How to Calculate a Percentage
A percentage is a part per hundred. Almost every percentage question is one of three operations: take a percent of a number, express one number as a percent of another, or measure how much a value changed. Work a small example for each, then use the matching calculator when you want the arithmetic filled in.
Find X percent of Y
Use this when you already know the percentage and the whole, and you need the part. Convert the percent to a decimal by dividing by 100, then multiply by the number.
(X / 100) × Y
- Write the percent as a fraction of 100. For 15%, that is 15/100, or 0.15.
- Multiply that decimal by the number.
- 15% of 80 is 0.15 × 80 = 12.
Find what percent X is of Y
Use this when you have a part and a whole and you need the percentage. Divide the part by the whole, then multiply by 100.
(X / Y) × 100
- Divide the first number by the second number.
- Multiply the result by 100 to put it on a per-hundred scale.
- 25 is what percent of 200? (25 / 200) × 100 = 12.5%.
Find a percentage increase or decrease
Use this when a value moved from an original amount to a new amount. The original value is the denominator, so the sign tells you whether the change is an increase or a decrease.
((new − original) / original) × 100
- Subtract the original value from the new value.
- Divide that difference by the original value.
- Multiply by 100. From 50 to 65 is ((65 − 50) / 50) × 100 = 30% increase.
Spreadsheet formulas
In Excel or Google Sheets, the same three operations are cell formulas. Keep the percent-of version as a multiply-and-divide, not a hardcoded 0.15, so the sheet stays editable.
- Percent of a number: =A1*B1/100
- What percent: =(A1/B1)*100
- Percentage change: =(B1-A1)/A1
Related reading
Questions about the method
Only when you are converting a percent into a decimal multiplier, such as 15% of 80. When you already have a part and a whole, you divide the part by the whole and then multiply by 100.
Yes. If the part is larger than the whole, the result is over 100%. 40 is 125% of 32 because 40/32 × 100 = 125.
No. Change uses a starting value as the base. Difference uses the average of the two numbers and has no direction.
Divide the final amount by 1 plus the percent for an increase, or by 1 minus the percent for a decrease. 110 after a 10% increase came from 100.
Use the percent-of calculator. This guide shows the method; the tool accepts any pair of numbers without a separate page for each example.