Percentage Calculation Tricks and Tips

A percentage is a number expressed as a fraction of 100. The word comes from Latin 'per centum' meaning 'by the hundred'. 50% = 50/100 = 0.5.

Three main calculations: 1) X% of Y = (X/100) × Y. Example: 25% of 200 = 0.25 × 200 = 50. 2) X is what % of Y = (X/Y) × 100. Example: 50 is 25% of 200. 3) % change = ((new - old) / old) × 100.

Mental math tricks: 10% of any number = move decimal one place left. 10% of 450 = 45. Then 5% = half of 10% = 22.5. 20% = double 10% = 90.

To find 15%: calculate 10%, then add half of it. 15% of 80 = 8 (10%) + 4 (5%) = 12. To find 25%: divide by 4. 25% of 120 = 30.

Discount example: A $200 item with 25% off. Savings = 200 × 0.25 = $50. Final price = $150. Or: Final = 200 × (1 - 0.25) = 200 × 0.75 = $150.

Use our Percentage Calculator in the Basic Calculators category — it handles all three percentage modes with step-by-step solutions.