Calculating a percentage is one of the most useful everyday math skills, and it shows up everywhere: discount signs in shops, tips at restaurants, grades on tests, tax on receipts, and growth figures in the news. The question behind nearly every one of those situations is the same — what percent is one number of another — and the good news is that a single formula answers all of them. Learn it once and percentage problems stop being guesswork.
The Basic Percentage Formula
A percentage is simply a fraction with a denominator of 100. To find what percent one number is of another, divide the part by the whole, then multiply the result by 100. The part is the value you care about, and the whole is the reference value it is being compared against. Getting those two the right way round is the most common mistake people make, so whenever you are stuck, ask yourself: "out of what is this percentage being taken?"
percentage = (part / whole) × 100
What percent is 45 of 300?
(45 / 300) × 100 = 15%
What percent is 15 of 60?
(15 / 60) × 100 = 25%Do the math in seconds
Skip the manual arithmetic and let the percentage calculator handle it.
Three Questions, One Formula
- What percent is X of Y? The core question: divide X by Y and multiply by 100. Example: 25 is 12.5% of 200.
- Percent of an amount: Multiply the amount by the percentage as a decimal. 18% of 250 is 250 × 0.18 = 45.
- Reverse it: If you know the result and the percentage, divide the result by the percentage as a decimal. 40% of $125 is $50, so $50 / 0.40 = $125.
- Compare two changes: Turn each change into a percentage with the same formula so they become directly comparable.
Sanity-Check Your Answer
Quick mental checks catch most errors before they matter. A 100% increase doubles the number, a 50% increase multiplies it by 1.5, and a 10% decrease multiplies it by 0.9. If your answer does not land in a believable range, you probably swapped the part and the whole. When the numbers get awkward — odd decimals, large totals, or percentages you need to double-check for a payment or a grade — reach for a calculator so the division is done right and you can focus on whether the result makes sense. This is especially handy when you are comparing offers or splitting something between people, where a wrong percentage can change a number by a noticeable margin.