The basic percentage formula works the same way on every calculator
To find a percentage of a number, multiply the number by the percentage, then divide by 100. On a calculator, this looks like: the number you're finding the percentage of, times the percentage amount, divided by 100, then press equals.
For example, to find 20% of 150: type 150, press the multiplication button (×), type 20, press the division button (÷), type 100, then press equals. The answer is 30.
This method works on a basic four-function calculator, a scientific calculator, a phone calculator app, or a computer calculator. The buttons are always in the same place, and the order never changes.
Key Takeaways
- Multiply the number by the percentage, then divide by 100 — this is the formula that works on every calculator type.
- A percentage button (%) on some calculators does the division by 100 automatically, so you only need to multiply and press %.
- 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 before a percentage was taken away) require dividing by the percentage, then multiplying by 100.
Using the percentage button if your calculator has one
Many calculators have a dedicated percentage button marked with the % symbol. This button does the division by 100 for you automatically. If your calculator has this button, the shortcut is: the number, times the percentage, then press the % button.
Using the same example: type 150, press ×, type 20, then press %. The calculator shows 30 without needing the extra division step. This is faster, but only works if the % button is present — basic calculators often do not have one.
Test your calculator's % button with a simple example first. Type 100, press ×, type 50, then press %. If it shows 50, the button works as expected. If it shows something else, your calculator may use a different method, and you should stick with the multiply-and-divide-by-100 approach.
Finding what percentage one number is of another
Sometimes you need to know what percentage one number represents of a larger number. For example, if you scored 45 points out of 60 on a test, what percentage is that? The formula is: divide the smaller number by the larger number, then multiply by 100.
On your calculator: type 45, press the division button (÷), type 60, press equals, then multiply by 100. The answer is 75, meaning 45 is 75% of 60.
This calculation is useful for test scores, discounts, and comparing parts to a whole. The order matters — if you divide the larger number by the smaller one, you will get a number larger than 100, which is not what you want.
Calculating a percentage increase or decrease
When a price goes up or down by a percentage, or a quantity grows or shrinks, you need to find the new amount after the change. The method depends on whether you are adding or subtracting the percentage.
For a percentage increase: multiply the original number by the percentage, divide by 100, then add the result to the original number. If a $50 item increases by 20%, type 50, ×, 20, ÷, 100, then note the answer (10). Then type 50 + 10 and press equals. The new price is $60.
For a percentage decrease: multiply the original number by the percentage, divide by 100, then subtract the result from the original number. If a $50 item decreases by 20%, find 20% of 50 (which is 10), then type 50 − 10 and press equals. The new price is $40.
Working backwards from a percentage to find the original number
Sometimes you know the final amount after a percentage was applied, but you need to find what the original number was. This is called a reverse percentage. For example, if an item costs $60 after a 20% discount, what was the original price?
The formula is: divide the final amount by the percentage (as a decimal), then multiply by 100. First, subtract the discount percentage from 100 to find what percentage of the original price remains. In this case, 100 − 20 = 80, so the final price is 80% of the original.
On your calculator: type 60, press ÷, type 0.8 (which is 80% as a decimal), then press equals. The answer is 75, meaning the original price was $75. To convert a percentage to a decimal, divide by 100 — so 80% becomes 0.8, and 25% becomes 0.25.
Avoiding common mistakes with percentages
The most common error is forgetting to divide by 100 when using the basic formula. If you multiply 150 by 20 and forget the division step, you get 3000, which is clearly wrong. Always include the ÷ 100 part, or use the % button if your calculator has one.
Another mistake is mixing up the order when finding what percentage one number is of another. Remember: the smaller number goes first (in the numerator), and the larger number goes second (in the denominator). Dividing 60 by 45 gives you a different answer than dividing 45 by 60.
When working with percentage increases or decreases, write down the intermediate answer before adding or subtracting. This prevents errors from happening in your head and gives you a chance to double-check the math before moving to the final step.
Checking your answer to make sure it is reasonable
After you calculate a percentage, pause and ask whether the answer makes sense. If you are finding 20% of 150, the answer should be smaller than 150 — in this case, 30 is correct. If your answer is larger than the original number, something went wrong.
If you are finding what percentage one number is of another, the answer should usually be between 0 and 100 (unless the first number is larger than the second). If you get 150 when you expected something like 75, you likely divided in the wrong order.
For percentage increases and decreases, the new amount should move in the direction you expect. A 20% increase should make the number bigger, and a 20% decrease should make it smaller. If your result goes the wrong direction, you likely added when you should have subtracted, or vice versa.
Frequently Asked Questions
What is the difference between the % button and doing the math manually?
The % button automatically divides by 100, so you skip one step. Both methods give the same answer. The % button is faster if your calculator has one, but the manual method (multiply, then divide by 100) works on every calculator and helps you understand what is happening.
Can I use a phone calculator app the same way?
Yes. Most phone calculator apps work exactly like physical calculators — type the number, press the operation button, type the next number, and press equals. Some phone apps have a % button, and some do not. The multiply-and-divide-by-100 method works on all of them.
How do I find a percentage of a percentage?
Multiply the first percentage by the second percentage, then divide by 100. For example, to find 50% of 20%, type 50, ×, 20, ÷, 100, and press equals. The answer is 10, meaning 50% of 20% is 10%.
What if the calculator shows a very long decimal?
Round to the number of decimal places that makes sense for what you are calculating. For money, round to two decimal places (cents). For test scores or survey results, round to the nearest whole number. The calculator is showing extra precision you do not need.
Is there a faster way to calculate common percentages like 10% or 50%?
Yes. 10% is always one-tenth of a number, so you can divide by 10. 50% is always half, so you can divide by 2. 25% is one-quarter, so divide by 4. These shortcuts are faster than the full formula, but the multiply-and-divide-by-100 method always works if you forget the shortcut.