Calculate the percentage difference between two values when neither one is the starting point, using their average as the reference.
The percentage difference formula
Percentage difference = |A − B| ÷ ((A + B) ÷ 2) × 100
Because it divides by the average of the two values, the answer is the same whichever value you list first. That makes it the right choice when comparing two things on equal footing, such as two store prices or two measurements.
Worked example
Comparing 100 and 120: the difference is 20 and the average is 110, so the percentage difference is 20 ÷ 110 × 100 = 18.18%.
Compare that with percentage change: going from 100 to 120 is a 20% increase, but going from 120 to 100 is a 16.67% decrease. Percentage difference sits between the two.
When to use which
- Percentage change: when there is a clear before and after, such as a price rise or growth in sales. Use the percentage calculator.
- Percentage difference: when comparing two values with no natural starting point, such as two lab results or two quotes.
Frequently asked questions
Can the percentage difference exceed 100%?
Yes, up to 200%, when one value is much larger than the other (for example, 1 and 100).
What about negative numbers?
The formula works best for positive values. With mixed signs, the average can be close to zero and the result becomes misleading.
Related tools
Results are estimates for planning and education only. They are not financial, tax, legal or medical advice. Check important numbers with a qualified professional.