scale. For 15% of 240: 10% is 24, 5% is half that (12), so 15% is 36.
Can a percentage be more than 100%?
Yes. If a value more than doubles, the change is over 100%. Going from 50 to 150 is a 200% increase. A share of a whole, like "what percent of the class passed", can't exceed 100%.
What's the difference between percent and percentage points?
If a rate goes from 4% to 5%, it rose by one percentage point, which is a 25% increase in the rate itself. News reports often mix the two up.
Why can't the original value be zero?
Percentage change divides by the original value, and dividing by zero has no answer. Describe a change from zero as an amount instead, for example "from 0 to 40 sales".
Formulas checked 29 October 2026. This calculator is for planning and education, not financial, tax or legal advice.