Calculate a percentage: formulas, examples and online tool
All percentage calculation formulas: % of a number, change, discount, increase and reverse percentage.
Published on January 14, 2026Calculating a percentage is one of the most commonly used mathematical operations in daily life: commercial discounts, salary changes, statistics, school grades, interest rates... Mastering the different percentage formulas avoids common errors and confusion.
Understanding the conversion
There are several types of percentage calculations depending on the need: calculating X% of a number, calculating what percentage A represents of B, calculating a percentage increase or decrease, calculating the percentage change between two values, or finding the original value before an increase/decrease.
📐 Formula
📊 Conversion table
| Calculation type | Formula | Example |
|---|---|---|
| X% of a number N | N × X / 100 | 15% of 80 = 12 |
| A is what % of B? | (A / B) × 100 | 30 is what % of 120 = 25% |
| Increase N by X% | N × (1 + X/100) | 200 + 15% = 230 |
| Decrease N by X% | N × (1 - X/100) | 200 - 20% = 160 |
| Change between A and B | ((B - A) / A) × 100 | (150-120)/120 = +25% |
| Price before X% discount | Sale price / (1 - X/100) | $80 after -20% → $100 |
💡 Practical examples
Discount = 79.99 × 30% = $24.00. Final price = 79.99 × (1 - 0.30) = $55.99.
Change = ((2,750 - 2,500) / 2,500) × 100 = (250/2,500) × 100 = 10% raise.
Sale price = $45. Original price = 45 / (1 - 0.25) = 45 / 0.75 = $60.