Percentage calculations show up everywhere — discounts, tips, taxes, interest rates, test scores, and business metrics. Here are all the formulas you'll ever need.
Basic Percentage: What Is X% of Y?
Formula: Result = Y × X / 100
Example: What is 15% of 200? → 200 × 15 / 100 = 30
Reverse Percentage: A Is What % of B?
Formula: Percentage = (A / B) × 100
Example: 45 is what percent of 180? → (45 / 180) × 100 = 25%
Percentage Increase
Formula: ((New - Old) / Old) × 100
Example: Price went from $80 to $100 → (100-80)/80 × 100 = 25% increase
Percentage Decrease
Formula: ((Old - New) / Old) × 100
Example: Price dropped from $100 to $75 → (100-75)/100 × 100 = 25% decrease
Common Percentage Mistakes
- A 50% increase followed by a 50% decrease does NOT return to the original number. $100 + 50% = $150. $150 - 50% = $75.
- Percentage points ≠ percent. Going from 10% to 15% is a 5 percentage point increase but a 50% relative increase.
Use our percentage calculator for instant calculations.