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Percentage Difference Calculator

Compare two values with the symmetric percentage difference formula |a - b| / average, plus directional percent change and ratio. Picks the right denominator so you stop conflating the two.

Compare two values

Percentage difference

40%

Absolute difference
50
Average of A and B
125
Percentage difference
40%
Percent change A to B
+50%
Percent change B to A
-33.3333%
Ratio (larger:smaller)
1.5:1
A greater than B
No
Exact 2x relationship
No

Symmetric percentage difference uses |a - b| divided by the average ((a + b) / 2). It is order-independent: swapping a and b gives the same result (40%). Use this when comparing two equivalent measurements where neither is a baseline.

Common confusion: a price moving from $100 to $150 is a 50% increase (percent change), but the percentage difference between $100 and $150 is 40% (50 / 125). Going from $150 back to $100 is a 33.33% decrease, not a 50% decrease. The two numbers differ because percent change uses the starting value as the denominator, while percent difference uses the average of both values.

Frequently Asked Questions about the Percentage Difference Calculator

What is the difference between percentage difference and percentage change?
Percentage difference is symmetric and uses the absolute average as the denominator: |a - b| / |(a + b) / 2| x 100. Percentage change is asymmetric and uses the starting value: (b - a) / a x 100. From 100 to 150, percentage difference is 40%, while percentage change is 50%.
When should I use percentage difference vs percentage change?
Use percentage difference when you are comparing two equivalent measurements with no baseline, like two lab results, two sensor readings, or two companies' revenue. Use percentage change when you have a clear before-and-after, like a stock price moving over time, an A/B test variant vs control, or a salary raise. The first question is "how far apart are these?"; the second is "how much did this move?"
Why is the percent change from A to B not the same as from B to A?
Because the denominator changes. Going from 100 to 150 is a 50% increase (50 / 100), but going from 150 back to 100 is only a 33.33% decrease (50 / 150). Each direction uses its own starting value as the base, and the bases are different. This is exactly why percent change is the wrong tool when neither value is a baseline: it picks one side arbitrarily and the answer flips when you swap them.
What is the most common math error with these formulas?
Treating percentage difference and percentage change as interchangeable. People will compute (b - a) / a for two unordered values, then call the result a "difference" and present it as symmetric. It is not. If your audience could sensibly swap a and b without changing the meaning of the comparison, you need the symmetric formula. A second common error is forgetting to take the absolute value of (a - b) for percentage difference; the symmetric form is always non-negative by definition.
Why is percentage difference symmetric?
The formula uses |a - b| in the numerator and the absolute average of a and b in the denominator. Swapping a and b changes neither value, so the result stays the same. Use this symmetry when neither measurement is a baseline. Use percentage change when one value is explicitly the starting point.

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