Most "percentage problems" people actually run into break down into three distinct calculations, each with its own formula — which is why this tool has three tabs instead of one input box:
| Tab | Formula | Default example |
|---|---|---|
| X% of Y | Y × (X ÷ 100) | 15% of 200 = 30 |
| X is what % of Y | (X ÷ Y) × 100 | 30 is 15% of 200 |
| % change | ((Y − X) ÷ X) × 100 | 80 → 100 = 25% increase |
"X% of Y" is what you want for tips, discounts, and commissions. "X is what % of Y" is for things like test scores or market share, where you already know both numbers and want the relationship between them. "% change" is for anything that moves over time — prices, populations, revenue — and it's the one people most often reach for without realizing there's a subtlety to watch for.
If an interest rate moves from 20% to 25%, that's a 5 percentage point increase — you just subtract one percentage from the other. But as a percent change, using the formula above, it's a 25% increase ((25 − 20) ÷ 20 × 100 = 25), because percent change is always relative to the starting value, not an absolute count. Both statements are true about the same move — they're just answering different questions — and mixing them up is one of the most common sources of confusion in news coverage of interest rates, tax changes, and survey results. When in doubt, the "% change" tab above gives you the percent-change version; a percentage-point difference is just simple subtraction of the two raw percentages.
"X% of Y" finds a portion of a number. "% change" tells you how much a number grew or shrank between two values, expressed as a percentage.
Because percentages are relative to different starting points — increasing 100 by 50% gives 150, but decreasing 150 by 50% gives 75, not back to 100.
Yes — run the calculator twice, using the result of the first calculation as the input to the second.