The basic percentage formula on a calculator
To find a percentage of a number, multiply the number by the percentage, then divide by 100. On a calculator, this means: enter the number, press multiply (×), enter the percentage, press divide (÷), enter 100, then press equals (=).
For example, to find 20% of 150: type 150, press ×, type 20, press ÷, type 100, press =. Your answer is 30. This works on any basic calculator — phone, computer, or handheld.
The reason this works is that a percentage is always "out of 100". When you say 20%, you mean 20 out of every 100. So 20% of 150 is the same as asking "what is 20 out of 100, applied to 150?"
Key Takeaways
- The percentage formula is: (number × percentage) ÷ 100, entered in that exact order on your calculator.
- A percentage button (%) on your calculator skips the division step — just enter the number, press ×, enter the percentage, then press the % button instead of ÷ 100.
- To find what percentage one number is of another, divide the first number by the second, then multiply by 100.
- Reverse percentages (finding the original number when you know the percentage) require division instead of multiplication.
Using the percentage button if your calculator has one
Many calculators have a dedicated % button that does the division by 100 automatically. If yours does, the shortcut is faster: enter the number, press ×, enter the percentage, then press % instead of pressing ÷ and 100.
Using the same example: type 150, press ×, type 20, press %. The calculator shows 30 immediately. You do not need to press = after the % button — most calculators display the result right away.
Test whether your calculator has this feature by trying it on a number you know. If you get the right answer without typing ÷ 100, your calculator has the shortcut. If nothing happens or you get a wrong number, your calculator does not have a % button, and you should use the full formula instead.
Finding what percentage one number is of another
Sometimes you know two numbers and need to find the percentage. For example, you scored 18 out of 25 on a test and want to know the percentage. The formula is: (smaller number ÷ larger number) × 100.
On your calculator: type 18, press ÷, type 25, press =. You get 0.72. Now multiply by 100: press ×, type 100, press =. Your answer is 72%. This means you scored 72% on the test.
The reason you multiply by 100 at the end is because percentages are always expressed as a number out of 100. When you divide 18 by 25, you get a decimal (0.72). Multiplying by 100 converts that decimal into the percentage form (72%).
Calculating a discount or sale price
When an item is on sale, you often see a percentage off. To find the sale price, first calculate the discount amount, then subtract it from the original price. A shirt costs $40 and is 25% off: find 25% of 40, then subtract from 40.
Type 40, press ×, type 25, press ÷, type 100, press =. You get 10 — that is the discount amount. Now subtract: press −, type 10, press =. The sale price is $30.
Alternatively, if you know the discount percentage, you can find what percentage you actually pay. If something is 25% off, you pay 75% of the original price (100% − 25% = 75%). Type 40, press ×, type 75, press ÷, type 100, press =. You get $30 directly, skipping the subtraction step.
Working backwards from a percentage to find the original number
Sometimes you know the percentage and the result, but need to find the original number. For example, you know that 15% of some number equals 45, and you want to find that number. The formula is: (result ÷ percentage) × 100.
Type 45, press ÷, type 15, press =. You get 3. Now multiply by 100: press ×, type 100, press =. The original number is 300. To check: 15% of 300 is 45, which is correct.
This is the reverse of the basic percentage formula. Instead of multiplying by the percentage and dividing by 100, you divide by the percentage and multiply by 100. The order matters — if you reverse it, you will get the wrong answer.
Common mistakes and how to avoid them
The most common mistake is forgetting to divide by 100 at the end of the basic formula. If you type 150 × 20 and press =, you get 3000, which is wrong. You must include the ÷ 100 step, or use the % button if your calculator has one.
Another mistake is using the wrong number as the divisor when finding what percentage one number is of another. Remember: the smaller number goes first (on top), and the larger number goes second (on the bottom). If you reverse them, your percentage will be backwards.
A third mistake is pressing = at the wrong time. On calculators with a % button, pressing = after the % button can sometimes change the result. Test your specific calculator first on a number you know, so you understand whether to press = or not.
Percentages on phone and computer calculators
The iPhone calculator app has a % button in the standard view. Android calculators vary by manufacturer, but most include one. On Windows, open the Calculator app, make sure you are in Standard view (not Programmer), and you will see a % button. On Mac, the Calculator app also has a % button.
The behavior is the same as a handheld calculator: enter the number, press ×, enter the percentage, press %, and the result appears. If you are unsure whether your phone or computer calculator has a % button, look for the % symbol on the screen.
Web-based calculators (like Google's built-in calculator) work the same way. Type your calculation into Google's search box and it will show you the result. The formula is identical: number × percentage ÷ 100, or number × percentage % if the calculator has a % button.
Frequently Asked Questions
Why do I divide by 100 instead of just typing the percentage?
Because a percentage is defined as a number out of 100. When you say 20%, you mean 20 out of every 100. Dividing by 100 converts the percentage into the decimal form the calculator needs to multiply correctly. Without this step, you would multiply by the full number (20) instead of the fraction (0.20), giving you a result 100 times too large.
Does the order matter when I multiply and divide?
For the basic percentage formula, no — you can type number × percentage ÷ 100 or number ÷ 100 × percentage and get the same answer. However, when finding what percentage one number is of another, order matters completely. The smaller number must be divided by the larger number, not the other way around.
What if my calculator does not have a % button?
Use the full formula: number × percentage ÷ 100. This works on every calculator, including very old or very basic ones. It takes one extra step, but the result is identical to using a % button. Write down the formula on a sticky note if you need to remember it.
Can I calculate percentages on a calculator without knowing the formula?
Not reliably. You need to understand what you are calculating — whether you are finding a percentage of a number, finding what percentage one number is of another, or working backwards from a percentage. Once you know which type of problem you have, the formula is straightforward, but guessing will give you wrong answers.