How to Calculate a Percentage (Free Calculator + Formulas)
Percentages come up everywhere — discounts, tips, test scores, interest, growth. Yet the formula you need changes depending on the question, which is where people get stuck. This guide explains the three percentage calculations you'll actually use, with simple formulas and real examples, and shows how to get the answer instantly with a free calculator.
➗ Try the Percentage Calculator now — freeOpen →Getting percentages right matters for money and decisions — a miscalculated discount, tip or interest figure has real consequences. Understanding the three basic formulas means you can sanity-check any percentage, and a calculator gives you the exact answer without second-guessing.
The three percentage questions
First: what is X% of a number? Multiply the number by the percentage as a decimal. 20% of 80 = 80 × 0.20 = 16. This is how you find a discount or a tip.
Second: X is what percent of Y? Divide and multiply by 100. 16 is what percent of 80? 16 ÷ 80 × 100 = 20%. This is how you find a score or a share of a total.
Percentage change (increase or decrease)
Third: how much did something change in percent? Subtract the old value from the new, divide by the old value, and multiply by 100. If a price rises from 50 to 60: (60 − 50) ÷ 50 × 100 = 20% increase. If it falls from 50 to 40: (40 − 50) ÷ 50 × 100 = −20%, a 20% decrease.
A common trap: a 20% increase followed by a 20% decrease does not return you to the start, because the second percentage is taken from a different, larger base. The calculator handles this correctly so you don't have to.
Step-by-step
Common mistakes to avoid
Pro tips
Frequently asked questions
How do I find X% of a number?
Multiply the number by the percentage as a decimal. 15% of 200 = 200 × 0.15 = 30.
How do I calculate percentage change?
Subtract the old value from the new, divide by the old value, and multiply by 100.
How do I find what percent one number is of another?
Divide the part by the whole and multiply by 100. 45 of 90 = 45 ÷ 90 × 100 = 50%.
Is the calculator free?
Yes — free with no sign-up, and it runs instantly in your browser.
Why don't a 20% rise and 20% fall cancel out?
Because the fall is calculated from a larger base than the rise, so you end up slightly below where you started.
Master the three percentage formulas — percent of a number, what percent one is of another, and percentage change — and you can handle any real-world percentage. For exact answers on money and scores, let the free calculator do the arithmetic.