The three percentage questions
Nearly every percentage problem in daily life is one of three shapes, and this tool answers all three at once. What is X% of Y? — tips, taxes, discounts, commissions. X is what % of Y? — scores, completion, proportions. What is the percentage change from A to B? — price rises, growth, weight change. Pick your question at the top; the other two are computed anyway and shown in the table, because in practice one answer usually prompts the next question.
The three formulas
A percent is just a count of hundredths, so every formula begins by converting between the percent and its decimal form. Percentage change deserves one caution: it is always measured against the starting value. Going from 80 to 100 is +25%, but going back from 100 to 80 is −20% — the same $20, a different base. That asymmetry surprises people in sales ("prices rose 25%, now 20% off!") and is exactly why the starting value is the denominator. For comparing two values without a before/after, the Math section's Percentage Difference Calculator splits the difference symmetrically.
Percent vs percentage points
If an interest rate moves from 4% to 5%, that is a rise of one percentage point — but a 25% increase in the rate. News reports mix these constantly. This calculator works in percents of numbers; when you read a change described in "points", it is the plain subtraction of the two percents, with no division involved.
Worked example
15% of 240 is 36 (the classic tip and discount question). And in reverse: 36 is 15% of 240. From 80 to 100 is a 25% increase.
How to use this calculator
- Choose which of the three questions you are asking.
- Fill in that question's two numbers (the other fields still update their answers too).
- Read your answer in the green panel.
- Check the table for all three answers and the step box below for the working.
Quick percent landmarks
Some percents are worth knowing cold: 10% is a tenth (move the decimal), 25% is a quarter, 33⅓% is a third, 1% is a hundredth. Combining them covers most mental math — 15% of anything is 10% plus half of 10%; 18% (a common tip) is 20% minus 2%. The calculator confirms these instantly when the bill arrives.