Percentage difference helps describe how different two values are relative to their average.
It can be useful when comparing measurements, prices, experimental results, statistics, or other values where neither value is necessarily considered the original one.
Percentage Difference Formula
The formula is:
Percentage Difference = |Value 1 − Value 2| ÷ ((Value 1 + Value 2) ÷ 2) × 100
Suppose the two values are 80 and 100.
The absolute difference is:
100 − 80 = 20
Their average is:
(80 + 100) ÷ 2 = 90
Then:
(20 ÷ 90) × 100 ≈ 22.22%
The percentage difference is approximately 22.22%.
Percentage difference versus percentage change
Choose the denominator to match the question. Percentage difference compares two values equally, so it uses their average. Percentage change describes movement from an original value to a later value, so it uses the original value as the base. Reversing the order of the inputs leaves percentage difference unchanged, but changes the percentage change.
For 80 and 100, the percentage difference is 22.22% in either order. An increase from 80 to 100 is 25%, while a decrease from 100 to 80 is 20%. These results answer different questions; none of them should be substituted for another without checking the context.
More worked examples
Two prices: 120 and 150. Their absolute difference is 30 and their average is 135. Calculate 30 ÷ 135 × 100 to get approximately 22.22%. Use this comparison when neither price is the original baseline.
Two measurements: 98 and 102. The difference is 4 and the average is 100, so the percentage difference is 4%. Whether that difference is acceptable depends on the measurement method and the tolerance you are working with.
Equal values: 50 and 50. The difference is zero and the average is 50. The percentage difference is therefore 0%.
Zero, negative values, and rounding
For nonnegative values, if one value is zero and the other is positive, this formula gives 200%. If both are zero, the average is zero and the formula involves division by zero, so the result is undefined. Negative values or values with opposite signs can make this average-based comparison misleading, especially when their average is zero or close to zero. Explain the context and consider an absolute difference instead.
Keep the full average during the calculation and round only the final result. For 80 and 100, using 90 as the average gives 22.222…%, which can be reported as 22.22%.
Calculate an increase or decrease instead
If you are comparing an old price, salary, or total with a new one, use the percentage calculator and select its increase/decrease calculation. For a step-by-step explanation of that baseline method, read how to calculate percentage increase.