How to Calculate Percentages: Formulas and Everyday Examples
Percentages show up in shopping, pay, exam results, interest rates and recipes. The calculations are not hard, but a few patterns cause most of the mistakes. This guide covers the formulas you need and shows each one with a worked example.
Written by the Toolseza team · Updated 2026-10-01
What a percentage means
Percent means per hundred. So 40% is 40 out of 100, which is the same as the fraction 2/5 and the decimal 0.4. To convert a percentage to a decimal, divide by 100. To convert a decimal to a percentage, multiply by 100.
Finding a percentage of a number
Multiply the number by the percentage as a decimal. To find 15% of 240, calculate 240 times 0.15, which is 36. A mental trick is to find 10% by moving the decimal point one place (24), then 5% by halving that (12), and adding them: 24 plus 12 is 36.
Working out what percent one number is of another
Divide the part by the whole, then multiply by 100. If you scored 42 out of 60, divide 42 by 60 to get 0.7, then multiply by 100: 70%.
Percentage increase and decrease
Subtract the old value from the new value, divide by the old value and multiply by 100. A price moving from 50 to 62 is a rise of 12 divided by 50, which is 24%. A drop from 50 to 40 is minus 10 divided by 50, which is minus 20%.
Always divide by the original value. This is the most common error: people divide by the new value and get a different, wrong answer.
Reverse percentages
If a price is 90 after a 25% discount, the original price is not 90 plus 25%. After a 25% discount you pay 75% of the original, so divide 90 by 0.75 to get 120. The same idea works for tax: a total of 120 that includes 20% VAT has a pre-tax price of 120 divided by 1.2, which is 100.
Common mistakes to avoid
Percentages are not symmetrical. Raising 100 by 50% gives 150, but lowering 150 by 50% gives 75, not 100. Successive percentages do not add: two discounts of 20% and 10% give a total of 28%, not 30%. And percentage points differ from percent: moving from 4% to 5% is one percentage point but a 25% increase.