Percentage Difference Calculator
The percentage difference between two numbers tells you how far apart they are relative to their average, without treating either one as the baseline. It is the symmetric counterpart to percentage change: you divide the absolute difference by the mean of the two values, then multiply by 100. This calculator returns the percentage difference, the plain difference, and the percentage change from the first value to the second, so you can pick the comparison that fits your situation, whether you are comparing two measurements, two prices, or two readings of the same quantity.
Percentage difference formula
Mean = (a + b) / 2
Absolute difference = |a - b|
Percentage difference = |a - b| / mean * 100
Percentage change (a to b) = (b - a) / a * 100
Percentage difference is symmetric: swapping a and b leaves it unchanged. Percentage change is directional and uses a as the baseline.
When to use which measure
- Use percentage difference when neither value is the obvious starting point (two lab readings, two quotes).
- Use percentage change when there is a clear before and after (last year vs. this year).
- Percentage difference can exceed 100 percent when values are far apart relative to their mean.
- Both values being zero makes the mean zero and the result undefined.
- A negative percentage change means the value fell from a to b.
Percentage difference: frequently asked questions
What is percentage difference?
Percentage difference measures how far apart two values are relative to their average. It is the absolute difference between the two numbers divided by their mean, expressed as a percentage. Because it uses the average as the reference, it is symmetric: swapping the two values gives the same result.
How is percentage difference different from percentage change?
Percentage change has a clear starting value and ending value: it divides the difference by the original value, so the order matters. Percentage difference treats both numbers equally by dividing by their mean, so order does not matter. Use change when one value came first; use difference when neither is the obvious baseline.
What is the formula?
Percentage difference = |a - b| / ((a + b) / 2) times 100. The vertical bars mean absolute value, so the result is always non-negative. For percentage change from a to b, the formula is (b - a) / a times 100.
Can the percentage difference exceed 100 percent?
Yes. If the two values are far apart relative to their mean, the percentage difference can exceed 100 percent. For example, comparing 1 and 9 gives a difference of 8 over a mean of 5, which is 160 percent.
What happens if both numbers are zero?
The mean is zero, so the formula divides by zero and the result is undefined. This calculator shows n/a in that case. Percentage difference is only meaningful when the two values are not both zero.
Official sources
- NIST Digital Library of Mathematical Functions: DLMF home.
- NIST: National Institute of Standards and Technology.
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.