Percent Difference Calculator

What is Percent Difference?

Percent difference measures the relative difference between two values without regard to which is larger. It uses the average of the two values as the reference point.

Formula: Percent Difference = |Value₁ - Value₂| ÷ ((|Value₁| + |Value₂|) ÷ 2) × 100%

Note: This is different from percent change, which uses one specific value as the reference (usually the original/starting value).



Worked example

Enter 50 as the first value and 75 as the second. Absolute difference: |50 - 75| = 25. Average of the absolute values: (50 + 75) / 2 = 62.5. Percent difference: 25 / 62.5 × 100 = 40%. Notice the result is symmetric — entering 75 first and 50 second gives the exact same 40%, because both the difference and the average treat the two values identically.

Why this differs from percent change

Percent change (used for things like "sales grew from 50 to 75") divides by the original/starting value: (75-50)/50 × 100 = 50%. Percent difference instead divides by the average of both values, and by design doesn't care which number came first or is "original." Using 50 and 75 through each formula gives two different, both-correct-for-their-purpose answers: 50% change if 50 is the baseline you're growing from, but 40% difference if you're just comparing two measurements with no inherent order — like two lab readings of the same quantity.

When to use which

Use percent change when one value is clearly the "before" and the other the "after" (revenue growth, weight loss, price increases). Use percent difference when comparing two independent measurements of the same thing with no natural starting point — comparing two thermometers' readings, or two survey results, where neither one is more "correct" than the other.

Constraints

If both values are exactly zero, the result is reported as 0% rather than the undefined 0/0 the formula would otherwise produce. Both inputs must be valid numbers.