How to Calculate Percentages: 4 Everyday Formulas

"Percent" means "per hundred", so 15% is 15 out of every 100, or 0.15. Almost every percentage question in daily life is one of four types. Once you spot which one you have, the math takes seconds.

1. What is X% of a number?

Turn the percentage into a decimal by dividing by 100, then multiply.

Example: 15% of 420 = 0.15 × 420 = 63.

2. What percent is one number of another?

Divide the part by the whole and multiply by 100.

Example: you scored 42 out of 60 on a test. 42 ÷ 60 = 0.7, so you scored 70%.

3. Percentage increase or decrease

Find the difference, divide by the original value, then multiply by 100.

Example: rent rises from $1,200 to $1,290. The difference is $90, and 90 ÷ 1,200 = 0.075, a 7.5% increase. A negative result means a decrease.

4. Reverse percentages: finding the original

If you know a value after a percentage was added or removed, divide instead of subtracting.

Example: a jacket costs $68 after 15% off. The original price is 68 ÷ 0.85 = $80. A common mistake is adding 15% of $68 back on, which gives $78.20. That's wrong, because the 15% was taken from the larger original price.

Quick mental shortcuts

• 10% of any number: move the decimal point one place to the left.
• 5% is half of 10%; 20% is double 10%.
• 1% is the number divided by 100.
• X% of Y equals Y% of X, so 8% of 50 is the same as 50% of 8, which is 4.

Percentage points are not the same as percent. An interest rate moving from 4% to 5% rose by one percentage point, but by 25 percent.