Percentage calculator
Three calculators, with the working shown — because knowing how beats copying an answer.
What is X% of Y?
Working: (15 ÷ 100) × 80 = 12
X is what % of Y?
Working: 12 ÷ 80 × 100 = 15%
Percentage change from X to Y
Working: (92 − 80) ÷ 80 × 100 = +15%
The three formulas, once and for all
| Question | Formula | Example |
|---|---|---|
| What is P% of N? | (P ÷ 100) × N | 15% of 80 = 12 |
| A is what % of B? | A ÷ B × 100 | 12 of 80 = 15% |
| Change from A to B | (B − A) ÷ A × 100 | 80 → 92 = +15% |
The mistake almost everyone makes
In percentage change, the old value is always the denominator. Going from 80 to 92 is a 15% increase, but going from 92 back to 80 is a 13% decrease — because the second change is measured against 92, not 80. This is why a price that rises 50% and then falls 50% ends up below where it started, and why "the discount was reversed" never quite returns the original price.
Mental shortcuts that actually work
- 10% first. Move the decimal one place: 10% of £64 is £6.40. From there, 5% is half of that, 20% is double, 15% is one-and-a-half times.
- 1% for precision. Move the decimal two places: 1% of £64 is £0.64, so 3% is £1.92.
- Flip it when it's easier. P% of N equals N% of P. 8% of 50 feels hard; 50% of 8 is instantly 4. Same answer, always.
- Tips and VAT. 12.5% is 10% plus a quarter of that. 20% is just double the 10% figure.
The three calculators above run entirely in your browser and work with no connection. This page sets no cookies and loads no third-party scripts.
Common questions
How do I add or subtract a percentage from a number?
To add P%: multiply by (1 + P÷100). To remove P%: multiply by (1 − P÷100). Adding 20% to £250 is 250 × 1.2 = £300. Note that removing 20% from £300 gives £240, not £250 — to reverse an added percentage you divide by 1.2 instead (see our VAT calculator, where this exact confusion costs people money).
What's a percentage point?
The absolute difference between two percentages. Interest moving from 4% to 6% is a rise of 2 percentage points — but a 50% rise in the rate itself. When a headline says "up 2%", check which one it means.
Why do exam mark schemes and discounts round differently?
There is no single rounding rule. Shops usually round prices to the penny in their favour; this page shows up to two decimal places and leaves rounding decisions to you.
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