Percentage Difference Explained
When to compare two values against their average — and how.
Published by QuickUtilaUpdated 2 min read
Percentage difference measures how far apart two numbers are, relative to their average. Unlike percentage change, it doesn’t treat either number as the starting point, so swapping the two values gives exactly the same answer.
The percentage difference formula
The vertical bars mean “absolute value” — ignore any minus sign. The formula takes the gap between the two numbers and divides it by the average of the two.
- 1Gap: |40 − 60| = 20
- 2Average: (40 + 60) ÷ 2 = 50
- 3Divide and multiply by 100: 20 ÷ 50 × 100 = 40
Therefore: The percentage difference between 40 and 60 is 40%.
Why not just use percentage change?
Percentage change depends on which number you start from. From 40 to 60 is a 50% increase, but from 60 to 40 is a 33.33% decrease. If neither value is “before” or “after”, choosing one as the base makes the answer arbitrary.
Percentage difference avoids that problem by using the average as the base. It gives one symmetric figure: 40%.
When to use percentage difference
- Comparing prices for the same product at two stores.
- Comparing two measurements of the same thing, such as readings from two scales.
- Comparing results from two groups or two experiments.
- Checking how closely two estimates agree.
Special cases
If both values are zero there is nothing to compare, and the formula would divide by zero, so the result is undefined. If one value is zero and the other isn’t, the percentage difference is always 200%.