The 6 Most Common Percentage Calculations
1. What is X% of Y?
Formula: Result = (X / 100) × Y
Example: What is 15% of 200? → 0.15 × 200 = 30
2. What percentage is X of Y?
Formula: Percentage = (X / Y) × 100
Example: What % is 30 of 200? → (30 / 200) × 100 = 15%
3. Percentage Change (Increase/Decrease)
Formula: Change % = ((New - Old) / Old) × 100
Example: Price went from $80 to $100. → ((100-80)/80) × 100 = +25%
4. Tip Calculator
Formula: Tip = (Tip% / 100) × Bill Amount
Example: 18% tip on $45 bill → 0.18 × 45 = $8.10
5. Discount Calculator
Formula: Sale Price = Original × (1 - Discount%/100)
Example: 30% off $120 → 120 × 0.70 = $84
6. Reverse Percentage
Formula: Original = Amount / (1 - Discount%/100)
Example: Final price after 20% discount is $80 → 80 / 0.80 = $100 original
How to Calculate Percentages Online
1Open the Percentage Calculator tool.
2Choose the calculation type from the 6 available modes.
3Enter your values — results update instantly.
💡 Quick Mental Math: To find 10% of any number, just move the decimal point one place to the left. Then multiply for other percentages: 20% = 10% × 2, 5% = 10% ÷ 2.
FAQ
How do I calculate percentage increase/decrease without a calculator?
Divide the change by the original value, then multiply by 100. For a price that went from $50 to $65: (65-50)/50 × 100 = 30% increase.