Math
Percentage Difference Calculator
Enter two values to compare them without either one being treated as the starting point.
The formula
difference % = |a − b| ÷ ((a + b) ÷ 2) × 100
Worked example
Two quotes, $40 and $60
- |40 − 60| = 20
- (40 + 60) ÷ 2 = 50
- 20 ÷ 50 × 100 = 40% difference
Where this goes wrong
Reaching for it when you meant percentage change
Difference compares two figures with no before and after, so it divides by their average and reads the same whichever order you enter them. Change has a direction and divides by the starting value. For 40 and 50 the difference is 22.2% while the change is 25% — same pair, different question. If one of your numbers came first in time, you want change.
Questions
- When should I use difference instead of change?
- Use percentage difference when neither value came first — two quotes, two measurements, two suppliers. Use percentage change when one value became the other over time.
- Why is 40 versus 60 a 40% difference and not 50%?
- Because the baseline is the average of the two, 50, not one of the values. Against 40 the gap is 50%, against 60 it is 33%, and the average settles it at 40% either way.