How to calculate percentage
Updated on June 18, 2026 · by Rafael Rossi
A percentage is a fraction of 100. "20%" means 20 out of 100, i.e. 20 ÷ 100 = 0.20. Almost every percentage problem is just turning the "%" into a decimal and multiplying.
The five essential operations
- A% of B = (A ÷ 100) × B
- A is what % of B = (A ÷ B) × 100
- Increase B by A% = B × (1 + A/100)
- Decrease B by A% = B × (1 − A/100)
- Change from A to B = ((B − A) ÷ A) × 100
Worked examples
15% of 80: 0.15 × 80 = 12. Mental shortcut: 10% of 80 is 8, half of that is 4, and 8 + 4 = 12.
Discount: a $200 item at 30% off costs 200 × 0.70 = $140.
What % a value is: 42 of 50 questions → 42 ÷ 50 × 100 = 84%.
Pitfalls to avoid
Up and down by the same % doesn't return to the start. 100 up 20% is 120, down 20% is 96, because the base changed.
Percentage points ≠ percent. A rate going from 10% to 12% rose 2 points — but that's a 20% increase.
FAQ
How to find the price before a discount? Divide by the factor: $68 at 15% off came from 68 ÷ 0.85 = $80.
Do two discounts add up? No. 20% + 20% is 36% (0.80 × 0.80 = 0.64), not 40%.