Percentage change vs percentage difference
Use percentage change when one amount is the starting value. Use percentage difference for two nonnegative values when neither is the baseline.
A starting value makes the comparison directional
From 80 to 100, percentage change is (100 − 80) ÷ 80 × 100 = 25%. From 100 to 80, it is (80 − 100) ÷ 100 × 100 = −20%. The denominator is the starting value.
A starting value of zero cannot give a finite percentage change. State the two amounts directly instead of inventing a percentage.
An average makes percentage difference symmetric
For 80 and 100, the average is 90. The absolute gap is 20, so percentage difference is 20 ÷ 90 × 100, or about 22.22%. Swapping the two inputs leaves that answer unchanged.
Use this comparison for nonnegative quantities. Do not silently apply it to signed values where the average can be zero or the interpretation changes.
Two percentage rates need another distinction
A move from 10% to 15% is a rise of 5 percentage points. It is also a 50% relative increase from the starting rate. The existing percentage-points guide explains when to use each wording.