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Percentage Difference Calculator

Find the percentage difference between two values.

How it is calculated

Percentage difference compares two values without deciding which one is the starting point. The tool divides the absolute gap by the average of the two numbers: diff% = |a − b| / ((a + b) / 2) × 100, where a and b are the two values in the same unit. Because the average is the reference, swapping a and b gives the same answer, unlike percentage change, which uses the old value as the base. The result is always positive. If the two numbers average to zero (for example 5 and −5) the formula is undefined and nothing is shown. Use percentage change instead when one value is clearly the original and the other the new one.

Worked example

Compare 80 and 120. The absolute gap is |80 − 120| = 40. The average is (80 + 120) / 2 = 100. Divide: 40 / 100 = 0.4, which is a percentage difference of 40.00%. Notice that treating 80 as the starting value would give a 50% increase, and treating 120 as the starting value a 33.3% decrease; the percentage difference sits between the two.

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Common mistakes

  • Using it when one number is the original and the other the new value; a price rise or a drop is a percentage change, which gives a different figure.
  • Dividing by one of the numbers instead of the average, which gives 50% or 33.3% for 80 and 120.
  • Comparing values in different units, such as kilograms and grams, without converting first.
  • Expecting a sign; the result is always positive and does not say which value is larger.
  • Mixing it up with percentage points, which is the plain subtraction of two percentages.

About this calculator

This tool calculates the percentage difference between two numbers, expressing how far apart they are relative to a reference point rather than as a raw gap — useful because a difference of 10 units means very different things depending on whether the numbers being compared are around 20 or around 20,000. Note this is distinct from percentage change: percentage difference typically treats the two values symmetrically (often relative to their average), whereas percentage change measures growth or decline from a specific starting value.

It is used when comparing two measurements, prices, or results where neither one is clearly the "before" and the other the "after" — for example comparing two competing product prices, two survey results, or two lab measurements of the same quantity taken by different methods. Analysts and researchers reach for this specific calculation when they want a relative sense of how much two figures diverge without implying a directional trend between them.

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