Quick answer
Percent difference = |a − b| ÷ ((a + b) ÷ 2) × 100. For 10 and 20 it is 10 ÷ 15 × 100 = 66.67%.
Enter two values. The calculator returns the percent difference.
Related tools: Percent Increase Calculator: Percentage increase between two values. Percentage Change Calculator: Find the percentage increase or decrease from an old value to a new value. Percent Decrease Calculator: Percentage decrease between two values.
How to use the Percent Difference Calculator
- Enter value 1.
- Enter value 2.
- Press Calculate to see the result, and use Reset to start again.
| Input | What to enter | Allowed values |
|---|---|---|
| Value 1 | A number | — |
| Value 2 | A number | — |
Formula and method
Formula
percent difference = |a − b| ÷ ((a + b) ÷ 2) × 100
IEEE-754 double precision; shown to at most 6 decimal places.
The difference is divided by the mean of the two values, so the order does not matter.
Worked examples
These examples are verified: each one is an automated test case that the live calculator must reproduce exactly.
Example 1
Inputs: Value 1 = 10; Value 2 = 20.
| Result | Value |
|---|---|
| Percent difference | 66.666667 % |
Example 2
Inputs: Value 1 = 5; Value 2 = 5.
| Result | Value |
|---|---|
| Percent difference | 0 % |
The first line is the headline result (percent difference). IEEE-754 double precision; shown to at most 6 decimal places.
Symmetric comparison
Percent difference uses the average of the two values as the base, so it doesn't depend on which value is first. Percent change uses the old value as the base: from 10 to 20 is a 100% increase, from 20 to 10 a 50% decrease.
When to use it
Use it to compare two values when neither is a reference, such as two measurements or two prices. If one is a known true value, use percent error. Both values 0, or values that sum to 0, are undefined.
How the result changes with value 1
Holding every other input at the first worked example, this table changes only value 1. It shows how sensitive the answer is to that one value; each row was produced by running this calculator.
| Value 1 | Percent difference |
|---|---|
| 5 | 120 % |
| 10 | 66.666667 % |
| 20 | 0 % |
| 40 | 66.666667 % |
What happens with invalid input
These messages are produced by the calculator itself, so you know what to fix.
- Value 2: entering "0" shows "The values must not sum to 0." beside that field and no result.
Choosing between the Percent Difference Calculator and similar calculators
Each row shows what a related calculator on this site asks for and what it returns, so you can pick the one that matches your question.
| Calculator | You enter | It returns |
|---|---|---|
| [Percent Increase Calculator](/math/percent-increase-calculator/) | original value and new value | Percentage increase |
| [Percentage Change Calculator](/math/percentage-change-calculator/) | old value and new value | Percentage change |
| [Percent Decrease Calculator](/math/percent-decrease-calculator/) | original value and new value | Percentage decrease |
| [Percentage Calculator](/math/percentage-calculator/) | what do you want to find?, a and b | 15% of 200 |
| [Percent Off Calculator](/math/percent-off-calculator/) | original price and discount | Sale price |
Background: rounding and basic arithmetic
Rounding to a place looks at the digit just to its right: 0 to 4 rounds down and 5 to 9 rounds up (the half-up convention, so 2.5 becomes 3). Rounding 838.274 gives 800 to the nearest hundred, 840 to the nearest ten and 838 to the nearest one.
Some systems use banker's rounding (half to even), which sends 2.5 to 2 and 3.5 to 4; check which convention your task requires, and round only at the end of a multi-step calculation.
Assumptions and limitations
- Both values 0, or values that sum to 0, are undefined
If a required value is empty, not a number, or outside its allowed range, the form marks that field with a message and shows no result instead of a misleading number. The automated checks deliberately try invalid entries for value 2 to confirm this.
Frequently asked questions
Is percent difference the same as percent change?
No. Percent difference uses the average of the two values as the base; percent change uses the old value.
What if the values are equal?
The difference is 0%.
Can it exceed 100%?
Yes, up to 200% for positive values.
What does the Percent Difference Calculator calculate?
Symmetric percent difference between two values. It returns Percent difference.
What formula does the Percent Difference Calculator use?
percent difference = |a − b| ÷ ((a + b) ÷ 2) × 100. The difference is divided by the mean of the two values, so the order does not matter.
What do I need to enter in the percent Difference Calculator?
You enter Value 1 and Value 2.
How is the result rounded or limited?
IEEE-754 double precision; shown to at most 6 decimal places. Both values 0, or values that sum to 0, are undefined.
What is an example of a percent Difference result?
With value 1 10 and value 2 20, the percent difference is 66.666667 %.
Related calculators
- Percent Increase Calculator — Percentage increase between two values.
- Percentage Change Calculator — Find the percentage increase or decrease from an old value to a new value.
- Percent Decrease Calculator — Percentage decrease between two values.
- Percentage Calculator — Calculate a percentage of a number, what percent one number is of another, or the whole from a part.
- Percent Off Calculator — Find the sale price and savings after a percentage discount.
- Percent Error Calculator — Compare a measured value with the true value.
- Decimal to Percent Calculator — Convert a decimal to a percentage.
- Percent to Fraction Calculator — Convert a percentage to a simplified fraction.
Sources
- OpenStax, Prealgebra 2e (Rice University): fractions, decimals, percents, ratios and rounding
Content last reviewed 2026-11-04. Formulas and worked examples are checked by automated tests against independently computed values.