How to use this calculator
Choose the percentage operation that matches your question: a part of an amount, one amount as a percentage of another, a change from an original amount, or an increase or discount. Enter the original and new values in the correct fields.
Method and interpretation
A percentage of an amount is amount × percentage / 100. A percentage change is (new - original) / original × 100. The original value is the denominator, so reversing the old and new values usually changes the answer.
Assumptions and limits
A zero denominator makes a ratio or percentage change undefined. Percentage points describe the difference between two rates, while percent change describes their relative change. Successive discounts are applied to a changing base and should not normally be added together.
Worked example
Increasing a price from $80 to $100 is a 25% increase: (100 - 80) / 80 × 100. Returning from $100 to $80 is a 20% decrease because the denominator is now $100.