In short: Percentages show up everywhere: discounts, tips, tax, grades, battery levels. The math is never hard, but under pressure it's easy to flip the formula.
Updated August 15, 2026, after a fresh review, with updated visuals and links.
Use the tool below for the three questions people actually ask, then the worked examples if you want to do it in your head next time.
How the three modes work
X percent of Y multiplies: 20% of 150 is 0.20 times 150, which is 30. This is the sale-price move: 20% off a $150 jacket means you pay 150 minus 30, or $120.
X is what percent of Y divides: 45 out of 150 is 45 divided by 150, times 100, which is 30%. Test scores and expense taxes live here.
Percent change compares before and after: from 80 to 100 is (100 minus 80) over 80, times 100: a 25% increase. Prices going the other way give a negative number, a decrease.
Worth remembering
- Quick 10% trick: move the decimal one place left. 10% of 86 is 8.6. Then scale: 20% is double that, 5% is half.
- A 50% discount followed by another 50% is not free: the second cut applies to the already-reduced price, leaving 25% of the original.
Frequently asked
How do I calculate a percentage increase?
Subtract the old value from the new one, divide by the old value, multiply by 100. Going from 80 to 100: 20 over 80 times 100, a 25% increase.
How do I find what percent one number is of another?
Divide the part by the whole and multiply by 100: 45 out of 150 is 45 divided by 150, times 100, equals 30%.
Find every related guide in our Online Calculators: Every Free Tool index.
