How to Calculate a Percentage

Every “what is X% of Y” question uses the same two steps: turn the percent into a decimal, then multiply. This guide shows why that works, walks through examples you can open in the calculator, and covers the shortcuts and slips worth knowing.

What “percent” means

Per cent is Latin for “per hundred.” A percentage is just a fraction with 100 on the bottom, written with a % sign instead. So 20% is another way of writing 20 / 100, which is the decimal 0.2.

That is the whole idea. “20% of something” means 0.2 of it — a fifth. To go from a percent to a decimal, divide by 100 (move the point two places left); to go back, multiply by 100.

The one formula

To take a percent of a number, convert the percent to a decimal and multiply by the whole:

part = (percent / 100) × whole

For 20% of 150: 20 / 100 = 0.2, then 0.2 × 150 = 30. Done. There is no separate method for “harder” percentages — 37% of 84 is the same two steps, 0.37 × 84 = 31.08.

Worked examples

Each of these opens the percentage calculator with the numbers already entered, so you can see the working:

What is 20% of 150?0.2 × 150 = 30Open in the calculator →What is 15% of 80?0.15 × 80 = 12Open in the calculator →What is 8% of 250?0.08 × 250 = 20Open in the calculator →

The three questions percentages answer

The same relationship — part = (percent / 100) × whole — answers three different questions depending on which piece is missing. Rearrange for the unknown:

What is 20% of 150? — you have the percent and the whole. 0.2 × 150 = 30.
30 is what percent of 150? — you have both amounts. Divide and multiply by 100: 30 / 150 × 100 = 20%. Open this.
30 is 20% of what number? — you have the part and the percent. Divide by the decimal: 30 / 0.2 = 150. Open this.

Mental-math shortcuts

10% — move the decimal point one place left. 10% of 46 is 4.6.
1% — move it two places left. 1% of 46 is 0.46.
5% is half of 10%. 20% is double it. 15% is 10% + 5% — the usual way to work out a tip.
Percentages flip. x% of y always equals y% of x. 8% of 250 looks awkward; 250% of 8 is obviously 20. Same answer.

Common mistakes

Forgetting to divide by 100. Multiplying by 20 instead of 0.2 makes the answer 100× too big.
Mixing up “20% of” with “20% more than.” 20% of 150 is 30. A number that is 20% more than 150 is 180. Working backwards from a total that already includes the increase is a reverse percentage.
Confusing a percentage of a number with a percentage change. “20% of 150” is a slice of one number; going “from 150 to 180” is a percentage change.
Adding percentages of different wholes. 10% of your rent plus 10% of your grocery bill is not 10% of anything useful — the two 10%s are measured against different amounts.

Percentage reference

Common percentages of a few round amounts:

Read a row across: 20% of 250 is 50.
Percent	of 20	of 50	of 100	of 250	of 1,000
1%	0.2	0.5	1	2.5	10
5%	1	2.5	5	12.5	50
10%	2	5	10	25	100
15%	3	7.5	15	37.5	150
20%	4	10	20	50	200
25%	5	12.5	25	62.5	250
50%	10	25	50	125	500

Frequently asked questions

What is the fastest way to calculate a percentage in your head?

Start from 10% by moving the decimal point one place left, then scale. For 15% of 80: 10% is 8, 5% is 4, so 15% is 12.

How do I turn a fraction into a percentage?

Divide the top number by the bottom number, then multiply by 100. 30 out of 150 is 30 / 150 = 0.2, which is 20%.

Is 8% of 250 the same as 250% of 8?

Yes. x% of y always equals y% of x, because both are x times y divided by 100. Flipping the numbers can make the arithmetic easier: 250% of 8 is clearly 20.

Can a percentage be more than 100%?

Yes. A percentage over 100% just means the part is larger than the whole it is compared against. 300 is 200% of 150.