Most calculators have a percent (%) button that handles the math for you, but the key sequence depends on what type of percentage calculation you’re doing. There are three common situations: finding a percentage of a number, adding or subtracting a percentage, and figuring out what percentage one number is of another. Here’s how to handle each one.
Finding a Percentage of a Number
This is the most common use. Say you want to find 20% of 80. On most calculators, you type:
- 80 × 20 %
The answer is 16. On some calculators, the order doesn’t matter, so 20 % × 80 also gives you 16. If your calculator doesn’t have a percent button, you can get the same result by dividing the percentage by 100 first to convert it to a decimal, then multiplying. So 80 × 0.20 = 16.
There’s another way to think about it that works on any calculator: divide the number by 100 to find 1%, then multiply by whatever percentage you need. For 20% of 80, that looks like 80 ÷ 100 = 0.8, then 0.8 × 20 = 16. This approach is especially handy when your calculator’s percent button behaves differently than you expect.
Adding a Percentage (Tips, Tax, Markups)
When you need to add a percentage on top of a number, like calculating a tip or sales tax, many calculators let you do it in a single line using the plus key and the percent button together. To add a 15% tip to a $20 bill:
- 20 + 15 %
The calculator returns 23. It figured out that 15% of 20 is 3, then added it to 20 for you. Without the percent button, you’d do the two-step version: multiply 20 × 0.15 to get 3, then add 3 to 20.
Subtracting a Percentage (Discounts, Deductions)
Subtracting a percentage works the same way, just with the minus key. To take a 20% discount off a $50 item:
- 50 – 20 %
The calculator returns 40. It calculated 20% of 50 (which is 10), then subtracted it. Without the percent button, multiply 50 × 0.20 to get 10, then subtract: 50 – 10 = 40.
Finding What Percentage One Number Is of Another
Sometimes you need to work backward. If you scored 35 out of 50 on a test and want to know your percentage, the formula is:
- Part ÷ Whole × 100
On your calculator, type 35 ÷ 50 × 100. The result is 70, meaning you scored 70%. The “part” is the number you’re measuring (35 correct answers), and the “whole” is the total possible (50 questions). Dividing gives you a decimal (0.7), and multiplying by 100 converts that decimal into a percentage.
This works for any situation where you’re comparing a smaller number to a larger one. Spent $450 out of a $2,000 budget? Type 450 ÷ 2000 × 100 to find you’ve used 22.5%.
Why the % Button Behaves Differently on Some Calculators
Not all calculators handle the percent button the same way. On a basic desktop or phone calculator, pressing % after a multiplication usually divides the number by 100 for you. But on some scientific calculators, the percent key may do nothing visible or may require you to press it in a different position in the sequence.
If you press a key sequence and get an unexpected result, try the manual approach: convert the percentage to a decimal yourself by dividing by 100, then multiply, add, or subtract as needed. For 25%, use 0.25. For 7.5%, use 0.075. This method works identically on every calculator, every phone app, and every spreadsheet. Once you’re comfortable converting percentages to decimals, you won’t need to rely on the percent button at all.
Quick Reference for Common Calculations
- Find X% of a number: number × X % (or number × X ÷ 100)
- Add X% to a number: number + X % (or number × 1.X for whole percentages)
- Subtract X% from a number: number – X % (or number × 0.X for the remaining portion)
- Find what % A is of B: A ÷ B × 100
For the add and subtract shortcuts without the percent button: adding 15% is the same as multiplying by 1.15, and subtracting 20% is the same as multiplying by 0.80. These one-step multiplications are faster when you’re doing several calculations in a row, like figuring out sale prices on a list of items.