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Percent Difference Calculator

Step 1 / 3 Percent difference
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How far apart are two measurements when neither one is the “right” answer? Percent difference compares two values symmetrically - it divides the absolute gap between them by their average, so swapping the order gives the same result. That makes it the tool of choice for lab repeats, A/B readings, and any pair where neither number is a baseline. Enter both values and this calculator returns the percent difference, the raw absolute difference, and the average it used - with the exact formula on display.

How to calculate percent difference

  1. 1

    Enter the first value

    One of the two numbers you want to compare. Order does not matter.

  2. 2

    Enter the second value

    The other number in the pair - it is treated as an equal, not a baseline.

  3. 3

    Read the result

    Percent difference, absolute difference, and the average appear instantly with the formula shown.

The formula

percent_difference = |a - b| / ((a + b) / 2) x 100

The denominator is the average of the two values, which is what makes percent difference symmetric: a vs b gives the same answer as b vs a. There is no “old” or “new” number here - both are equal partners. This is the key distinction from percentage change, which always measures movement away from a single reference value.

Worked example

Compare 80 and 100.

  • Average = (80 + 100) / 2 = 90
  • Absolute difference = |80 − 100| = 20
  • Percent difference = 20 / 90 x 100 ≈ 22.22%

Note that the percentage change from 80 to 100 is 25% (20 / 80), and from 100 to 80 it is −20% (20 / 100). Percent difference splits the difference and reports one symmetric figure: 22.22%.

Percent difference vs percent change

Concept Formula Use when
Percent difference ` a - b
Percent change `(new - old) / old
Percent error ` measured - true

Pitfalls people actually hit

  • Confusing it with percent change. If one number is genuinely a starting point, use percent change instead - percent difference will understate the move because it divides by the larger average.
  • Both values negative or mixed signs. When a + b is near zero the average collapses and the percentage explodes or becomes meaningless. Check that an average makes physical sense for your data.
  • Average of zero. If the two values are equal and opposite (e.g. 5 and −5), the average is 0 and percent difference is undefined; this calculator returns 0 in that edge case.
  • Reporting too many digits. Percent difference inherits the precision of your inputs - rounding to 2 decimals is usually honest enough.

Frequently Asked Questions

Percent change needs a baseline (an old value) and tells you direction - up or down. Percent difference treats both numbers as equals, divides by their average, and is always positive. Swapping the order changes a percent change result but never a percent difference.

No. Because the formula uses the absolute difference over the average, comparing a to b gives exactly the same percent difference as comparing b to a.

The percent difference is mathematically undefined when the average is zero (for example, values of 5 and −5). The calculator returns 0 in that case rather than dividing by zero.

They are used to update the calculation and, in the step-by-step version, may appear in the page URL so you can move between steps. Avoid entering confidential data.

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