Look for an original value
Percentage change answers an old-to-new question. The formula is (new − original) ÷ |original| × 100. Because the original value is the denominator, moving from 80 to 100 is a 25% increase, while moving from 100 to 80 is a 20% decrease.
Use difference when neither value comes first
Percentage difference compares two measurements without calling either one original. The formula is |A − B| ÷ [(|A| + |B|) ÷ 2] × 100. Swapping A and B leaves the answer unchanged because the comparison uses their average magnitude.
Handle zero before interpreting the result
Percentage change from an original value of zero is undefined because its denominator is zero. Percentage difference between 0 and 0 is also undefined because the average magnitude is zero. With one value at zero and the other nonzero, the percentage difference is 200%; that result follows the symmetric formula and should not be described as an old-to-new growth rate.
Match the method to the real question
Use change for a price moving over time, a salary before and after a raise, or an investment balance with a known starting point. Use difference for two independent quotes, laboratory measurements, forecasts, or estimates when neither observation is the baseline. If the real question is simply what portion one value represents of a total, use the part-to-whole percentage mode instead.
Example: 80 and 100 produce two valid answers
From 80 to 100, percentage change is (100 − 80) ÷ 80 × 100 = 25% increase. As two peer measurements, percentage difference is |100 − 80| ÷ 90 × 100 = 22.2222%. The numbers are the same, but the question and denominator are different.
What to check before you decide
- State the baseline in words whenever you report percentage change.
- Negative values can be calculated with an absolute original or average-magnitude convention, but their real-world interpretation may need domain-specific context.
Sources behind this guide
These official and primary sources let you verify rules, definitions, and terms that may change.
NIST — The percent symbol represents one part in one hundred↗NIST Dataplot — Percent difference↗