How to calculate percentage
To calculate a percentage, divide the part by the whole and multiply by 100: 30 out of 150 is 30 ÷ 150 × 100 = 20%. To find a percentage of a number, divide the percentage by 100 and multiply by the number: 20% of 150 is 0.2 × 150 = 30.
Short on time? The percentage calculator does it for you and shows every step.
What a percentage is
Per cent means “per hundred”. 20% is 20 out of every 100: the fraction 20/100, which simplifies to 1/5, and the decimal 0.2. Every percentage problem involves three numbers: a part, a whole and a rate. You always know two of them and look for the third.
The three basic questions
| You want | Formula | Example |
|---|---|---|
| The part: what is 20% of 150? | rate ÷ 100 × whole | 0.2 × 150 = 30 |
| The rate: 30 is what % of 150? | part ÷ whole × 100 | 30 ÷ 150 × 100 = 20% |
| The whole: 30 is 20% of what? | part ÷ (rate ÷ 100) | 30 ÷ 0.2 = 150 |
Method 1: turn the percentage into a decimal
Divide the percentage by 100, which moves the decimal point two places to the left, then multiply. 35% becomes 0.35, so 35% of 60 is 0.35 × 60 = 21. This method works for every case, including awkward ones like 12.5% or 0.4%.
Method 2: use a fraction you know
Some percentages are simple fractions, which makes them easy to do in your head:
- 50% = 1/2, so 50% of 84 is 42.
- 25% = 1/4, so 25% of 80 is 20.
- 20% = 1/5, so 20% of 45 is 9.
- 12.5% = 1/8, so 12.5% of 64 is 8.
- 33⅓% = 1/3, so a third of 90 is 30.
Method 3: build it from 10% and 1%
10% of a number moves its decimal point one place left, and 1% moves it two places. Combine them: 35% of 60 is 30% (3 × 6 = 18) plus 5% (half of 6 = 3), which gives 21. 17% of 200 is 10% (20) + 5% (10) + 2% (4) = 34.
Try it: type any percentage and number.
Percentage change
To see how much something grew or shrank, subtract the old value from the new one, divide by the old value and multiply by 100. From 40 to 50 is (50 − 40) ÷ 40 × 100 = 25% more. From 50 to 40 is −20%: the base changed, so the percentage is different. Use the percentage change calculator to check.
A mental math shortcut
x% of y is always the same as y% of x. 8% of 50 sounds hard, but 50% of 8 is 4, and so is 8% of 50. Pick whichever version is easier.
Percent vs. percentage points
When a percentage changes, say whether you mean the gap or the relative change. An interest rate moving from 5% to 7% went up 2 percentage points, which is a 40% increase in the rate.
Common mistakes
- Dividing by the wrong whole. The whole is the amount the percentage is “of”.
- Adding percentages in a row. +10% and then +10% is +21%, because the second 10% applies to a bigger number.
- Undoing a percentage by subtracting it. After +20%, divide by 1.2 to go back; the reverse percentage calculator does it for you.
How we calculate: exact decimal math (no rounding errors), you choose the decimal places, and your numbers never leave your browser. Methodology →
Updated October 2026