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 15% of 200: type 200, press ×, type 15, press ÷, type 100, press =. The answer is 30. This works the same way on a phone calculator, a basic four-function calculator, or a scientific calculator.
The reason this works is that "percent" literally means "per hundred." When you divide by 100 at the end, you are converting the percentage into its decimal form, then multiplying it by your original number.
Key Takeaways
- The percentage formula is: number × percentage ÷ 100, entered in that exact order on your calculator.
- Many calculators have a dedicated percentage button (%) that does the division by 100 automatically, so you can skip the ÷ 100 step.
- To find what percentage one number is of another, divide the first number by the second, then multiply by 100.
- Percentages are useful for calculating tips, discounts, tax amounts, and comparing parts of a whole.
Using the percentage button if your calculator has one
Many calculators—including most phone calculators and dedicated devices—have a % button that shortcuts the division by 100. If your calculator has this button, you can skip the last two steps.
To use it: enter the number, press multiply, enter the percentage, then press the % button instead of dividing by 100. For the same example (15% of 200), you would type 200, press ×, type 15, press %, then press =. The answer is still 30.
The percentage button is a convenience feature—it does not change the math, only saves you two keystrokes. If you are not sure whether your calculator has one, look for a button labeled with the % symbol. On a phone, it is usually visible on the standard calculator app without needing to switch to scientific mode.
Finding what percentage one number is of another
Sometimes you need to work backwards: you know two numbers and want to know what percentage the first is of the second. This is common when comparing test scores, calculating what portion of a budget went to one category, or figuring out how much of a goal you have reached.
The formula is: (first number ÷ second number) × 100. On a calculator, enter the first number, press divide, enter the second number, press multiply, enter 100, then press equals. For example, if you scored 18 out of 25 on a test, type 18, press ÷, type 25, press ×, type 100, press =. The answer is 72, meaning you scored 72%.
If your calculator has a percentage button, you can also enter the first number, press divide, enter the second number, then press %. This gives you the decimal form (0.72 in the test example), which you can then multiply by 100 if you want the percentage as a whole number.
Calculating discounts and sale prices
When an item is on sale, you often see a percentage off—for example, 20% off a $50 shirt. To find the discount amount, use the basic percentage formula: multiply the original price by the discount percentage, then divide by 100.
For a $50 shirt with 20% off: type 50, press ×, type 20, press ÷, type 100, press =. The discount is $10. To find the final price you pay, subtract this from the original: $50 − $10 = $40.
Alternatively, if you want the final price in one calculation, you can find what percentage you are paying (100% minus the discount percentage) and multiply by that. For 20% off, you pay 80% of the original price. Type 50, press ×, type 80, press ÷, type 100, press =. This gives $40 directly, which is faster if you only care about what you actually pay.
Calculating tips and tax
Tipping and tax both use the percentage formula. For a $35 meal with an 18% tip, type 35, press ×, type 18, press ÷, type 100, press =. The tip is $6.30. Add this to the original amount to get your total: $35 + $6.30 = $41.30.
The same method works for sales tax. If an item costs $25 and your local tax rate is 7%, the tax amount is found by typing 25, press ×, type 7, press ÷, type 100, press =. The tax is $1.75, so the total is $25 + $1.75 = $26.75.
Many people calculate the total price in one step instead: multiply by (100 + the tax or tip percentage), then divide by 100. For a $25 item with 7% tax, type 25, press ×, type 107, press ÷, type 100, press =. This gives $26.75 directly.
Common mistakes and how to avoid them
The most common error is forgetting to divide by 100 at the end. If you type 200, press ×, type 15, and press = without the division step, you get 3000, which is wrong. Always include the ÷ 100 unless your calculator has a percentage button that does it for you.
Another mistake is entering the numbers in the wrong order. The formula is always (number × percentage ÷ 100), not (percentage × number ÷ 100). Multiplication is commutative—meaning the order does not matter mathematically—but entering them in the standard order makes it easier to catch mistakes and easier to explain your work to someone else.
A third error happens when calculating backwards (finding what percentage one number is of another). People sometimes divide the second number by the first instead of the first by the second. Remember: the number you are measuring goes first, and the total goes second. If you scored 18 out of 25, the 18 goes first.
Frequently Asked Questions
What if my calculator does not have a percentage button?
You can still calculate percentages using the basic formula: number × percentage ÷ 100. The percentage button is only a shortcut that skips the division by 100. Without it, you straightforward type all the steps. Any calculator that can multiply and divide can calculate percentages.
How do I calculate a percentage increase or decrease?
Find the difference between the new number and the old number, divide by the old number, then multiply by 100. For example, if a price went from $20 to $25, the difference is $5. Type 5, press ÷, type 20, press ×, type 100, press =. The price increased by 25%.
Can I use a percentage calculator on my phone instead of doing it manually?
Yes. Most phones have a built-in calculator app that works the same way as a physical calculator. Some phones also have dedicated percentage calculator apps in their app store, though the standard calculator is usually sufficient for basic percentage work.
What does the percentage button actually do?
The percentage button automatically divides by 100, so you do not have to type it yourself. It is purely a convenience feature that saves keystrokes. The math is identical whether you use the button or type ÷ 100 manually.