The percentage difference formula
Percentage difference = |A − B| ÷ ((|A| + |B|) ÷ 2) × 100
The gap between the two numbers is divided by their average, so neither number is treated as the original. That makes the result symmetric: comparing 50 with 70 gives the same answer as comparing 70 with 50.
Percentage difference or percent change?
Use percentage change when one value is clearly the earlier one, such as a price last year and this year. Use percentage difference when you compare two values side by side, such as the heights of two buildings, the results of two lab measurements or the prices of two shops.
| Numbers | Percentage difference | Percent change, first to second |
|---|---|---|
| 50 and 70 | 33.33% | +40% |
| 70 and 50 | 33.33% | −28.57% |
Choosing the basis
The standard method divides by the average of the two numbers, and it is the default here. The selector also lets you divide by the first value, the second value, the larger one or the smaller one. This is useful for homework that names a specific rule or for comparing against a reference value. Note that these bases make the answer depend on which number you treat as the reference.
Special cases
- If the two numbers are equal, the difference is 0%.
- If both numbers are 0, the average is 0 and the result is undefined.
- Negative numbers are compared by their size, using absolute values in the basis.
Need a before and after figure instead? Use the percentage change calculator. The percentage change guide compares all the methods.