The four kinds of percentage problem
Most percentage questions fall into one of four shapes, and this tool handles all of them: finding X% of Y (a discount or tip), finding what percent X is of Y (a test score or statistic), finding the percentage change from one value to another (a raise, a price drop), and finding what number X% of equals Y (working backward from a known result). Pick the mode that matches your question, and the result updates as you type — no submit button needed.
Worked examples, computed by this exact tool
20% of 150 is 30.00, using the formula (20 ÷ 100) × 150. Going the other direction, 30 is 20.00% of 150. A price that drops from 100 to 80 is a change of -20.00% — shown as a signed number, since a decrease and an increase are different answers, not just different words for the same thing. And if 20% of some number is 30, that number is 150.00.
Negative numbers and percentage decreases
Percentage change can be negative, and this tool shows that sign rather than hiding it: a value that went down is reported as a negative percentage, not rephrased as a positive "percentage decrease" you have to interpret yourself. Negative inputs are accepted anywhere they're mathematically meaningful.
What happens with zero
Some percentage questions have no real answer when a particular value is zero — asking what percent one number is of zero, or asking for the percentage change starting from zero, are both undefined in ordinary arithmetic (you'd be dividing by zero). Rather than showing "Infinity" or crashing, this tool tells you plainly that the calculation is undefined and why.
What this tool doesn't do
This calculator handles each of the four percentage problems on its own — it doesn't chain them together automatically. If you need to apply a discount and then a tax, or compute compound interest over several periods, that's a sequence of separate calculations (or a different, dedicated calculator) rather than something this single formula does for you. It also doesn't attach currency symbols or units to the result; the numbers are yours to apply to dollars, points, kilograms, or anything else.
Privacy
All calculation happens directly in your browser using plain arithmetic. Nothing you type is sent anywhere, logged, or stored.