Practical guide

How to Calculate Percentage Difference

Measure relative difference between two values using midpoint normalization.

Quick Answer

Percentage difference = |A − B| ÷ ((A + B) ÷ 2) × 100. It divides the gap between two values by their average, which makes it symmetric — you get the same answer regardless of which value you list first. This is different from percentage change, which divides by the original value.

Step-by-Step Method

Find the absolute difference between the two values, ignoring which is larger.
Calculate the midpoint by averaging the two values: (A + B) ÷ 2.
Divide the difference by the midpoint and multiply by 100.
Confirm percentage difference is the right metric — if one value is genuinely a starting point and the other a result, use percentage change instead.

Worked Example

Comparing 40 and 60: difference = 20, midpoint = (40 + 60) ÷ 2 = 50, so percentage difference = 20 ÷ 50 × 100 = 40%. Compare that with percentage change: going from 40 to 60 is a 50% increase, while going from 60 to 40 is a 33.3% decrease. Percentage difference returns 40% either way.

Detailed Explanation

Symmetry is the whole reason this metric exists. When neither value has a claim to being the reference point — two lab instruments measuring the same sample, two suppliers quoting the same part, two regions reporting the same statistic — using either one as the denominator would be an arbitrary choice that changes the answer. Averaging the two removes that arbitrariness, which is why the formula is standard in measurement and quality-control work.

The trade-off is that percentage difference has no direction. It tells you the values differ by 40% but not which is larger, and it cannot express growth, decline, or any before-and-after relationship. Reporting a sales figure as a percentage difference from last quarter is a category error — there is a clear original value there, so percentage change is the correct and more informative metric.

Two practical cautions. The formula behaves badly when values are close to zero or have opposite signs: the midpoint can approach zero and inflate the result toward infinity, or land at zero and make it undefined. It also loses meaning at large separations — the maximum possible percentage difference between two positive numbers approaches 200%, when one is near zero, so a figure like 180% signals values on entirely different scales rather than a meaningful comparison.

Use the Calculator

Run your real values in the interactive tool and review assumptions before taking action.

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FAQ

What is the difference between percentage difference and percentage change?

Percentage change divides by the original value and shows direction, so it suits before-and-after comparisons. Percentage difference divides by the average of both values and is symmetric, so it suits comparing two independent measurements where neither is the baseline.

Can percentage difference exceed 100%?

Yes. For two positive numbers it can approach 200% as one nears zero. Values above roughly 100% usually indicate the two figures are on quite different scales.

Which one should I use to compare two prices?

If you are comparing two competing quotes, percentage difference is appropriate since neither is a baseline. If you are comparing a new price against a previous price, use percentage change.

Related Guides

How to Calculate Percentages