The basic method: multiply, then divide by 100
To find a percentage of any number on a calculator, multiply the number by the percentage, then divide by 100. For example, to find 20% of 150, you would enter: 150 × 20 ÷ 100, which gives you 30.
The order matters slightly depending on your calculator. On most standard calculators, you can enter the numbers left to right as shown above. On some calculators, you may need to enter the percentage first, then the number. Test with a simple example like 10% of 100 (which should equal 10) to see how your specific calculator works.
This method works for any percentage and any starting number. Whether you're calculating a tip, a discount, a tax amount, or a grade, the formula stays the same: the number you're finding a percentage of, times the percentage, divided by 100.
Key Takeaways
- Multiply the number by the percentage, then divide by 100 to find what that percentage equals.
- A percentage button (%) on your calculator can do this in one step — just enter the number, press %, then enter the percentage.
- 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 for faster calculation
Most modern calculators have a dedicated percentage button marked with the % symbol. This button does the division by 100 automatically, which saves a step. To use it, enter your starting number, press ×, enter the percentage you want, then press the % button instead of =.
For example, to find 15% of 80: enter 80, press ×, enter 15, press %. The calculator shows 12 immediately. This is faster than the multiply-and-divide method because you skip the final division step.
Not all calculators have a percentage button, and some work differently. If your calculator has one, try it with a number you already know the answer to — like 50% of 100, which should be 50 — to confirm how it works on your specific model.
Finding what percentage one number is of another
Sometimes you need to work backwards: you know two numbers and want to find what percentage the first is of the second. For this, divide the first number by the second, then multiply by 100. For example, if you scored 18 out of 25 on a test, you would enter: 18 ÷ 25 × 100, which gives you 72.
This calculation tells you what portion of the total your number represents, expressed as a percentage. It works for any comparison: what percentage of your budget went to groceries, what percentage of employees work part-time, or what percentage of a recipe's ingredients is flour.
Calculating percentage increases and decreases
When something changes from one amount to another, you can find the percentage change by subtracting the original from the new amount, dividing by the original, then multiplying by 100. For example, if a price went from $40 to $50, you would enter: (50 − 40) ÷ 40 × 100, which gives you 25% increase.
For a decrease, the same formula works. If a price dropped from $50 to $40, you enter: (40 − 50) ÷ 50 × 100, which gives you −20%, or a 20% decrease. The negative sign tells you it went down rather than up.
These calculations are useful for tracking price changes, salary increases, weight loss or gain, or any situation where you want to know the percentage difference between a starting point and an ending point.
Working backwards from a percentage to find the original number
If you know a percentage of a number but not the original number itself, you can find it by dividing the percentage amount by the percentage, then multiplying by 100. For example, if you know that 30% of something equals 45, you would enter: 45 ÷ 30 × 100, which gives you 150.
This type of calculation comes up when you see a discount amount but not the original price, or when you know how much tax you paid but not the pre-tax total. The process reverses the original percentage calculation: instead of multiplying by the percentage, you divide by it.
Common percentage calculations you'll encounter
Calculating tips on a bill is one of the most frequent uses. A 15% tip on a $32 bill: 32 × 15 ÷ 100 = $4.80. A 20% tip: 32 × 20 ÷ 100 = $6.40. Many calculators have a tip button that does this automatically, but the manual method works on any calculator.
Sales tax and discounts use the same method. A 10% discount on a $75 item: 75 × 10 ÷ 100 = $7.50 off, so you pay $67.50. Sales tax of 8% on a $100 purchase: 100 × 8 ÷ 100 = $8 tax, so the total is $108. Grade percentages work the same way: if you got 92 out of 100 points, that's 92%, and if you got 23 out of 25, that's 92% as well.
Frequently Asked Questions
What's the difference between the % button and doing ÷ 100?
The % button does the division by 100 automatically. Using 150 × 20 % gives the same result as 150 × 20 ÷ 100, but the % button saves you one step. Both methods are correct; the % button is just faster if your calculator has one.
Why do I divide by 100 instead of just using the percentage number?
A percentage is always out of 100. When you say 20%, you mean 20 out of 100, or 20/100. To convert any percentage to its decimal form, you divide by 100. That's why 20% of 150 is 150 × 20 ÷ 100, not 150 × 20.
Can I calculate percentages on my phone's calculator?
Yes. Most phone calculators have a % button that works the same way as a physical calculator. On some phones, you may need to rotate to landscape mode to see the % button. Test it with a simple calculation like 50% of 100 to confirm it works on your phone.
How do I calculate a percentage of a percentage?
Multiply the two percentages together, then divide by 100. For example, 20% of 50% is: 20 × 50 ÷ 100 = 10%. This comes up rarely in everyday math but is useful for compound discounts or nested calculations.
What if my calculator doesn't have a % button?
You can still calculate percentages using the multiply-and-divide method: multiply the number by the percentage, then divide by 100. This works on any calculator, even basic ones without a % button. It just takes one extra step.