How to calculate a percentage
A percentage expresses a number as a fraction of 100. To find X% of a number, convert the percentage to a decimal and multiply:
For example, 20% of 150 is (20 ÷ 100) × 150 = 30. This is the calculation used by the first tab above.
Finding what percent one number is of another
To answer "30 is what percent of 150?", divide the part by the whole and multiply by 100:
This is common when reporting scores, survey results, or completion rates.
Percentage increase and decrease
To measure how much a value changed, subtract the original from the new value, divide by the original, and multiply by 100. A positive result means the value increased; a negative result means it decreased.
This is the calculation behind price changes, salary raises, and year-over-year comparisons.
Reverse percentage
Sometimes you know the result and the percentage, and need the original number. For example, if 30 is 20% of a number, divide the value by the percentage (as a decimal):
This mode is useful for working backward from a discounted price, a tax-inclusive total, or a commission payout to find the original amount.
Where percentages show up day to day
- Shopping: discounts, sales tax, and tip calculations.
- Finance: interest rates, investment returns, and loan APRs.
- Work: commission rates, project completion, and performance metrics.
- School: test scores and grade weighting.
Frequently asked questions
Why does the reverse percentage mode give a different number than expected?
Make sure you're answering the right question. "30 is 20% of what number?" gives 150. But if the question is "a $30 item is discounted 20%, what was the original price?", that's a different formula, since $30 is 80% of the original, not 20%.
Can I use negative numbers?
Yes, the "From / To" mode handles negative values correctly for percentage change calculations, which is useful for tracking losses or reductions.