Step 01
Understand the formula.
Percentage difference uses the average magnitude of the two values as the denominator, so neither value is treated as the baseline. This makes the result symmetric: comparing 100 with 80 gives the same percentage difference as comparing 80 with 100.
The formula uses absolute values so the denominator stays positive even when one input is negative.
- Absolute difference = |first value − second value|.
- Average magnitude = (|first value| + |second value|) ÷ 2.
- Percentage difference = absolute difference ÷ average magnitude × 100.
Step 02
Worked example: 100 and 80.
The absolute difference is 20. The average magnitude is 90. The percentage difference is 20 ÷ 90 × 100 = 22.2%.
Because the result is symmetric, it does not say whether the second value is higher or lower; the direction label uses the two entered values.
Step 03
Know when to use percentage change instead.
If one value is clearly the starting point—such as last year's revenue compared with this year's—use percentage change, which divides by the old value. Percentage difference is for comparing two measurements without a designated baseline.
Mixing the two methods is a common reporting mistake because they produce different numbers.
Step 04
Handle zero and opposite signs explicitly.
If both magnitudes are zero, the average is zero and the percentage difference is undefined; the calculator reports 0% for that boundary. If the two values have opposite signs, the average magnitude can be small and the percentage difference very large, so interpret the absolute difference as well.
Record the direction and context in the worksheet rather than relying on one percentage alone.
Free reusable resource
Keep a practical worksheet.
Download a blank, editable file and keep the plan with your own records. No email address is required.
Percentage difference worksheet
Record both values, the absolute difference, average magnitude, percentage difference, direction and context.