A percentage is simply a way to describe part of a whole out of 100. Once you recognise three basic patterns, discounts, tips, test scores, and price changes become much easier to understand.
Find a percentage of a number
Divide the percentage by 100, then multiply by the number. To find 15% of 80, calculate 15 ÷ 100 × 80. The answer is 12. This is the pattern behind tips, taxes, commissions, and finding a portion of a total.
Find what percentage one number is of another
Divide the part by the whole, then multiply by 100. If 18 of 24 tasks are complete, calculate 18 ÷ 24 × 100. The answer is 75%. Be careful to use the complete total as the denominator.
Calculate a percentage change
Subtract the old value from the new value, divide the difference by the old value, then multiply by 100. A move from 50 to 65 is (65 − 50) ÷ 50 × 100, which is a 30% increase.
Check whether the answer makes sense
Estimate before accepting a result. Ten percent of 80 is 8, so 15% must be a little more than 8. A quick estimate catches misplaced decimals and reversed numbers before they become real mistakes.
A calculator is often the quickest way to check your mental math. The important part is using the right starting value.
Open a calculator →Common questions
What is the easiest way to work out 10%?
Move the decimal point one place to the left. Ten percent of 340 is 34.
How do I turn a decimal into a percentage?
Multiply the decimal by 100 and add the percent sign. For example, 0.42 becomes 42%.
Can a percentage be more than 100%?
Yes. A value that is 150% of another value is one and a half times as large.
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