Guide · Difference
Percentage Difference Calculator
Updated: June 2026
Percentage difference answers a subtly different question from percentage change: how far apart are two numbers, when neither one is the obvious reference? Because there is no "before" and "after", the formula divides by the average of the two values rather than by one of them. That makes the result symmetric — and that symmetry is exactly why scientists and engineers reach for it when comparing two independent measurements.
Free · No upload · Instant in the browser
The percentage difference formula
For two values a and b:
The numerator is the absolute gap between the two numbers, so the result is always positive. The denominator is their average — the midpoint between them — which is what makes the calculation treat both values as equals. Swapping a and b leaves the answer untouched.
A worked example
Compare 40 and 60. The gap is |40 − 60| = 20. The average is (40 + 60) ÷ 2 = 50. So the percentage difference is 20 ÷ 50 × 100 = 40%. Notice this sits between the two percentage-change figures: going from 40 to 60 is a 50% increase, going from 60 to 40 is a 33% decrease, and the difference of 40% lands between them — because the average base is between the two starting points.
Difference vs change — when to use which
The two are easy to confuse but answer different questions:
| Percentage change | Percentage difference | |
|---|---|---|
| Base | starting value | average of both |
| Order | matters | doesn't matter |
| Sign | can be ± | always positive |
| Use for | trends over time | comparing two figures |
If your two numbers are "last month" and "this month", use change. If they are "machine A" and "machine B", or "my estimate" and "the measured value", use difference.
A note on its limits
Percentage difference works best when both numbers are positive and of similar magnitude. When the two values are wildly different, the average sits far from both and the figure can feel unintuitive. And if the two values sum to zero, the average is zero and the formula breaks down entirely — there is simply no meaningful midpoint to divide by.
Where you'll use it
- Comparing two independent measurements in a lab or quality check.
- Reporting the spread between two quotes, bids or estimates.
- Expressing how close an experimental result is to a reference value.
- Any side-by-side comparison where neither figure is "first".
Frequently asked questions
What is the percentage difference formula?
Take the absolute difference between the two values, divide by their average, then multiply by 100: |a − b| ÷ ((a + b) ÷ 2) × 100.
When should I use percentage difference instead of percentage change?
When the two values have no natural before-and-after order, so neither is the obvious reference point.
Why does percentage difference use the average?
Dividing by the average makes the result symmetric — comparing a to b gives the same figure as comparing b to a.
Can percentage difference be negative?
No. It uses the absolute gap, so it is always zero or positive.