These two terms sound alike but answer different questions — and mixing them up gives a different number, not just a differently-labeled one.
Percent change: there's a reference value
Percent change asks "how much did this grow or shrink, relative to where it started?" It's direction-sensitive: going from 80 to 95 is a +18.75% change ((95−80)÷80×100), but going from 95 back to 80 is a −15.79% change ((80−95)÷95×100) — not the same magnitude, because the reference value is different each time.
Percent difference: neither value is the reference
Percent difference instead compares the two values symmetrically, using their average as the base: |value1 − value2| ÷ ((value1 + value2) ÷ 2) × 100. For 80 and 95, that's |80−95| ÷ 87.5 × 100 = 17.14% — and entering them in the reverse order (95 and 80) gives the exact same 17.14%, because neither is treated as "before" or "after."
When to use which
- Use percent change when there's a clear before/after, old/new, or baseline — prices over time, exam scores, growth rates.
- Use percent difference when comparing two independent measurements where neither is the "correct" or "original" one — two lab readings, two estimates, two competing prices with no inherent order.
Common mistakes
- Using percent change's asymmetric formula when the two values have no natural order — this makes the result depend on an arbitrary choice of which value goes first.
- Expecting percent difference to match percent change — they use different denominators (the average vs. one specific value) and will rarely produce the same number.
Compare any two values with the Percent Difference Calculator — the result is the same regardless of which value you enter first.