"What is 20% of 150?" It is one of the most common math questions we all run into. You see it when you shop, when you work out a tip, when you check a tax bill or help with homework.

The good news is that it always works the same way. Once you know the trick, you can do it in a few seconds. And if you'd rather skip the math, you can use our percentage calculator and type in the two numbers.

The simple method

To find X% of Y:

1. Divide X by 100 to turn it into a decimal.

2. Multiply that decimal by Y.

X% of Y = (X ÷ 100) × Y

Example 1: 20% of 150

20 ÷ 100 = 0.20 0.20 × 150 = 30

Example 2: 7.5% of 240

7.5 ÷ 100 = 0.075 0.075 × 240 = 18

Example 3: 35% of 90

35 ÷ 100 = 0.35 0.35 × 90 = 31.5

Another way to think about it

You can also multiply first and divide after. For 12% of 450:

12 × 450 = 5,400 5,400 ÷ 100 = 54

Both ways give the same answer. Pick whichever feels easier.

Do it in your head

You don't always need a calculator. Break the percentage into easy pieces.

• 10% of a number: move the decimal one place to the left.

• 5%: half of the 10%.

• 1%: move the decimal two places to the left.

• 25%: divide by 4.

• 50%: divide by 2.

Let's try 15% of 60. First, 10% of 60 is 6. Half of that, 5%, is 3. Add them: 6 + 3 = 9.

Another one: 30% of 80. 10% is 8, and 30% is three times that, so 24.

Where you will use this

• Sales tax. Sales tax of 8% on a $40 item is 0.08 × 40 = $3.20.

• Tips. A 20% tip on a $55 meal is 0.20 × 55 = $11.

• Test scores. If a test is worth 40% of your grade and you want to know how many points that is out of 200, it is 0.40 × 200 = 80.

• Savings. If you want to save 15% of a $3,000 paycheck, you put away $450.

Mistakes to avoid

• Forgetting to divide by 100. If you multiply 20 × 150 you get 3,000, which is clearly too big. Always turn the percent into a decimal first.

• Mixing up the two numbers. In "20% of 150", 20 is the percent and 150 is the whole.

• Using the wrong tool. If you need the reverse, like "30 is what percent of 150?", divide 30 by 150 and multiply by 100. That gives 20%.

How to use the calculator

1. Open the Percentage calculator.

2. Choose the "what is X% of Y" option.

3. Type your percent and your number.

4. Read the answer.

Common questions

What is 1% of a number? Divide the number by 100. So 1% of 4,500 is 45.

Is X% of Y the same as Y% of X? Yes, it is. 8% of 50 is 4, and 50% of 8 is also 4. This trick can make some problems easier. For example, 4% of 75 is the same as 75% of 4, which is 3.

Can I find a percentage of a decimal? Yes. The method is the same. 10% of 2.5 is 0.25.

Explore more

For more everyday tools, visit the Advanced Calculator home page. If you like numbers, our math calculators page has plenty more.