Compare two values on equal footing. Percent difference divides the gap by the average of the two numbers, so it gives the same answer whichever order you enter them.
How to use the percent difference calculator
Enter any two values — prices from two shops, measurements from two instruments, results from two experiments — and the calculator gives the percent difference between them. It also shows the absolute difference, the average and, for comparison, the percent change in each direction so you can see why the three measures differ.
The percent difference formula
The top of the fraction is the size of the gap between the numbers. The bottom is their average. Because the average does not depend on which number you call A and which you call B, the result is symmetric: swapping the values never changes the answer. That is the key reason to use percent difference instead of percent change.
Worked example
A product costs 10 in one shop and 12 in another. The absolute difference is |10 − 12| = 2. The average is (10 + 12) ÷ 2 = 11. Divide: 2 ÷ 11 = 0.1818, so the percent difference is 18.18%.
Compare that with percent change. Going from 10 up to 12 is a 20% increase; going from 12 down to 10 is a 16.67% decrease. Percent difference (18.18%) sits between the two, because it uses the midpoint as the reference rather than either end.
Percent difference vs percent change vs percent error
| Measure | Divide by | Use when |
|---|---|---|
| Percent difference | Average of both values | Neither value is the reference — two labs, two brands, two samples |
| Percent change | The old (original) value | There is a clear before and after — prices over time, growth |
| Percent error | The true or accepted value | Comparing a measurement against a known correct value |
If you have a clear starting point, use the percentage increase calculator. If one value is the accepted "correct" answer, use the percent error calculator.
Interpreting the result
Percent difference ranges from 0% (identical values) up to a maximum of 200%, which occurs when one value is zero and the other is not. Some useful rules of thumb:
- Under about 5% — the values are close; in lab work two trials this close are often considered in agreement.
- 5–20% — a noticeable gap worth investigating, e.g. a meaningful price difference.
- Above 50% — the values are very different in scale; an absolute difference or ratio may communicate it better. The ratio calculator can express two numbers as a simple ratio.
A note on negative numbers
Percent difference is designed for positive quantities such as lengths, masses and prices. If one value is negative, the plain average can be zero or misleading, so this calculator uses the average of the magnitudes (absolute values). Results involving opposite signs should be read with care.
Where it is used
Scientists use percent difference to check whether two measurements of the same quantity agree. Shoppers and analysts use it to compare prices or performance between two options without favouring either. Quality engineers use it to compare readings from two gauges. In every case it answers the question "how far apart are these, relative to their typical size?"
Why not just divide by the bigger number?
Some people divide the gap by the larger value, or by the smaller one. Both are legitimate ways to describe a gap, but each answers a different question and gives a different number (16.67% and 20% in the example above). Percent difference, with the average as its base, is the convention in physics and chemistry labs, so use it whenever an instructor or standard asks for "percent difference" by name.
Cite or link to this page
Using this percent difference calculator in an article, report or homework? Copy a citation or a ready-made link:
Calcivia. (2026). Percent Difference Calculator. Retrieved October 31, 2026, from https://www.calcivia.com/percent-difference-calculator/"Percent Difference Calculator." Calcivia, 2026, https://www.calcivia.com/percent-difference-calculator/. Accessed 31 Oct. 2026.<a href="https://www.calcivia.com/percent-difference-calculator/">Percent Difference Calculator</a> by Calcivia