Percent Difference Calculator
Percent difference tells you how far apart two values are, measured as a percentage of their average. Unlike percent change, it treats both values as equals with no baseline, so it is the right tool when comparing two measurements, two prices, or two readings where neither is the correct reference. This tool takes two values and returns the percent difference, the raw absolute difference between them, and their average so you can see how the percentage is built. The result is symmetric: swapping the two inputs leaves the answer unchanged.
Percent difference formula
Absolute difference = |a - b|
Average = (a + b) / 2
Percent difference = (absolute difference / |average|) * 100
Here a and b are the two values being compared. Dividing by the average makes the result symmetric, so swapping a and b does not change the percent difference.
How percent difference works
- Percent difference compares two values when neither is the accepted reference.
- It divides the absolute difference by the average of the two values.
- The result is symmetric, so the order of the inputs does not matter.
- The standard definition uses the absolute difference, giving a non-negative result.
- If the average is zero the calculation is undefined, so the tool returns n/a.
Percent difference: frequently asked questions
What is the formula for percent difference?
Percent difference is the absolute difference between two values divided by their average, then multiplied by 100. In symbols: percent difference = |a - b| / ((a + b) / 2) * 100. It compares two values symmetrically, so swapping which value is first does not change the result.
How is percent difference different from percent change?
Percent change measures movement from an old value to a new value, dividing by the original starting value, so order matters and it has a direction. Percent difference compares two values with no privileged reference, dividing by their average, so it is symmetric. Use percent difference when neither value is the baseline.
Why divide by the average rather than one of the values?
Dividing by the average makes percent difference symmetric: comparing a to b gives the same answer as comparing b to a. If you divided by just one value the result would change depending on which you chose, which is why the average is used when neither value is the accepted reference.
Can percent difference be negative?
The standard definition uses the absolute value of the difference, so percent difference is always reported as a non-negative number. Because the comparison is symmetric, there is no natural sign: it simply measures how far apart the two values are relative to their average.
When are the values invalid for this calculation?
Percent difference divides by the average of the two values. If the average is zero, for example when the values are equal in magnitude but opposite in sign, the calculation is undefined and this tool returns n/a. Otherwise any pair of real numbers is valid.
Official sources
- National Institute of Standards and Technology: Guidelines for Evaluating and Expressing Uncertainty.
- National Aeronautics and Space Administration: Mathematics reference.
Reviewed by the CalculatorHub team, edited by James Graham, 16 June 2026. See our methodology.