Percentage change vs percentage difference: which to use
Percentage change measures movement from a starting point, so it cares which number came first: from 80 to 100 is a 25% increase, but from 100 to 80 is a 20% decrease. Percentage difference compares two values against their average, so order is irrelevant and the answer never flips: between 80 and 100 it is 22.22%, whichever you name first. Use change when one value is a baseline and the other is what happened to it. Use difference when neither value has any claim to being first.
The same numbers, three answers
Take 80 and 100. Depending on the question you are actually asking, they produce three different percentages.
| Question | Formula in words | Answer |
|---|---|---|
| How much did 80 grow to reach 100? | change relative to the start, 80 | +25% |
| How much did 100 fall to reach 80? | change relative to the start, 100 | −20% |
| How far apart are 80 and 100? | gap relative to their average, 90 | 22.22% |
The two change figures disagree because they divide the same gap of 20 by a different base: 20 out of 80 is a quarter, 20 out of 100 is a fifth. The difference figure sidesteps the argument by dividing by 90, the average of the two, so there is one answer instead of two.
When change is right
Reach for percentage change whenever one number came before the other. A price last month and this month. A population in 2010 and in 2020. A reading before treatment and after. In every case there is a baseline the reader already has in mind, and the question is how far things moved from it.
Change is directional by design, and that direction carries meaning you would lose by discarding it. A 25% rise and a 20% fall are not mirror images even though they involve the same two numbers, because they measure from opposite ends. This asymmetry is why a value that falls and then rises by the same headline percentage does not return to where it began: 100 reduced by 20% is 80, and getting from 80 back to 100 needs a 25% rise, not another 20%. The effect grows with the size of the move. Recovering from a 50% fall does not need a 50% gain; it needs a 100% rise, because you are now building on a base half the size.
If your sentence contains the words “from”, “to”, “grew”, “fell”, “since” or “was”, you want percentage change.
When difference is right
Reach for percentage difference when two things sit side by side and neither is the starting point. Two sensors reporting the same quantity. Two suppliers quoting the same job. Two measurements of the same object. There is no “before” and no “after”, so there is no natural base to divide by, and picking one of the two values as the base would be arbitrary.
Percentage difference solves this by dividing the gap by the average of the two values. Because the average treats both numbers identically, the result is symmetric: it does not matter which reading you write down first, and swapping them leaves the answer untouched. Between 80 and 100 the difference is 22.22%, full stop. That single, stable number is exactly what you want when the two values are peers rather than a sequence.
The quiet abuse
Percentage change has a lever built in, and it gets pulled. Because the figure depends on which value you divide by, the same absolute gap can be dressed up or played down by choosing the smaller or larger base. A gap of 20 between 80 and 100 is “25% bigger” if you measure against 80 and “20% smaller” if you measure against 100. Anyone wanting a gap to sound dramatic quotes the version divided by the smaller number, and both figures are technically correct, which is what makes the trick durable. Percentage difference removes the lever entirely: with the average fixed as the base, there is no smaller number to choose and no way to inflate the gap by picking your denominator. When two values genuinely have no privileged order, the symmetric figure is the honest one.
A related idea is worth keeping separate from all of this: a change measured in percentage points, which is the plain arithmetic gap between two percentages rather than a proportion of either one.