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Percent difference measures how far apart two values are when neither is the "correct" or "starting" one — you're just comparing two measurements on equal footing. It divides their difference by their average, so swapping the two values gives the same answer.
Enter the two values and get the percent difference, plus the raw difference and their average.
How is it calculated?
The percent difference formula
percent difference = |a − b| ÷ ((a + b) ÷ 2) × 100
| Result | Meaning | |
|---|---|---|
| Percent difference | The gap as a percentage of the average of the two | |
| Absolute difference | a − b | , the raw gap |
| Average | (a + b) ÷ 2, the reference base |
Why it divides by the average
With no "original" value to divide by, using either one as the base would give a different answer depending on which you picked. Dividing by the average treats both symmetrically, so the percent difference between 40 and 60 is the same whichever order you enter them.
Percent difference vs percent change
They answer different questions. Percent change has a clear before and after and divides by the starting value — a price going from 40 to 60 is a +50% change. Percent difference has no direction, divides by the average, and gives 40% for the same pair. Use change when time or a reference is involved, and difference when comparing two peers.
Where it helps
Comparing two lab measurements, two prices, two estimates, or results from two methods — anywhere neither number is the accepted truth. When you do have an accepted value and a measured one, percent error is the right tool instead.
Worked example
Compare 40 and 60. The absolute difference is 20 and the average is (40 + 60) ÷ 2 = 50, so the percent difference is 20 ÷ 50 × 100 = 40%. Enter them the other way round — 60 and 40 — and you get exactly 40% again, because the formula is symmetric. (By contrast, the percent *change* from 40 to 60 is +50%, and from 60 to 40 is −33%.)
FAQ
How do I calculate percent difference?+
Take the absolute difference of the two values, divide by their average, and multiply by 100: |a − b| ÷ ((a + b) ÷ 2) × 100. For 40 and 60 that is 40%.
What is the difference between percent difference and percent change?+
Percent change has a defined starting value and direction; percent difference compares two values symmetrically by dividing by their average. The same pair can give 50% change but 40% difference.
Why is percent difference symmetric?+
Because it divides by the average of the two values rather than by one of them, swapping the two inputs leaves the result unchanged. Neither value is treated as the reference.
When should I use percent difference?+
When comparing two measurements or values where neither is the "true" or "original" one — two lab results, two estimates, or two methods. If one is an accepted value, use percent error.
Can percent difference be more than 100%?+
Yes, if the two values are far apart relative to their average. Comparing 10 and 90 gives |80| ÷ 50 × 100 = 160%.