How to calculate percent difference
Percent difference measures how far apart two values are relative to their average: |a − b| ÷ ((a + b) ÷ 2) × 100. For 40 and 60, the absolute difference is 20 and the average is 50, so the percent difference is 20 ÷ 50 × 100 = 40%.
Work it in three steps. Subtract to get the gap and drop any minus sign, add the two values and halve them to get the reference average, then divide and multiply by 100. Both values should be in the same units and positive. When one of the two values is clearly the starting point, what you actually want is the percentage change calculator.
Percent difference vs. percent change
Percent difference is symmetric — it compares two values without treating either as the starting point. Percent change, by contrast, measures movement from an original value to a new one. If you need a directional figure, look at percentage change tools instead. See also the proportion calculator for related ratio work.
The two answers differ noticeably. Going from 40 to 60 is a 50% increase, while going from 60 to 40 is a 33% decrease — yet the percent difference between them is 40% either way. Use percent difference when comparing two independent measurements, such as readings from two instruments.
How to use this calculator
Enter the two values you want to compare in fields A and B. The result is the percent difference along with the absolute gap and the average used in the calculation. Order doesn't matter — swapping A and B gives the same answer.
Because there is no fixed baseline, the figure says nothing about direction. Comparing a measurement against a known true value instead is the job of the percent error calculator.
Frequently asked questions
- What is the percent difference between 40 and 60?
- It is 40%. The gap of 20 divided by the average of 50, times 100, equals 40%.
- Does it matter which value I put first?
- No. Percent difference uses the absolute gap and the average, so swapping the values gives the same result.
- How is percent difference different from percent change?
- Percent difference is symmetric between two values; percent change measures movement from a specific starting value.
- Why does it divide by the average?
- Dividing by the average treats both values fairly, since neither is taken as the fixed reference point.
- Can percent difference be more than 100%?
- Yes. Once one value is more than three times the other, the gap exceeds their average and the result passes 100%. The maximum approaches 200%, which is reached only when one of the two values is zero.