Find the percentage difference between two values — no direction, just magnitude.
Percentage difference measures how far apart two values are relative to their average, expressed as a percentage. Unlike percentage change, it has no direction — it does not identify which value is "higher" or which came "first." The result is always positive, simply quantifying the magnitude of separation between two numbers.
The formula uses the average of the two values as the denominator: |A - B| / ((A + B) / 2) × 100. This averaging makes the formula symmetric — swapping A and B gives identical results. This symmetry is exactly what you want when neither value has a privileged "original" status.
Use percentage difference when comparing two peer values with no implied before/after relationship. Comparing prices at two competing stores: neither price is the "original." Comparing the scores of two students on the same test: neither score is the reference point. Comparing the weight of two objects: neither is the baseline.
Percentage difference is also appropriate in scientific contexts when comparing two experimental measurements or two different instruments' readings of the same quantity. If two thermometers read 98.4°F and 99.1°F, the percentage difference quantifies their disagreement without implying either is the "correct" baseline.
Using the average (rather than one of the two values) as the denominator achieves symmetry. If you used A as the denominator, the result would change when you swapped A and B. The average treats both values equally, which is the right mathematical approach when neither value has priority.
This is fundamentally different from percentage change, where the original value is the correct denominator because you're measuring movement from a specific starting point. The choice of formula should reflect the underlying relationship between your numbers, not just which calculation you remember.
In laboratory settings, percentage difference (also called percent error in some contexts) is used to compare experimental results against theoretical or accepted values. When a physics experiment measures the speed of light at 299,100,000 m/s against the accepted 299,792,458 m/s, the percentage difference is |299,100,000 - 299,792,458| / ((299,100,000 + 299,792,458)/2) × 100 ≈ 0.23%. This quantifies measurement accuracy in a standardized way across different experiments and research groups.