Percentage Difference Calculator
Use this when neither number is "the original". It tells you how far apart two values are, as a percent of their average.
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The formula
Percentage difference = (|A - B| / ((|A| + |B|) / 2)) x 100
When to use percentage difference
Percentage difference is for comparing two things on equal footing: two shops quoting different prices, two lab measurements, or the heights of two buildings. Because there is no old and new, we divide by the average of the two numbers so that swapping them gives the same answer.
If one number really is the starting point, such as last year versus this year, use the percentage increase or decrease calculator instead.
A worked example
Two laptops cost 900 and 1,100. The gap is 200 and the average price is 1,000. 200 / 1,000 = 0.2, so they differ by 20%.
Compare that with percentage change from 900 to 1,100, which is 22.2%. The numbers are close but not equal, and mixing them up in a report is a common source of confusion.
Examples to check
Type these into the calculator above to see how it behaves. Every result below was produced by the same calculator code that runs on this page.
| What you enter | What you get |
|---|---|
| Percentage difference between X and Y; First value (X): 900; Second value (Y): 1100 | Percentage difference: 20% |
| Percentage difference between X and Y; First value (X): 50; Second value (Y): 75 | Percentage difference: 40% |
| Percentage difference between X and Y; First value (X): 3.2; Second value (Y): 3.8 | Percentage difference: 17.1429% |
| Percentage difference between X and Y; First value (X): 10; Second value (Y): 10 | Percentage difference: 0% |
Percentage Difference Calculator: frequently asked questions
Why divide by the average?
It keeps the answer the same whichever number you call A or B. If you divided by one of them, the result would change depending on which you picked.
Is percentage difference the same as percent error?
Not quite. Percent error compares a measurement to a known true value and divides by that true value. Percentage difference compares two values with no known truth.
How do I turn a percentage into a decimal?
Divide by 100, or move the decimal point two places to the left. So 15% is 0.15, 5% is 0.05 and 120% is 1.2. Multiplying by that decimal is exactly how every percentage calculation works.
Can a percentage be more than 100?
Yes. 150% of a number is one and a half times that number. Percentages above 100 show up all the time with growth: if sales went from 40 to 100, they grew by 150%.
Why does a 50% rise followed by a 50% fall not bring me back to where I started?
Because the second percentage is taken from a bigger number. Start at 100, rise 50% to 150, then fall 50% and you land on 75. The same trap catches people with investments and sale prices.
How accurate is this calculator?
It uses standard double-precision arithmetic, accurate to about 15 significant digits. Results are rounded only for display.
Last reviewed: October 3, 2026