How to use it
- Choose the question: a percent of a number, what share one number is of another, the change between two values, or a number plus or minus a percent.
- Type the two numbers. Decimals work with a dot or a comma.
- Read the answer on the right. It updates on every keystroke.
Formulas
| Question | Formula | Example |
|---|---|---|
| X% of Y | X ÷ 100 × Y | 15% of 200 = 30 |
| X is what % of Y | X ÷ Y × 100 | 30 of 120 = 25% |
| Change from X to Y | (Y − X) ÷ |X| × 100 | 80 → 100 = +25% |
| Y plus / minus X% | Y × (1 ± X ÷ 100) | 250 ± 20% = 300 / 200 |
The change is measured against the absolute value of the old number, so a move from −50 to −25 shows as +50%: the value went up.
Worked example
A jacket costs 250 and the shop adds 20% sales tax. Choose Add or subtract %, enter 250 and 20: with tax it costs 300. If instead the price was cut by 20%, the result under “Minus the percent” is 200.
Common mistakes
- Reversing a discount with the same percent. A price cut by 20% (100 → 80) needs a 25% rise to get back to 100, because the base is now 80.
- Mixing up percent and percentage points. An interest rate from 4% to 5% is +1 point but +25%.
- Dividing by the wrong base. “What percent is X of Y” divides by the whole (Y), not by the part.