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Percentage Difference Calculator

Compare two values where neither came first. Percentage difference measures the gap between them against their average, which is what makes it symmetric — swap the inputs and the answer is unchanged.

What is the percentage difference?

Enter your numbers and press Calculate — the full working appears here.

The percentage difference formula

The gap between the two values goes on top; the average of the two goes underneath. Using the average is the whole point — it refuses to privilege either value as the baseline.

(|A − B| ÷ ((A + B) ÷ 2)) × 100

(|100 − 120| ÷ 110) × 100 = 18.18%

The vertical bars mean “ignore the sign” — a gap of 20 is a gap of 20 regardless of which value is larger.

When percentage difference is the right question

The test is simple: does one of your two values come before the other? If it does, you want percentage change. If the two values are simply two readings, two quotes or two options sitting side by side, percentage difference is the honest comparison.

  • Two quotes for the same job — a builder at $4,200 and another at $3,800. Neither is “the original”.
  • Repeated measurements — two lab readings of the same sample, where the difference indicates measurement error rather than a change over time.
  • Comparing two products — two packs at 340 g and 400 g.
  • Comparing two regions or groups — average salary in two cities, or scores for two cohorts sitting the same exam.

Using percentage change for these would force you to nominate one value as the baseline, and your answer would shift depending on which you picked. That is not a rounding quirk — it is a different question producing a different number.

Worked example: two quotes

Two contractors quote $4,200 and $3,800 for the same work. How far apart are they?

  1. Find the gap, ignoring which is larger.|4,200 − 3,800| = 400
  2. Find the average of the two quotes.(4,200 + 3,800) ÷ 2 = 4,000
  3. Divide the gap by the average and multiply by 100.400 ÷ 4,000 × 100 = 10%

Answer10% difference

Open this example in the calculator →

Frequently asked questions

What is percentage difference?
It is how far apart two values are, expressed as a percentage of their average. Because the average treats both values equally, the answer is the same whichever order you enter them in.
What is the percentage difference between 100 and 120?
18.18%. The gap is 20 and the average of the two values is 110, so 20 ÷ 110 × 100 = 18.18%. Note that this is not 20% — that would be the percentage change from 100 to 120.
Why divide by the average instead of one of the values?
Because neither value is more authoritative than the other. Dividing by one of them would make that value the reference point and produce a different answer depending on which you picked — which is exactly what percentage change does, on purpose.
When should I use percentage difference instead of percentage change?
Use difference when the two values are peers with no before-and-after relationship: two suppliers’ quotes, two measurements of the same thing, two products’ weights. Use change when one value genuinely preceded the other.
Can percentage difference be more than 100%?
Yes. Comparing 10 and 90 gives a gap of 80 against an average of 50, which is 160%. Differences above 100% simply mean the gap is larger than the typical size of the two values.

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