The three questions
- X% of Y — e.g. 15% of 200 = 30. Discounts, tips, taxes, commissions.
- X is what % of Y — e.g. 30 of 150 = 20%. Test scores, completion rates, survey results.
- Percentage change — e.g. 80 → 100 = +25%. Growth, inflation, stock moves, salary raises.
Common traps
Percent change isn't symmetric: going 100 → 80 is −20%, but going back 80 → 100 is +25%. Always anchor to the starting value. "Percentage points" ≠ percent: a rate rising from 10% to 12% is a 20% increase but only 2 percentage points. And percent of what matters: a tip on the pre-tax amount differs from post-tax.
Need compound growth or reverse calculations? See the Percentage Change Calculator and Compound Interest Calculator.