Percentage Difference Calculator
Compare two values symmetrically — the percentage difference divides by their average, so neither one is treated as the baseline.
Percentage difference and percentage change answer different questions, and mixing them up is the most common error on this topic.
How the percentage difference calculator works
Percentage difference and percentage change answer different questions, and mixing them up is the most common error on this topic.
Percentage change has a direction: it asks how much a value moved from an old figure to a new one, dividing by the old. Going 40 → 50 is a 25% increase; going 50 → 40 is a 20% decrease. Same pair, different answers.
Percentage difference has no direction. It divides by the average of the two, so comparing 40 and 50 gives 22.22% whichever way round you enter them. Use it when neither value is "the original" — two measurements, two products, two labs.
Formula: |a − b| / ((a + b) / 2) × 100
Worked examples
| Inputs | Percentage difference | Note |
|---|---|---|
| 40 compared with 50 | 22.2222% | 22.22% |
| The same pair reversed | 22.2222% | still 22.22% |
| Two close measurements | 0.102% | 0.102% |
FAQFrequently asked questions
What is percentage difference?
The gap between two values as a percentage of their average — a symmetric comparison with no baseline.
How is it different from percentage change?
Percentage change divides by the starting value and has a direction. 40 → 50 is +25%, 50 → 40 is −20%. Percentage difference gives 22.22% either way.
Which should I use?
Percentage change when one value came first. Percentage difference when the two are just two measurements and neither is the original.
Can it exceed 100%?
Yes, up to 200% as one value approaches zero — which is why it is unreliable when the two are far apart.
What if one value is negative?
The formula stops being meaningful once the average crosses zero; compare the raw difference instead.
Where these figures come from
- NIST Digital Library of Mathematical Functions — reference definitions for elementary and special functions
- Wolfram MathWorld — definitions and formulas for every topic on this page
- NIST/SEMATECH e-Handbook of Statistical Methods — the statistical formulas (mean, variance, z, confidence intervals, sample size)
- Australian Curriculum (ACARA) — Mathematics — the terms and methods taught in Australian schools
Last checked: September 2026. Formulas are fixed by mathematics and do not change with tax years or regulations.