# Percent Difference Calculator

> Percent difference calculator compares two values to show relative difference with worked steps. Free, private, runs locally in your browser, no signup.

- **Canonical URL:** https://boringtoolskit.com/percent-difference-calculator/
- **Category:** Everyday Calculators
- **Evidence level:** authored-methodology
- **Reviewed:** 2026-09-20
- **Last verified:** 2026-10-12
- **Privacy:** Inputs and results stay in this browser unless the page clearly labels an external request.

## How it works

Percent difference = |A − B| ÷ ((A + B) ÷ 2) × 100. The denominator is the average of the two values, which makes the measure symmetric — swapping A and B changes nothing.

## Worked example

Two labs measure the same part as 180 mm and 220 mm. Difference is 40, average is 200, so percent difference = 40 ÷ 200 × 100 = 20.00%.

## Assumptions

- Both values are measurements of the same quantity with no known 'true' reference among them.
- The symmetric (average-denominator) convention is used, matching most statistics references.
- Both-zero input is reported as undefined rather than approximated.

## Common questions

### What is the percent difference formula?

Absolute difference divided by the average of the two values, times 100: |A − B| ÷ ((A + B)/2) × 100. Both values get equal weight.

### How is it different from percent change?

Percent change divides by the starting value and carries a sign (growth or decline). Percent difference divides by the average and has no direction — 180 vs 220 is 20.0% either way, while the change from 180 to 220 is +22.2%.

### When should I use percent error instead?

When one of the two numbers is a known or accepted value. Percent error = |measured − true| ÷ true × 100 — the denominator is the reference, not the average.

### Why is my result over 100%?

That is normal for values far apart. A = 0, B = 100 gives 200%. Percent difference is unbounded — it just measures relative separation.

### Does the order of inputs matter?

No. |A − B| and the average are both symmetric, so swapping the two values gives the identical result.

### Why do both zeros give an error?

The denominator is the average, which is zero — division is undefined. The calculator shows an explicit message instead of a fake number.

### Can I use it with negative numbers?

The math runs, but interpretation gets shaky — opposite-sign pairs can average near zero and explode the result. For signed quantities, percent change is usually the right measure.

## Related tools

- /percentage-calc/
- /percentage-change-calculator/
- /percent-difference-calculator/

## Sources

- [Calculator methodology and reference cases](https://boringtoolskit.com/methodology/)
- [Relative change and difference](https://en.wikipedia.org/wiki/Relative_change_and_difference)

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