Percentage Calculator
The five percentage questions people actually ask, each with its formula spelled out so you can check the working.
How to use the Percentage Calculator
- Choose the type of calculation from the buttons at the top.
- Fill in the two numbers — the labels change to match the mode you picked.
- Press Calculate. The working is shown underneath so you can verify it.
The five formulas
| Question | Formula |
|---|---|
| What is X% of Y? | (X ÷ 100) × Y |
| X is what percent of Y? | (X ÷ Y) × 100 |
| Percentage change from A to B | ((B − A) ÷ |A|) × 100 |
| Add X% to Y | Y × (1 + X ÷ 100) |
| Subtract X% from Y | Y × (1 − X ÷ 100) |
Percentage points are not percentages
If a rate moves from 4% to 6%, that is a rise of two percentage points — but a 50% increase. Both statements are true and they describe the same change. Headlines routinely blur the two, usually in whichever direction sounds more dramatic.
The rule: percentage points measure the arithmetic gap between two percentages; percentage change measures the proportional difference. When the underlying quantity is already a percentage, say which one you mean.
Why increases and decreases do not cancel
Take 100, add 20%, then subtract 20%. You get 96, not 100. The increase is calculated on 100 and the decrease on 120, so the two operations work on different bases.
To reverse a 20% increase you divide by 1.20, which is a 16.67% decrease. This is why a shop's "50% off, then 20% off" is a 60% discount rather than 70% — and why a stock that falls 50% needs to double to recover.
Frequently asked questions
How do I reverse a percentage increase?
Divide rather than subtract. If a price of 120 includes a 20% markup, the original is 120 ÷ 1.20 = 100. Subtracting 20% from 120 gives 96, which is wrong.
What does a negative percentage change mean?
The value fell. Change from 250 to 200 is −20%, meaning a decrease of one fifth.
Why is percentage change from zero undefined?
The formula divides by the starting value. Any growth from zero is infinite in proportional terms, so report it as an absolute change instead.
How many decimal places does it show?
Results are displayed to two places, and the working shows up to six so you can see rounding before it happens.
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