They look similar and get swapped constantly, but they answer different questions. The deciding factor is simple: is one number a starting point, or are the two numbers just being compared?
Percentage change divides by the original baseline; percentage difference divides by the average of two values. Learn when each applies, plus percent error and relative variance for data work.
See also: Percentage calculator · Formula cheat sheet
Ask: does one value come before the other?
| Percentage change | Percentage difference | |
|---|---|---|
| Formula | (New − Old) ÷ Old × 100 | |A − B| ÷ ((A + B) ÷ 2) × 100 |
| Divides by | the original value | the average of both values |
| Sign | + up, − down | always positive |
| Symmetric? | No | Yes |
Compare 40 and 60.
The direction matters for change (50% up is not the mirror of 33.3% down) but is irrelevant for difference.
A stock that rises 50% then falls 50% does not return to where it started: 100 → 150 → 75. The second percentage acts on a bigger base than the first. This is why "up 50%, down 50%" leaves you down 25% overall — a trap worth remembering whenever gains and losses are chained.
In science and measurement, percent error is just percentage change where the accepted (true) value is the baseline:
Percent error = |Measured − Accepted| ÷ Accepted × 100
A pendulum experiment measures g = 9.8 m/s² against the accepted 9.81: |9.8 − 9.81| ÷ 9.81 = 0.10%. You divide by the accepted value, never by the measurement, because the accepted value is the reference point.
When you compare a result against a plan or forecast, the plan is the baseline, so you use percentage change: (Actual − Forecast) ÷ Forecast × 100. Reserve percentage difference for comparing two peers where neither is the "truth" — for example, agreement between two independent instruments. Choosing the wrong denominator quietly distorts every figure in the report.
Percentage change has a clear before-and-after and divides by the original value, so it's directional. Percentage difference compares two values with no natural order and divides by their average, so it's always positive and symmetric.
Always the original (older or starting) value: (New - Old) / Old x 100. Dividing by the new value gives a different, incorrect result.
Because the second percentage applies to a larger amount. 100 rises 50% to 150, then falls 50% of 150 (which is 75), leaving you at 75 — a net 25% loss.
Yes, it's percentage change with the accepted or true value as the baseline: |Measured - Accepted| / Accepted x 100.
Links point to primary sources and standards bodies. Tax rates and official formulas change over time — verify against the source for current figures.