A percentage expresses a number as a part of one hundred, and calculating one comes down to dividing a part by a whole and multiplying by 100. Once you know the basic formula, you can find percentages, increases, and decreases. This guide explains the core calculations with clear examples.
What you'll need
- A calculator (optional)
Step by step
- Understand the formulaTo find what percent one number is of another, divide the part by the whole and multiply the result by 100.
- Find a percentage of a numberConvert the percentage to a decimal by dividing by 100, then multiply it by the number. For example, 20 percent of 50 is 0.20 times 50, which is 10.
- Work out what percent one number is of anotherDivide the part by the whole and multiply by 100. For example, 15 out of 60 is 15 divided by 60 times 100, which is 25 percent.
- Calculate a percentage increaseSubtract the original from the new value, divide by the original, and multiply by 100 to get the percent it grew.
- Calculate a percentage decreaseSubtract the new value from the original, divide by the original, and multiply by 100 to get the percent it dropped.
- Double-check your resultSanity-check the answer by seeing if it makes sense, since a percentage of a number should be smaller than the whole for values under 100 percent.
Handy tips
- To find 10 percent quickly, just move the decimal point one place to the left.
- Percentages can be reversed: 25 percent of 80 equals 80 percent of 25, so pick the easier one to compute.
- Convert the percentage to a decimal first, as it makes the multiplication straightforward.
FAQ
How do I turn a percentage into a decimal?
Divide it by 100 or move the decimal point two places to the left. So 45 percent becomes 0.45.
What is the difference between percentage points and percent?
A change from 10 percent to 15 percent is a 5 percentage point rise, but a 50 percent increase relative to the original 10. They measure change differently.
How do I find the original number before a percentage change?
Divide the final amount by 1 plus the increase as a decimal, or by 1 minus the decrease. For example, if 120 is after a 20 percent rise, divide 120 by 1.20 to get 100.