Percent Change vs Percentage Difference: When to Use Each

The two formulas side by side

	Percent change	Percentage difference
Formula	(new − old) ÷ old × 100	|A − B| ÷ ((A+B)/2) × 100
Symmetric?	No	Yes
Direction?	Yes (+ or −)	No (always positive)
Use when	Clear baseline / chronology	Comparing equals

When percent change is right

Anytime there’s a clear “before” and “after”, or one value is treated as the baseline:

• Stock prices over time
• Year-over-year sales
• Treatment vs. control in a clinical trial
• Inflation calculations
• Pay raises and salary changes

When percentage difference is right

When neither value is the baseline — typically when comparing two measurements of the same thing or two independent quantities:

• Gas prices in two cities
• Two students’ scores on a standardized test
• Heights of two trees
• Two companies’ revenue (without a chronological direction)
• Lab measurements from two different instruments

Why symmetry matters

Imagine two scientists measure the same plant: one gets 40 cm, the other gets 60 cm. Should the discrepancy depend on whose result is labeled “first”?

• If you treat 40 as “old”, percent change is +50%.
• If you treat 60 as “old”, percent change is −33.3%.

Two different numbers for the same discrepancy — unsatisfying. Percentage difference gives a single answer (40%) regardless of order.