Percentages turn raw numbers into a shared scale, which is why they show up everywhere from restaurant tips to test scores. This free calculator handles the three questions people ask most, and updates the moment you change a number.
The three ways to use it
The first mode answers "what is X% of Y" — perfect for tips, discounts, and commissions. The second mode flips the question to "X is what percent of Y," which is how you turn a score of 45 out of 50 into 90%. The third mode measures percent change between an old value and a new one, so you can see that a price rising from $80 to $100 is a 25% increase.
Why the math works
A percentage is simply a fraction with 100 on the bottom. "20 percent" literally means 20 per hundred, or 0.20. To take a percentage of a number you multiply by that decimal; to express one number as a percentage of another you divide and multiply the result by 100. Percent change compares the size of a difference to the value you started from, which keeps increases and decreases on a fair footing.
Common mistakes to avoid
The most frequent error is measuring change against the wrong base — a jump from 100 to 120 is a 20% gain, but the drop back from 120 to 100 is only a 16.7% loss, because the starting points differ. Another trap is stacking percentages: taking 10% off twice is not the same as taking 20% off. When in doubt, convert to plain decimals, do the arithmetic, and convert back at the very end. Prices with tax are a percentage problem too — the sales tax calculator handles them by state, and the online scientific calculator covers everything else.
More worked examples
A discount: a $79.99 jacket is marked 30% off. In the X% of Y mode, enter 30 and 79.99. The result is 23.997, so the markdown is about $24.00 and you subtract it from the tag price.
A test score: you answered 42 questions correctly out of 48. In the What percent mode, enter 42 as the value and 48 as the total. The result is 87.5%.
A percent change: rent goes from $1,250 to $1,400 a month. In the Percent change mode, enter 1250 as From (A) and 1400 as To (B). The increase is 12%, a difference of 150. Swap the two values and the same 150 is a 10.7143% decrease, because the change is measured against the starting value, which is now larger.
When to use this calculator
Pick the mode by the question you are asking. Use X% of Y for discounts, tips, commissions, and tax on a price. Use What percent when you have a part and a whole, such as a score, a budget share, or how much of a goal you have reached. Use Percent change for anything measured over time: prices, salaries, rent, or a portfolio balance from one year to the next.
Reading the result panel
Every mode shows the result first, then the inputs it used and the formula with your numbers filled in, so you can copy the working into homework or a spreadsheet. The X% of Y mode also shows the decimal form of the percentage, which is the number you multiply by. In Percent change, the panel reports the difference between the two values alongside the percentage, and labels the result as an increase or a decrease so the sign is never ambiguous.