Practical guide
How to calculate percentages without mixing up the formulas
Percentage questions look similar but they are not all the same. The correct formula depends on whether you are finding a part, comparing two values or working backward from a final amount.
Find a percentage of a number
To find 15% of 240, convert 15% to 0.15 and multiply: 240 × 0.15 = 36. The same calculation can be written as 240 × 15 ÷ 100.
Find what percentage one value is of another
If 36 items out of 240 meet a condition, divide the part by the total and multiply by 100: 36 ÷ 240 × 100 = 15%.
Calculate percentage increase or decrease
Percentage change compares the difference with the original value. If a price rises from 80 to 92, the increase is 12. Divide 12 by the original 80 and multiply by 100: 15%.
The starting value matters. A fall from 92 back to 80 is not a 15% decrease; it is 12 ÷ 92 × 100, or about 13.04%.
Reverse a percentage
If a final price of 92 includes a 15% increase, do not simply subtract 15% of 92. Divide by 1.15: 92 ÷ 1.15 = 80. This is a common source of mistakes in taxes, markups and discounts.
Percentage points are different
If a rate moves from 20% to 25%, the change is 5 percentage points. Relative to the original 20%, however, the rate increased by 25%. Keeping these two descriptions separate prevents misleading comparisons.
Practical checks
- Identify the original or reference value before calculating a change.
- Keep enough decimal places until the final step.
- Use percentage points when comparing two percentages directly.
- For reverse calculations, divide by the multiplier rather than subtracting the same percentage.
Calculate it online
Use the Percentage Calculator for percentage of a number and related percentage calculations, or the Discount Calculator for sale prices.