These two calculations get mixed up constantly, even in professional contexts. The confusion is understandable -- both involve two numbers and produce a percentage. But they answer different questions, use different formulas, and are appropriate in different situations. Using the wrong one doesn't just give you a different answer; it gives you a wrong answer for the question you were actually trying to ask.
The Core Distinction
- Percentage change measures movement from a defined starting point to an end point. It has direction (positive or negative) and requires that one value is clearly "before" and the other is "after."
- Percentage difference compares two values as equals -- neither came first. It measures how far apart they are relative to their average, and it is always positive (or zero).
The Formulas
Percentage Change = ((New - Old) / Old) x 100
Percentage Difference = (|Value A - Value B| / ((Value A + Value B) / 2)) x 100
The key structural difference: percentage change divides by the old (starting) value. Percentage difference divides by the average of both values.
Why the Average? Why Not One Value?
Using the average as the denominator makes percentage difference symmetric -- the result is the same regardless of which number you call "A" and which you call "B." If you used one value as the base instead, swapping the inputs would give you a different answer, which would be logically inconsistent for a comparison where neither value is the reference point.
A Decision Guide
| Situation | Use | Because |
|---|---|---|
| Salary last year vs. this year | Percentage Change | Last year is the clear starting point |
| Two competing job offers | Percentage Difference | Neither offer is the "original" |
| Stock price 3 months ago vs. today | Percentage Change | 3 months ago is the before state |
| Your height vs. a friend's height | Percentage Difference | No temporal or causal relationship |
| Sales in Q1 vs. Q2 of the same year | Percentage Change | Q1 is the reference period |
| Two lab measurements of the same object | Percentage Difference | Comparing simultaneous measurements |
| Revenue before vs. after a campaign | Percentage Change | Before is the baseline |
| Store A price vs. Store B price | Percentage Difference | Neither price is the reference |
The Same Numbers, Different Results
Here's why using the wrong formula matters. Compare $80 and $100:
- Percentage change from $80 to $100: (100-80)/80 x 100 = +25%
- Percentage change from $100 to $80: (80-100)/100 x 100 = -20%
- Percentage difference between $80 and $100: |100-80| / ((100+80)/2) x 100 = 20/90 x 100 = 22.2%
Three different answers for the same two numbers. Each is correct for a different question. If you're reporting on price change over time, 25% or -20% is right (depending on direction). If you're comparing two simultaneous prices with no starting point, 22.2% is right.
When Context Is Ambiguous
Sometimes it's genuinely unclear which calculation is appropriate. When in doubt, ask: "Was one of these values a starting point, and did the other value come from it over time?" If yes, use percentage change. If the two values are independent data points being compared side by side with no before/after relationship, use percentage difference.
If you're writing a report or analysis, it's worth naming which calculation you used, since a reader might assume the other one. Our Percentage Change Calculator and Percentage Difference Calculator are each built for their specific formula so you always get the right answer.