How to calculate percentages (with examples)
Updated 23 October 2026
Tool for this guide: Percentage calculator
Short answer: to find X% of a number, multiply by X and divide by 100: 15% of 200 = 200 × 15 ÷ 100 = 30. The percentage calculator does this and three other common percentage sums as you type.
The four percentage formulas
| Question | Formula | Example |
|---|---|---|
| What is X% of Y? | Y × X ÷ 100 | 15% of 200 = 30 |
| X is what % of Y? | X ÷ Y × 100 | 30 of 120 = 25% |
| % change from A to B | (B − A) ÷ A × 100 | 80 → 100 = +25% |
| Add or remove X% | Y × (1 ± X ÷ 100) | 50 + 20% = 60; 50 − 20% = 40 |
Everyday examples
- Discount: a 25% discount on 80 → 80 × 0.75 = 60.
- Tax or VAT: 15% on 200 → 200 × 1.15 = 230.
- Tip: 10% of 45 → 4.50.
- Exam score: 42 out of 50 → 42 ÷ 50 × 100 = 84%.
- Pay rise: 2,000 to 2,200 → (2,200 − 2,000) ÷ 2,000 × 100 = +10%.
- Price before discount: 60 after 25% off → 60 ÷ 0.75 = 80.
Step by step with the calculator
- Open the percentage calculator.
- Pick the card that matches your question.
- Type the two numbers. The answer updates instantly; copy it with one click.
Common mistakes
- Percentage change uses the starting value. From 80 to 100 is +25%, but from 100 to 80 is −20%.
- Percent vs percentage points. A rate going from 10% to 12% rose by 2 percentage points, which is a 20% increase.
- Two discounts don't add up. 20% off, then another 10% off, is 28% off in total, not 30%.
Questions
How do I calculate 20% of a number in my head?
Find 10% by moving the decimal point one place left, then double it: 20% of 85 is 8.5 × 2 = 17.
How do I reverse a percentage?
Divide by the multiplier: a price of 115 including 15% tax was 115 ÷ 1.15 = 100 before tax.
Try it now
Work out percentages instantly: X% of a number, what percent one number is of another, percentage increase or decrease, and adding or taking off a percentage.
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