Percentage calculator
Calculate percentages every way: X% of Y, increase, discount and percentage change.
How the calculation works
A percentage is simply 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 most common cases:
- 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
Step by step: 15% of 80
Turn 15% into a decimal (0.15) and multiply: 0.15 × 80 = 12. Mental shortcut: 10% of 80 is 8, half of that (5%) is 4, and 8 + 4 = 12.
Increase and discount
To increase 80 by 15%: 80 × 1.15 = 92. To discount 15%: 80 × 0.85 = 68.
Percentage change
From 80 to 92? Change is (92 − 80) ÷ 80 × 100 = +15%. Careful: going up 15% then down 15% does not return to the start (92 × 0.85 = 78.2), because the base changed.
Examples
- 15% of 80 = 0.15 × 80 = 12.
- 30 is what % of 120? 30 ÷ 120 × 100 = 25%.
- A R$200 price up 10%: 200 × 1.10 = R$220.
Frequently asked questions
How to calculate a store discount?
Use the "Decrease B by A%" mode: B is the price and A is the discount percent. E.g. R$200 at 30% off = 200 × 0.70 = R$140.
How to find what percent a value is?
Divide the part by the whole and multiply by 100. 42 out of 50 questions: 42 ÷ 50 × 100 = 84%.
Does +50% then −50% return to the original?
No. 100 rises to 150 then falls to 75, because the second percentage applies to a larger base.
Percentage points vs percent?
If a rate goes from 10% to 12%, it rose 2 percentage points — but that is a 20% increase (2 ÷ 10).
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Updated on June 17, 2026 · by Rafael Rossi · Methodology & sources