Percentage difference calculator
Percentage difference is not the same as percentage change. Use it when neither number is the 'before' — it compares both against their own average, so the answer is the same whichever order you enter them.
Result
- Percentage difference
- 40.00%
- Absolute difference
- 20.00
- Mean of the two
- 50.00
Worked examples
Comparing two measurements
Two scales read 40 kg and 60 kg for the same object.
- |40 − 60| = 20
- (40 + 60) ÷ 2 = 50
- 20 ÷ 50 × 100 = 40
A 40% difference.
When to use difference rather than change
Percentage change needs a before and an after. Percentage difference does not — it compares two values where neither has priority, such as two instruments measuring the same thing, or two branches of the same business.
Because it divides by the mean of the two values rather than by one of them, the answer is the same whichever order you enter the numbers. That symmetry is the whole point, and it is what distinguishes it from percentage change.
Why the answers differ from percentage change
Comparing 40 and 60 gives a 40% difference, because 20 is 40% of their mean of 50. Treating 40 as a starting point and 60 as an endpoint gives a 50% increase instead, because 20 is half of 40.
Neither is wrong. They answer different questions, and quoting one when you meant the other is a common way for a report to overstate a gap.
Formula
difference % = |a − b| ÷ ((a + b) ÷ 2) × 100
Difference against change, on the same pair
The two measures answer different questions and give different numbers. Quoting one where the other is meant is a common way to overstate a gap.
| Values | % difference | % change a→b | % change b→a |
|---|---|---|---|
| 40 and 60 | 40% | +50% | −33.33% |
| 10 and 100 | 163.64% | +900% | −90% |
| 90 and 100 | 10.53% | +11.11% | −10% |
| 1 and 2 | 66.67% | +100% | −50% |
| 50 and 50 | 0% | 0% | 0% |
Frequently asked questions
What is the formula for percentage difference?
Divide the absolute difference between the two values by their mean, then multiply by 100: |a − b| ÷ ((a + b) ÷ 2) × 100.
Is percentage difference the same as percentage change?
No. Percentage change measures movement from a known starting value. Percentage difference compares two values where neither is the baseline, so the order does not matter.
Can percentage difference exceed 100%?
Yes, when the two values are far apart. Comparing 10 and 100 gives 163.6%, because the difference of 90 is larger than their mean of 55.
Why is the mean used as the denominator?
To keep the result symmetric. Dividing by either value alone would give a different answer depending on which you picked first.
Can percentage difference be negative?
No. The numerator is an absolute difference, so the result is always zero or positive. If a tool reports a negative value it is computing percentage change instead.
Which should I use for comparing two lab measurements?
Percentage difference, because neither reading is the reference. Use percentage error instead when one value is a known true value.
What is percentage error?
The difference between a measured value and the accepted true value, divided by the true value. Unlike percentage difference it has a defined baseline.
Why does my percentage difference exceed 100%?
Because the gap is larger than the mean of the two values. Comparing 10 and 100 gives 163.6%, which is correct.