Percentage Difference Calculator
Enter two values to find how different they are as a percentage of their average. Unlike percentage change, the result is symmetric: it does not matter which value you enter first. This makes it the right tool for comparing two measurements, two prices or two results that do not have a natural "before" and "after" relationship.
Enter two values
Enter two values, then press Calculate difference.
The percentage difference formula
percentage difference = |a − b| / ((a + b) / 2) × 100
The numerator is the absolute difference between the two values; the denominator is their arithmetic mean. Dividing by the mean (rather than by one of the values) is what makes the result symmetric — the answer is the same whether you enter 80 and 100 or 100 and 80.
| Symbol | Meaning |
|---|---|
a, b | The two values being compared |
|a − b| | Absolute difference between the values (always ≥ 0) |
(a + b) / 2 | The average (mean) of the two values |
Use percentage difference when neither value is naturally the "starting point" — for example, comparing two people's heights, two thermometers' readings, or two suppliers' quotes. Use percentage change instead when there is a clear before-and-after relationship.
Worked examples
Example 1 — Two measurements
Value A = 80, value B = 100.
Difference = |80 − 100| = 20. Mean = (80 + 100) / 2 = 90.
Percentage difference = (20 / 90) × 100 ≈ 22.22%.
Result: 22.22%
Example 2 — Order does not matter
Value A = 100, value B = 80.
Difference = |100 − 80| = 20. Mean = (100 + 80) / 2 = 90.
Percentage difference = (20 / 90) × 100 ≈ 22.22%.
Result: 22.22%
Same result as Example 1 — this is the key property of percentage difference.
Example 3 — One value is zero
Value A = 0, value B = 50.
Difference = |0 − 50| = 50. Mean = (0 + 50) / 2 = 25.
Percentage difference = (50 / 25) × 100 = 200%.
Result: 200%
Interpreting the result
- 0% — the two values are identical.
- Small percentage (e.g. under 5%) — the values are close together; useful for spotting measurement agreement or small discrepancies.
- Large percentage (e.g. over 100%) — one value is much larger than the other; the difference exceeds the average, so it usually signals a substantial gap.
- Percentage difference is not bounded at 100%. Unlike percentage of a whole, there is no upper limit — for example, comparing 1 and 100 gives roughly 197.98%.
Common mistakes
- Using it when there is a clear "before" and "after". If one value is genuinely the starting point (a price last month, a baseline reading), use percentage change instead — it is the correct directional measure.
- Confusing it with "percentage of". Percentage difference describes how far apart two values are relative to their average; "percentage of" describes a part of a whole. They answer different questions.
- Expecting a sign. The result is always non-negative. Direction is not encoded — if you need to know which value is larger, check the inputs directly.
- Using the difference divided by one of the values. That is percentage change in one direction, not percentage difference. Percentage difference always divides by the mean of the two values.
Related calculators
Frequently asked questions
Percentage difference = |a − b| / ((a + b) / 2) × 100. The numerator is the absolute difference between the two values; the denominator is their average.
Percentage change uses the original value as the denominator and is directional. Percentage difference uses the average of the two values and is symmetric — swapping the two inputs gives the same result.
No. Percentage difference is symmetric by design, so entering 80 and 100 gives exactly the same result as entering 100 and 80.
Yes. There is no upper limit. For example, comparing 1 and 100 gives approximately 197.98% because the difference (99) is nearly twice the average (50.5).
The calculation still works as long as the two values do not average to zero. If both values are zero, the denominator becomes zero and the calculator returns an error.