About this tool
What does it compute?
The three percentage questions: X% of Y, X is what % of Y, and percent change between two numbers — plus increase/decrease with the formula shown, so results are auditable.
How to use it
- Pick the question type, enter the numbers.
- Read both the result and the shown formula.
- Flip between increase and decrease modes explicitly.
Examples
- 20% tip on an $84.50 bill.
- Price change $49 → $39: −20.4%.
- Test score 17/20 = 85%.
Important limitations
- “Percent” needs a stated base: “up 200%” vs “200% of” differ by 100 points of meaning.
- Compounding percentages do not add: +10% then −10% ≠ break-even.
- Rounding money: decide half-up vs bankers rounding deliberately.
Privacy
Local arithmetic. Your numbers never leave the browser.
Frequently asked questions
Increase vs margin?
Increase is relative to the old value; margin relative to revenue — different denominators, different answers.
Why isn’t +50% then −50% zero?
Bases differ: 100→150→75. Percentages chain multiplicatively.
Percentage points?
The arithmetic difference of two percentages (20%→25% is +5 points, +25% relative).