Percent Difference Calculator
Compare two numbers when neither one is the original.
Details
The order does not matter — swapping them gives the same answer
Percent difference
40%
A gap of 25 against an average of 62.5
If the first came first
50%
If the second came first
-33.33%
Those last two are percent change, and they differ from each other because each divides by a different starting value. Percent difference divides by the average instead, which is why it gives one answer rather than two.
This compares two numbers when neither one is the 'right' or 'original' value, and tells you how far apart they are as a percentage.
It also shows what the percent change would be in each direction, because that is the number people usually reach for by mistake.
What percent difference means
Percent difference tells you how far apart two numbers are, as a percentage, when neither of them is the starting point. Two lab readings, two quotes for the same job, two sensors measuring the same thing — none of those has a 'before' and an 'after'.
That is the whole reason it exists. If one number came first, you want percent change, which divides by that starting value. If neither did, dividing by either one would be an arbitrary choice, so you divide by the average of the two instead.
The practical effect is that percent difference gives one answer whichever way round you enter the numbers, and percent change gives two different answers depending on which you call the original.
Same two numbers, three different answers. Which one is correct depends entirely on whether either number came first.
What to enter
- First value
- One of the two numbers. There is no 'correct' one here, so it does not matter which you put first.
- Second value
- The other number. Swap the two and the percent difference is unchanged.
Which formula you actually want
- Percent change
- One value came first. Divide the gap by that original value. Prices, salaries, populations.
- Percent difference
- Neither came first. Divide the gap by the average of the two. Comparing two measurements or two quotes.
- Percent error
- One value is known to be correct. Divide the gap by that true value. Lab work against a published figure.
What this assumes
The answer is always positive. Percent difference measures size, not direction, so there is no such thing as a negative one.
Negative inputs are handled by their size, and two zeros leave nothing to divide by, so the result is undefined.
How to calculate the percent difference between two numbers
Three steps, and the middle one is the only thing that separates this from percent change.
- a, b
- The two values being compared, in either order.
- | |
- Absolute value — ignore the minus sign.
Find the gap. Subtract one value from the other and drop the minus sign. For 50 and 75 the gap is 25.
Find the average. Add the two values and halve them. (50 + 75) ÷ 2 = 62.5. This is the step that makes it a difference rather than a change.
Divide and multiply by 100. 25 ÷ 62.5 = 0.4, and 0.4 × 100 = 40%.
See a worked example: Two quotes for the same job
- First quote
- $50
- Second quote
- $75
Gap: 75 − 50 = 25.
Average: (50 + 75) ÷ 2 = 62.5.
25 ÷ 62.5 = 0.4, which is 40%.
The two quotes are 40% apart.
Frequently asked questions
- What is the difference between percent difference and percent change?
Percent change has a direction: one value is the starting point, and you divide the gap by it. Percent difference has no direction, so you divide by the average of the two values instead.
For 50 and 75, the percent difference is 40%, while the percent change is +50% going up and −33.33% coming back down. Same numbers, three answers.
- How do you calculate percent difference?
Subtract one value from the other and ignore the minus sign, then divide that gap by the average of the two values, then multiply by 100.
As a formula: |a − b| ÷ ((|a| + |b|) ÷ 2) × 100.
- Can percent difference be negative?
No. It measures how far apart two numbers are, not which is larger, so the answer is always zero or positive.
If you need to know the direction, you want percent change instead.
- Why divide by the average instead of one of the numbers?
Because choosing either number as the denominator would be arbitrary when neither is the baseline, and the two choices give different answers.
Dividing by the midpoint treats both values equally, which is why the result stays the same when you swap them.
- When should I use percent error instead?
When one of the two values is known to be correct — a published constant, a calibrated standard, an accepted result.
Then the true value is the right denominator, because the question is how far your measurement strayed from it. That is the percent error calculator.
- Is percent difference the same as percentage points?
No. Percentage points describe the gap between two percentages: a rate moving from 4% to 6% has risen by two percentage points, which is a 50% change and a 40% difference.
Percentage points are a subtraction. Percent difference is a ratio.
Problems people actually run into
Using percent change when nothing came first
Comparing two suppliers, two machines or two readings has no natural 'before'. Picking one as the baseline makes the gap look bigger or smaller depending on which you picked.
If swapping your two numbers would change the answer, and it should not, you want percent difference.
Expecting a fall and a rise to cancel out
A value that drops 50% needs to rise 100% to get back, because the second percentage is taken from a smaller base. This catches people out with investments and with discounts.
Percent difference sidesteps it entirely: 100 and 50 are 66.67% apart whichever direction you travel.
Reporting percent difference as a comparison over time
Sales this year against last year is a change, not a difference, and readers will assume you mean change.
Use difference only where the two values genuinely sit side by side rather than one after the other.
Results are estimates for general information only and are not professional financial, medical, or legal advice. Read our full disclaimer.
Last updated: September 12, 2026
