Percentage difference answers a symmetric question: how different are two numbers, relative to their size? Unlike percentage increase, it does not treat one number as the starting point. That symmetry makes it the right tool for comparing two measurements, two prices, or two results with no natural “before” and “after”.
The formula
Percentage difference = |a – b| / ((a + b) / 2) x 100. Take the absolute difference between the two numbers, divide by their average, and multiply by 100. The average in the denominator is what makes the result symmetric: swapping a and b gives the same answer.
Why the average? Because with percentage increase, the answer depends on which number you start from: 50 to 70 is a 40% increase, but 70 to 50 is a 28.6% decrease. Percentage difference avoids that argument entirely by using the midpoint as the reference.
Worked example
Compare 50 and 70. Difference: |50 – 70| = 20. Average: (50 + 70) / 2 = 60. Percentage difference = 20 / 60 x 100 = 33.33%. So the two numbers differ by about a third of their average size.
Second example: two lab measurements, 98 and 102. Difference = 4. Average = 100. Percentage difference = 4%. That small figure is the point: for measurements near 100, a 4-unit gap is small in relative terms, even though 4 sounds large on its own.
Difference vs increase vs change
Use percentage difference when neither value is the baseline: comparing two products, two readings, or two people’s results. Use percentage increase or decrease when there is a clear before and after, like a price rise or a weight change. Use percentage points when comparing two percentages themselves: a rise from 20% to 25% is 5 percentage points, but a 25% relative increase. Mixing these three up is the most common percentage error in reporting.
Edge cases: if one value is zero, the average is half the other value and the difference is 200%, which is correct but often confusing, so explain it when it appears. If both values are zero, the formula divides by zero and is undefined.
Quick checks
The answer is always positive, since the absolute difference is used. It can exceed 100% when the numbers are far apart: 10 and 100 differ by 163.6%. And it is symmetric: the difference between 50 and 70 equals the difference between 70 and 50. If your calculation is not symmetric, you used the increase formula by mistake.
Percentage difference in measurement and quality
Percentage difference is the standard way to compare two measurements of the same thing, for example two lab readings or a measured value against a specification. If a part should measure 100 mm and measures 96 mm, the difference is |96 – 100| / 98 x 100 = 4.08%.
Quality control often sets a tolerance as a percentage difference: parts within 2% of specification pass, parts beyond it fail. Because the formula is symmetric, it does not matter which measurement you call first, which is exactly why it suits quality checks where neither reading is the reference.
A final use case: comparing prices across shops or countries. If the same item costs $80 in one shop and $95 in another, the percentage difference is 15 / 87.5 x 100 = 17.1%. That figure is fairer than either shop’s markup framing, because it treats neither price as the baseline, which is exactly what a shopper comparing two options needs.
Try it faster with the tool
You can follow every step above by hand, but the free Percentage Difference Calculator does the arithmetic for you. Enter your numbers and it gives the answer right away, so you can check your hand calculation or skip straight to the result.
Quick tips
- Difference = |a – b| / average x 100.
- Use it when neither value is the baseline.
- A rise from 20% to 25% is 5 percentage points, not 5%.