Percentage Difference Calculator
Compare two values symmetrically — the gap relative to their average, so the order you enter them never changes the answer.
Enter your values to see instant results:
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The percentage difference formula
|a − b| ÷ ((a + b) ÷ 2) × 100. Take 50 and 70: they differ by 20, their
average is 60, and 20 ÷ 60 × 100 = 33.33%. Swap them — 70 and 50 — and
nothing changes, which is the whole reason this formula exists.
Contrast that with percentage change, where 50 → 70 is +40% but 70 → 50 is −28.57%. Change privileges one value as the baseline; difference refuses to. That makes difference the right tool for comparisons with no timeline: quoting how far apart two vendors' bids are, checking two instruments against each other, or comparing the same measurement from two methods.
A worked example
Two labs measure the same sample at 4.8 mg and 5.2 mg. The percentage difference is |4.8 − 5.2| ÷ 5.0 × 100 = 8% — a single, order-independent number for "how far apart are these readings." Reporting "lab B read 8.33% higher than lab A" is also valid, but it's a percentage change statement and needs the direction spelled out.
The common mistake
The classic error is using percentage difference where a baseline exists — comparing this quarter's revenue to last quarter's with the symmetric formula understates real growth (the average is dragged upward by the newer, larger value). If time flows between your two numbers, use percentage change; keep difference for genuine peer-to-peer comparisons.
Percentage difference FAQ
What is the percentage difference formula?
|a − b| ÷ ((a + b) ÷ 2) × 100 — the gap between the values, divided by their average. For 50 and 70: 20 ÷ 60 × 100 = 33.33%. Because the base is the average, the answer is the same whichever number you put first.
When should I use percentage difference instead of percentage change?
Use difference when neither value is the natural baseline: two shops' prices, two labs' measurements, two candidates' salaries. Use change when one value came first in time and the other followed — sales last year vs. this year. Difference is symmetric; change is directional.
Why divide by the average and not one of the values?
Dividing by either value would smuggle a direction back in: 20 ÷ 50 gives 40% but 20 ÷ 70 gives 28.57%, so the "difference" would depend on which number you happened to type first. The average is the only neutral base, which is why it's the convention in science and engineering.
Why doesn't percentage difference work for opposite-sign values?
Percentage difference compares magnitudes, and with one positive and one negative value the average heads toward zero, inflating the result toward meaninglessness (−5 vs. +5 would be an "infinite" difference). If your values straddle zero, what you almost certainly want is the directional percentage change — the calculator says so instead of printing a junk number.