Everyday Math guide

How do I calculate a percentage?

To find X% of a number, multiply the number by X/100. For example, 20% of 150 = 150 × 0.20 = 30. A percentage is simply a fraction out of 100, so "20%" means 20 per 100. That single multiply-by-a-decimal move handles most everyday percentage questions, from tips to discounts to test scores.

Run your own numbers with the Percentage CalculatorOpen →

What a percentage actually is

A percentage is a fraction with 100 on the bottom. The word comes from "per cent," meaning per hundred, so 20% is just another way of writing 20/100, or the decimal 0.20. Everything else about percentages follows from that one idea.

Because a percent is a fraction out of 100, converting between the two directions is quick: divide a percent by 100 to get a decimal (75% becomes 0.75), or multiply a decimal by 100 to get a percent (0.4 becomes 40%). Once a percentage is a decimal, you can multiply and divide with it like any other number.

Finding X% of a number

This is the most common question, and the rule is: multiply the number by X/100. To find 20% of 150, turn 20% into the decimal 0.20 and multiply: 150 × 0.20 = 30.

The same method scales to anything. A 15% tip on a $60 bill is 60 × 0.15 = $9. A 30%-off discount on an $80 jacket takes off 80 × 0.30 = $24, leaving $56. If mental math feels shaky, running the same figures through the Percentage Calculator confirms the result in one step.

Finding what percentage one number is of another

When you already have two numbers and want to know how they relate, divide the part by the whole and multiply by 100. So 30 as a share of 150 is 30 ÷ 150 × 100 = 20%. That matches the first example in reverse: 30 is 20% of 150.

This is the move behind a test score (18 correct out of 24 is 18 ÷ 24 × 100 = 75%) or a completion rate. The order matters, though: it's always part divided by whole, not the other way around.

Working backward to the whole

Sometimes you know the part and its percentage but not the total. To reverse it, divide the part by the percentage-as-a-decimal: whole = part ÷ (percent/100).

If 30 represents 20% of some total, then the total is 30 ÷ 0.20 = 150. This is handy when a receipt shows a tax amount and its rate, or when a survey reports "45 people, which was 30% of respondents" and you want the full group: 45 ÷ 0.30 = 150 respondents.

Frequently asked questions

How do I find 20% of 150?

Multiply 150 by 0.20, which gives 30. The rule for finding X% of any number is to multiply that number by X/100, so 20% becomes the decimal 0.20.

How do I turn a percentage into a decimal?

Divide it by 100. 20% becomes 0.20, 75% becomes 0.75, and 8% becomes 0.08. To go the other way, multiply the decimal by 100: 0.4 is 40%.

How do I find what percentage one number is of another?

Divide the part by the whole and multiply by 100. For example, 30 out of 150 is 30 ÷ 150 × 100 = 20%. Always divide the part by the total, not the reverse.

How do I find the whole when I only know a part and its percentage?

Divide the part by the percentage written as a decimal. If 30 is 20% of a total, the total is 30 ÷ 0.20 = 150.

Sources: Math is Fun — Percentage; Khan Academy — Percentages.

Last reviewed July 4, 2026 · Editorial policy · This is general information, not financial advice.