A symmetric comparison between two values, with no “before” or “after” — unlike percentage change, swapping the two numbers gives the same answer.
How it works
The absolute difference between the two values is divided by their average, then converted to a percentage.
What this does not include
This does not include directional percentage change — for that (where the order of the two values matters), use this site’s percentage calculator’s “% change” mode instead.
How to use this calculator
- Enter both values.
A worked example
Two lab instruments report a sample as 40 and 60. Neither is the “before” — they are two independent readings of the same thing.
Percentage difference = |40 − 60| ÷ ((40 + 60) ÷ 2) × 100 = 20 ÷ 50 × 100 = 40%
Now notice what percentage change would have said about the same pair. Going 40 → 60 is a 50% increase. Going 60 → 40 is a 33.33% decrease. Same two numbers, two different answers, and neither one is wrong — they simply answer a directional question. Percentage difference returns 40% whichever order you enter the values in, which is exactly what you want when there is no “starting” value to divide by.
What the variables mean
| Variable | Meaning | In the example |
|---|---|---|
| Value 1, Value 2 | Two comparable measurements; order does not matter | 40 and 60 |
| Absolute difference | The gap between them, sign discarded | 20 |
| Average | The midpoint of the two values — the denominator | 50 |
| Result | Difference ÷ average, as a percentage | 40% |
Edge cases worth knowing
Two values that sum to zero have no answer. If the values average out to zero — say 5 and −5 — the denominator is zero and percentage difference is undefined, not infinite. The calculator returns no result rather than a misleading number.
The ceiling is 200%, not 100%. Because the denominator is the average rather than the larger value, the maximum percentage difference between two positive numbers approaches 200% as one of them approaches zero — so a result above 100% is possible and not a mistake.
Identical values give exactly 0%. The numerator is zero, so any two equal values return 0% regardless of how large they are.
Frequently asked questions
How is this different from percentage change?
Percentage change is directional and divides by the original (starting) value; percentage difference is symmetric and divides by the average of both values, giving the same result no matter which value is entered first.
When should I use percentage difference instead of percentage change?
When comparing two measurements with no clear “before” and “after,” like two lab results or two competing estimates, rather than tracking a value’s change over time.
Can percentage difference be negative?
No — because it uses an absolute value in the numerator, percentage difference is always zero or positive, unlike percentage change which can be negative.
