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Percentage Difference Calculator

Compare two values symmetrically: percentage difference divides the gap by the average of the two, so it doesn't matter which number you call "first". The steps show why this differs from percentage change.

Difference betweenand

Percentage difference formula

Percentage difference is for comparing two values when neither is the "original" — two prices from different stores, two measurements, two cities. Take the absolute difference and divide by the average:

That's different from percentage change, which has a direction (old → new) and divides by the starting value. Between 40 and 60: the difference is 40% (20 over the average 50), while the change is +50% one way and −33% the other.

% difference = |a − b| ÷ ((a + b) ÷ 2) × 100

Worked examples

Example 1: comparing two prices

The same laptop costs $900 in one store and $1,100 in another.

  1. Absolute difference.
    |900 − 1,100| = 200
  2. Average of the two.
    (900 + 1,100) ÷ 2 = 1,000
  3. Divide and convert.
    200 ÷ 1,000 × 100 = 20%

A 20% difference between the stores.

Example 2: why it's symmetric

Compare 40 and 60 both ways.

  1. Difference over average.
    20 ÷ 50 × 100 = 40% either way
  2. Contrast with percentage change.
    40→60: +50%; 60→40: −33.3%

Percentage difference gives one answer (40%); change gives two, depending on direction.

Example 3: measurements in an experiment

Two instruments read 9.8 and 10.2.

  1. Difference and average.
    0.4 and 10.0
  2. Divide.
    0.4 ÷ 10 × 100 = 4%

A 4% difference between the readings.

FAQ

When should I use percentage difference instead of percentage change?+

Use difference when the two values are peers with no before/after order (two products, two labs, two regions). Use change when one value is the baseline — prices over time, growth, discounts.

Why divide by the average and not one of the values?+

Dividing by one value makes the answer depend on which you pick (50% vs 33.3% for 40 and 60). The average is the neutral reference, which is why scientific and engineering practice uses it.

Can percentage difference exceed 100%?+

Yes, up to 200% in the limit. Comparing 1 and 100: difference 99, average 50.5, so 196%. Values far apart push the ratio toward twice the gap.

What about percentage error?+

Percentage error divides by the TRUE or accepted value, not the average: |measured − true| ÷ true × 100. Use it when one number is the known correct one.

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