How to Calculate a Percentage of a Number

To calculate a percentage of a number, change the percent to a decimal and multiply. For example, 15 percent of 240 is 240 × 0.15 = 36. Divide the percent by 100 to get the decimal, such as 15 ÷ 100 = 0.15. This method works for discounts, tips, taxes, grades and any share of a total. This guide shows the three main types of percentage problem, with worked examples and a table you can check.

The basic formula

Use this formula to find a part of a whole:

Result = Number × (Percent ÷ 100)

The percent sign means "out of 100." So 15% means 15 out of every 100, or 0.15 of the number. The number you start with is the base. The result is the part that the percent gives you.

Three types of problem use this idea:

1. What is X percent of Y? Find the part. Example: 15 percent of 240.
2. What percent is X of Y? Find the percent. Example: 45 is what percent of 180?
3. X is P percent of what number? Find the whole. Example: 30 is 15 percent of what number?

Each type uses the same numbers in a different order. Learn the formula for each type, and the checks become easy.

Type 1: find a part

Find 15 percent of 240.

1. Change the percent to a decimal: 15 ÷ 100 = 0.15.
2. Multiply: 240 × 0.15 = 36.

The answer is 36. Check it: 10 percent of 240 is 24, and 5 percent is half of that, which is 12. Add them: 24 + 12 = 36. The two checks agree.

Another example: a jacket costs 80, and the sales tax is 8 percent. The tax is 80 × 0.08 = 6.40. The total is 86.40.

Type 2: find the percent

Find what percent 45 is of 180.

1. Divide the part by the whole: 45 ÷ 180 = 0.25.
2. Multiply by 100: 0.25 × 100 = 25.

The answer is 25 percent. Check it: 25 percent of 180 is 180 × 0.25 = 45. The check matches the start.

Type 3: find the whole

Thirty is 15 percent of what number?

1. Change the percent to a decimal: 15 percent is 0.15.
2. Divide the part by the decimal: 30 ÷ 0.15 = 200.

The answer is 200. Check it: 15 percent of 200 is 30. Do not multiply 30 by 0.15, because that gives 4.5, which is the wrong type of problem.

Worked table

The table below shows the three types with real numbers. Check each row with the formula.

Check the last row: 350 × 0.12 = 42. Ten percent of 350 is 35, and 2 percent is 7. Add them: 35 + 7 = 42.

Mental math tricks

You can find many percentages in your head.

• 10 percent: move the decimal point one place to the left. 10% of 370 is 37.
• 5 percent: find 10 percent, then halve it. 5% of 370 is 18.5.
• 1 percent: move the decimal point two places to the left. 1% of 370 is 3.7.
• 15 percent: find 10 percent, then add half of it. 10% of 370 is 37, and half of 37 is 18.5. Add them: 37 + 18.5 = 55.5.
• 20 percent: find 10 percent and double it. 20% of 370 is 74.

Use the trick that fits the number. For a quick check, round the base first. For example, 19 percent of 500 is close to 20 percent of 500, which is 100. The exact answer is 95.

Common mistakes

• Dividing by the wrong number. When you find a part, multiply by the decimal. When you find the whole, divide by the decimal.
• Forgetting to change the percent. Use 0.15, not 15, in the multiplication.
• Mixing up the base. The percent applies to the number after "of". Check which number is the whole.
• Rounding too early. Keep the full decimal until the end, then round the answer.

How Percent & Finance Calc Pro helps

Percent & Finance Calc Pro is a percentage calculator app for iPhone. The basic percentage calculators are free. These include what is X percent of a number, what percent one number is of another, and how to find the whole from a part and its percentage. Each result shows the formula with lettered inputs, so you can see how the answer was reached.

Results update as you type, and you can copy or share any result in one tap. The app also keeps your most used calculations at the top, and you can star a calculation to pin it. The tip and bill split tools are in the Business group, which needs the Pro plan. For the full list, see the app page.

For a tip calculation in more detail, read how to calculate a tip and split the bill. For change over time, read how to calculate percentage increase.

If you want to check a percentage in your own numbers, open Percent & Finance Calc Pro. You can find more tools on the apps page.

Frequently asked questions

How do I calculate 20 percent of a number in my head?

Find 10 percent by moving the decimal point one place to the left, then double it. For 20 percent of 370, 10 percent is 37, and double that is 74.

What is the difference between percent of a number and percent increase?

A percent of a number gives a part of the number. A percent increase compares a new value with an old value. Use percent of a number for a discount or a tip. Use percent increase for a price change.

Why does my answer look too big when I find the whole?

You probably multiplied by the percent when you should have divided. To find the whole from a part, divide the part by the decimal percent.

Does the order of the numbers matter?

Yes. "15 percent of 240" and "240 percent of 15" give different answers. The number after "of" is the base, so check it before you calculate.