The standard formula
The formula is: (percent divided by 100) multiplied by the number. Dividing by 100 converts the percent into its decimal form, and multiplying applies that fraction to the number.
Step by step with 25 percent of 240: first, 25 divided by 100 is 0.25. Then 0.25 times 240 equals 60. So 25 percent of 240 is 60. Another way to think about it: 25 percent means 25 out of every 100, and 240 contains two full hundreds plus 40, which is 50 plus 10, totaling 60.
Try a trickier one: 7.5 percent of 640. First, 7.5 divided by 100 is 0.075. Then 0.075 times 640 equals 48. The formula does not care whether the percent is a round number or the number is awkward. Convert, multiply, done.
Method two: the 10 percent shortcut
For everyday math, start with 10 percent, which is just the number with the decimal point moved one place left. Ten percent of 640 is 64. From there you can build any nearby percent by adding or halving.
Twenty percent is double 10 percent: 2 times 64 is 128. Five percent is half of 10 percent: 64 divided by 2 is 32. Fifteen percent is 10 percent plus 5 percent: 64 plus 32 is 96. Thirty percent is three times 10 percent: 192.
This is why waiters and shoppers love it. A 15 percent tip on an 84 dollar bill? Ten percent is 8.40, half of that is 4.20, add them for 12.60. You got the tip without touching a calculator.
Method three: the fraction trick
Many common percents are simple fractions in disguise, and swapping them in can be faster than multiplying decimals. Fifty percent is one half. Twenty-five percent is one quarter. Twenty percent is one fifth. Seventy-five percent is three quarters. Ten percent is one tenth.
So 20 percent of 135 is 135 divided by 5, which is 27. Seventy-five percent of 800 is 800 times 3 divided by 4, which is 600. This method is strongest when the number divides cleanly, and the standard formula covers everything else.
Worked examples from real life
Example 1, a tip: 18 percent on a 52 dollar bill. Convert: 0.18. Multiply: 0.18 times 52 equals 9.36, so about 9.36 dollars. A fast mental check with the 10 percent shortcut: 10 percent is 5.20, 5 percent is 2.60, and 3 percent is roughly 1.56, which sums to about 9.36. Same answer.
Example 2, sales tax: 6.25 percent on a 1,299 dollar laptop. Convert: 0.0625. Multiply: 0.0625 times 1299 equals 81.19, so 81.19 dollars of tax, and 1,380.19 total at checkout.
Example 3, a commission: 3 percent on a 425,000 dollar home sale. Convert: 0.03. Multiply: 0.03 times 425,000 equals 12,750 dollars in commission.
Example 4, a raise: 4 percent on a 68,000 dollar salary. Convert: 0.04. Multiply: 0.04 times 68,000 equals 2,720, so the new salary is 70,720 dollars.
Common mistakes to avoid
The number one mistake is multiplying by the percent itself instead of the decimal: 20 times 80 is 1,600, not the answer. Always divide by 100 first. The number two mistake is rounding too early. If you are adding tax and a tip, keep two or three decimal places through the middle steps and round once at the end.
Also watch the direction of the problem. "What is 20 percent of 80?" is this problem. "20 is what percent of 80?" is the reverse problem, answered by 20 divided by 80 times 100, which is 25 percent. If you want that direction, read our guide on how to find what percent one number is of another.