Percentage Difference Calculator
Find the percentage difference between two values relative to their average — the right measure when neither number is the baseline.
How it works
Percentage difference divides by the average of the two values, not by one of them. That makes it symmetric: comparing 40 with 60 gives the same answer as comparing 60 with 40, which is what you want when neither figure is a baseline.
Percentage change is the other tool, and it is not interchangeable. Change has a definite before and after and divides by the original, so it is directional. Using difference where you meant change — or the reverse — produces a plausible number that answers a question you did not ask.
The formula
Percentage difference
diff = |a − b| ÷ ((a + b) ÷ 2) × 100
Percentage change (for contrast)
change = (b − a) ÷ |a| × 100
Worked examples
| Scenario | Working | Result |
|---|---|---|
| 40 compared with 60 | 20 ÷ 50 × 100 | 40% difference |
| The same pair as a change | 20 ÷ 40 × 100 | +50% change |
| Two quotes, 1,200 and 1,000 | 200 ÷ 1100 × 100 | 18.18% difference |
When you'd use it
- Comparing two quotes or two measurements
- Reporting the gap between two readings with no natural order
- Comparing results from two instruments or methods
- Any comparison where swapping the two values should not change the answer
Common questions
When should I use difference rather than change?
Use difference when neither value is a starting point — two competing quotes, two lab measurements, two branches. Use change when there is a clear before and after, such as last month against this month.
Why divide by the average?
So the answer is the same whichever value you list first. Dividing by one of them would make the result depend on an arbitrary choice, which is exactly what you are trying to avoid in a symmetric comparison.
Why is the percentage difference smaller than the percentage change?
Because the average sits between the two numbers and is therefore larger than the smaller one. Dividing by a bigger denominator gives a smaller percentage. Both are correct; they measure different things.