Percent Difference Calculator
Calculate the percentage difference between two values relative to their average
By Waqar Mushtaq · Last updated:
Percent difference tells you how far apart two numbers are, as a percentage of their average. It's the right comparison when neither value is the starting point or the correct answer -- two lab measurements of the same quantity, the price of the same item at two stores, two cities' populations -- because it gives the same answer whichever number you list first. This calculator returns the percent difference, the plain difference, and the average it's measured against, and puts it side by side with the percent change in each direction so you can see exactly why the three numbers disagree and pick the one that fits your question.
How Percent Difference Is Calculated
O(1) calculationA, B = the two values being compared
Take the absolute difference between the two values, divide it by their average, and multiply by 100.
Example: two stores sell the same item for $40 and $50.
- Difference: |40 − 50| = 10
- Average: (40 + 50) / 2 = 45
- Percent difference: 10 / 45 × 100 = 22.22%
Compare that with percent change, which divides by one of the two values instead of their average:
- From $40 to $50 is a +25% change (10 / 40)
- From $50 to $40 is a −20% change (10 / 50)
Percent difference sits between the two and doesn't depend on order. Because the denominator is an average, the result can't exceed 200% -- that's reached when one of the values is 0. This calculator averages the absolute values, so two numbers of opposite sign (like −5 and 5) still produce a meaningful result rather than dividing by zero.
Note that some textbooks use the phrase "percent difference" loosely to mean percent change or percent error. If one value is a known correct value, use the Percent Error Calculator; if one is a starting point, use the Percent Change Calculator.
Use Cases
Comparing Two Lab Measurements
When two trials or two instruments measure the same quantity and neither is the accepted value, percent difference quantifies how well they agree -- a common requirement in physics and chemistry lab reports.
Price Comparison
Compare the price of the same product at two retailers, or two quotes for the same job, without implying that either one is the baseline.
Comparing Populations, Salaries, or Statistics
Two cities' populations, two job offers' salaries, or two survey results can be compared symmetrically -- the answer is the same regardless of which you mention first.
Checking Agreement Between Estimates
Engineers and analysts use percent difference to check whether two independent estimates (two models, two methods) agree within a tolerance, such as "within 5%."
Frequently Asked Questions
What is the difference between percent difference and percent change?
Percent change divides by the starting value, so it has a direction and depends on which value came first (40 to 50 is +25%, but 50 to 40 is −20%). Percent difference divides by the average of the two, so it has no direction and gives the same result either way (22.22% for 40 and 50).
What is the difference between percent difference and percent error?
Percent error compares a measured value against a known, accepted value and divides by the accepted value. Percent difference compares two values when neither is known to be correct, and divides by their average.
Can percent difference be more than 100%?
Yes, up to a maximum of 200%. For example, 10 and 30 have a difference of 20 and an average of 20, which is a 100% difference; 0 and any nonzero number is a 200% difference. It can never exceed 200% because the difference between two values can never be more than twice their average.
Is percent difference ever negative?
No. It uses the absolute difference, so it's always zero or positive. If you need a signed result showing which value is larger, use percent change instead.
Methodology
The formula above is implemented as a standalone module covered by unit tests, so its worked examples are re-checked on every change to the site. It is verified against the authoritative sources listed below.
Maintained by Waqar Mushtaq. Spotted a result that looks wrong? Report it and it becomes a failing test.