The basic method: multiply, then divide by 100

To find the percentage of a number on a calculator, multiply the number by the percentage, then divide by 100. For example, to find 20% of 150: enter 150, press multiply (×), enter 20, press divide (÷), enter 100, then press equals (=). The answer is 30.

This works on any calculator — basic, scientific, or the calculator app on your phone. The order matters: always multiply first, then divide by 100. If you divide first, you will get the wrong answer.

A second way to think about it: convert the percentage to a decimal first, then multiply. Twenty percent becomes 0.20. So 20% of 150 is 150 × 0.20 = 30. Both methods give the same result. Use whichever feels more natural to you.

Key Takeaways

  • Multiply the number by the percentage, then divide by 100 — for example, 150 × 20 ÷ 100 = 30.
  • Alternatively, convert the percentage to a decimal (20% = 0.20) and multiply — 150 × 0.20 = 30.
  • The order of operations matters: multiply before you divide, or you will get the wrong answer.
  • This method works on any calculator, including phone calculator apps and basic four-function models.
  • Double-check by estimating: 20% of 150 should be roughly one-fifth of 150, which is about 30.

Working backwards: finding what percentage one number is of another

Sometimes you need to know what percentage one number represents of a larger number. For instance, if you scored 45 points out of 60 on a test, what percentage is that? Divide the smaller number by the larger number, then multiply by 100.

Enter 45, press divide (÷), enter 60, press multiply (×), enter 100, then press equals (=). The answer is 75. You scored 75% on the test.

This is the reverse of the first method. When you know the percentage and want the number, you multiply then divide by 100. When you know both numbers and want the percentage, you divide then multiply by 100.

Finding percentage increases and decreases

Percentage increases and decreases are common in real life — sale prices, salary raises, population changes. The method is slightly different because you are comparing the change to the original amount, not to 100.

To find a percentage increase: subtract the original number from the new number to get the change. Then divide the change by the original number and multiply by 100. For example, if a shirt originally cost $40 and now costs $50, the change is $10. Divide 10 by 40 to get 0.25, then multiply by 100 to get 25%. The price increased by 25%.

For a percentage decrease, the process is the same. If the shirt is on sale for $30 (down from $40), the change is $10. Divide 10 by 40 and multiply by 100 to get 25%. The price decreased by 25%.

Using the percentage button on scientific calculators

Many scientific calculators and phone calculator apps have a percentage button (%) that does some of this work for you. On most models, you can enter the number, press ×, enter the percentage, press %, and the calculator shows the result without needing to divide by 100.

For example, to find 20% of 150 on a calculator with a % button: enter 150, press ×, enter 20, press %. The display shows 30 when ready. This saves a step, but the result is the same.

However, the % button behaves differently on different calculators. On some models, pressing % after a division gives you a percentage of the first number rather than a true percentage. Test your calculator with a straightforward example you already know the answer to — like 10% of 100, which should be 10 — before relying on the % button for important calculations.

Checking your work by estimating

Always estimate before or after you calculate. This catches mistakes. If you are finding 20% of 150, you know that 20% is roughly one-fifth. One-fifth of 150 is 30. Your calculator answer should be close to that.

If you are finding 15% of 80, estimate: 15% is between 10% and 20%. Ten percent of 80 is 8, and 20% is 16. So 15% should be between 8 and 16. If your calculator shows 12, that is reasonable. If it shows 120, something went wrong.

Estimation takes five seconds and catches almost every entry error. It is the fastest way to know whether to trust your answer.

Common mistakes and how to avoid them

The most common mistake is entering the numbers in the wrong order. Remember: the number you are finding a percentage of comes first. If you want 20% of 150, enter 150 first, not 20. Reversing them gives you 150% of 20, which is 30 — the same answer in this case, but only by coincidence.

Another mistake is forgetting to divide by 100 when you multiply. If you enter 150 × 20 and stop there, you get 3000, which is wrong. You must divide by 100 to get 30.

A third mistake is pressing the wrong operation button. If you mean to press divide (÷) but press multiply (×) instead, the answer will be completely wrong. Slow down and look at the button before you press it. Most calculators show what you entered on the display, so you can catch the error before pressing equals.

Frequently Asked Questions

What if my calculator does not have a divide button?

Basic calculators always have a divide button, usually marked ÷ or /. If you are using a phone calculator app and cannot find it, try rotating your phone to landscape mode — many phone apps hide buttons in portrait mode. If you are using a very old or unusual calculator, check the manual or look for a button marked "÷" or a forward slash.

Can I find a percentage on a calculator without dividing by 100?

Yes, if your calculator has a percentage button (%). Enter the number, press ×, enter the percentage, then press %. However, this button works differently on different calculators, so test it first with an example you know the answer to. The multiply-then-divide method works the same way on every calculator.

How do I find what percentage a number is of a total?

Divide the smaller number by the larger number, then multiply by 100. For example, 45 out of 60 is 45 ÷ 60 × 100 = 75%. This is the reverse of finding a percentage of a number.

What is the difference between percentage increase and percentage change?

They are the same thing. Percentage change, percentage increase, and percentage decrease all use the same method: subtract the original from the new, divide by the original, and multiply by 100. The word "increase" or "decrease" just tells you whether the number went up or down.

Why do I divide by 100 instead of just moving the decimal point?

Moving the decimal point two places to the left is the same as dividing by 100. If you multiply 150 × 20 to get 3000, moving the decimal two places left gives 30. Both methods work. Use whichever is faster for you — some people find decimal movement easier to visualize, others prefer the divide button.