Percentage Calculator

Percentages show up in more places than most people realize — tips, discounts, grades, interest rates, growth statistics — and the math behind them is simple once you know which of three common questions you're actually asking. This calculator handles all three in one tool instead of making you remember a different formula for each.

"X% of Y" is the most familiar version: what is 20% of 80? You multiply the percentage, converted to a decimal, by the number, giving 16. This is the calculation behind tips, sales tax, and commission.

"X is what % of Y" flips the question around: if you scored 42 out of 50 on a test, what percentage is that? Here you divide the part by the whole and multiply by 100, giving 84%. This version is common for grades, survey results, and any time you have two raw numbers and want to express their relationship as a percentage.

"Percent change" answers a third, different question: how much did something grow or shrink, expressed as a percentage of where it started? Going from a $50 stock price to $65 is a change of (65-50)/50 × 100, or 30%, an increase. Going the other direction, from $65 back to $50, is roughly a 23% decrease — the percentages aren't symmetric, which trips people up. This version shows up in financial reports, population statistics, and price changes.

Mixing these up is a common source of errors: mistaking "X is what % of Y" for "percent change," for instance, can produce a wildly wrong answer that still looks plausible. Picking the right mode for the question you're actually asking is the most important step, and often the only hard part of what is otherwise very simple arithmetic.