Find the percentage difference between two numbers — a symmetric comparison that divides the absolute difference by the average of the two values. Enter any two numbers and the calculator returns the percentage difference and the plain difference between them.
The percentage difference calculator tells you how far apart two numbers are, expressed as a percentage of their average. Unlike percentage change, which measures how much one value grew or shrank relative to a starting point, percentage difference is a symmetric comparison: it treats both numbers equally and gives the same answer no matter which one you list first. Enter two values and the calculator returns the percentage difference and the plain (absolute) difference between them. It is the right tool whenever you want to compare two quantities of the same kind without treating either one as the "reference."
This page explains what percentage difference means, the exact formula the calculator uses, worked examples you can reproduce, how it differs from percentage change and percentage error, and answers to the questions people ask most. By the end you will know exactly when to reach for percentage difference and how to interpret the number it produces.
The percentage difference between two numbers is the absolute value of their difference, divided by the average of the two numbers, expressed as a percentage. Because it uses the average of the two values as the reference point — rather than one specific value — it is a fair, direction-free way to compare quantities when neither is more "correct" or "original" than the other. For example, comparing the heights of two people, the prices of two similar products, or the readings from two instruments are all situations where percentage difference makes sense: there is no natural starting value, just two numbers you want to compare. The result is always positive, and it is symmetric, meaning the percentage difference between A and B equals the percentage difference between B and A.
The formula the calculator uses is:
% difference = 100 × |V₁ − V₂| / ((V₁ + V₂) / 2)
In words: take the absolute difference between the two values (ignore the sign), divide it by the average of the two values, and multiply by 100 to get a percentage. The absolute value bars, written |…|, are what make the result symmetric and always positive — the comparison has no direction. The denominator, (V₁ + V₂) / 2, is simply the mean of the two numbers, and using it as the base is the defining feature that separates percentage difference from percentage change.
Suppose you want the percentage difference between 70 and 85. First find the absolute difference and the average:
|70 − 85| = 15 · (70 + 85) / 2 = 77.5
Then divide and convert to a percentage:
% difference = 100 × 15 / 77.5 ≈ 19.355%
So the percentage difference between 70 and 85 is about 19.355%, and the plain difference between them is 15. These are the calculator's default values, so you can press Calculate to confirm the result and then enter your own numbers. Notice that if you swap the inputs — 85 and 70 — you get exactly the same 19.355%, because the formula uses the absolute difference.
These two are often confused, but they answer different questions and use different formulas. Percentage change measures how much a value has increased or decreased from a specific starting point, so it divides the difference by the original value and it has a direction (positive for an increase, negative for a decrease). It is the right choice for things that evolve over time — a stock price, a population, a salary. Percentage difference, by contrast, compares two values symmetrically, dividing by their average, and is always positive with no direction. Use it when neither value is the "before" or the "after," just two comparable quantities. A quick way to remember it: percentage change asks "how much did it change?" while percentage difference asks "how far apart are they?"
The choice of denominator is what makes percentage difference special. If you divided the difference by just one of the two values, you would get different answers depending on which value you picked as the base — the comparison would no longer be symmetric. By dividing by the average of the two numbers, the formula treats both values as equally important, so the result does not depend on the order in which you enter them. This symmetry is exactly what you want when comparing two things of the same kind with no natural reference point. The trade-off is that percentage difference can behave oddly when the two numbers have very different magnitudes or opposite signs, which is why it is best used for comparing values that are reasonably close and of the same sign.
Percentage difference is also sometimes confused with percentage error, but they serve different purposes. Percentage error compares a measured or experimental value against a known, accepted "true" value, dividing the difference by that true value — so it has a clear reference and is used to judge accuracy. Percentage difference has no such reference; it simply compares two values on equal footing. If you are checking how close an experimental result is to a textbook value, use percentage error. If you are comparing two independent measurements or two comparable quantities where neither is the standard, use percentage difference. Choosing the correct one matters because they can give noticeably different numbers for the same pair of values.
Percentage difference is the right tool whenever you are comparing two values of the same type and neither is a baseline. Common situations include comparing the prices of two similar products to see how far apart they are, comparing measurements taken by two different instruments or observers, comparing the performance of two comparable machines or processes, or comparing statistics between two groups or regions. In science and engineering it is often used to express the agreement (or disagreement) between two experimental results. In everyday life it helps you make sense of comparisons like "this option is X% different from that one." Whenever the phrase "difference between A and B" fits better than "change from A to B," percentage difference is the measure you want.
Percentage difference is simple and useful, but it has limits. Because it divides by the average of the two values, it becomes unstable when the numbers are far apart: comparing 1 and 100 gives a percentage difference near 196%, which can feel counterintuitive. It also breaks down if the two values sum to zero (the average is zero and the formula divides by zero), and it can be misleading when the values have opposite signs. For values of very different magnitudes, percentage change or a simple ratio may communicate the comparison more clearly. As with any single number, the percentage difference is most meaningful when you understand the context of the values you are comparing — it summarizes the gap but not the reason for it.
Once you know what percentage difference measures, you start to see uses for it everywhere. Shoppers use it to compare the prices of two similar products and judge whether the gap is trivial or substantial — a 5% difference between two brands is minor, while a 40% difference is worth a second look. Home cooks and dieters compare the calorie or nutrient content of two foods. Fitness enthusiasts compare this month's performance with a friend's, where neither number is a "before." Investors compare two portfolios or two funds of similar size. In each case the two quantities are of the same kind and neither is a natural baseline, which is exactly the situation percentage difference is designed for. Expressing a comparison as a single percentage makes it easy to communicate: saying two options differ by 8% is far more intuitive than quoting both raw numbers and leaving the listener to work out how close they are.
In laboratories and workshops, percentage difference is a standard way to express how well two independent results agree. When an experiment is repeated, or when two different instruments, methods, or observers measure the same thing, the percentage difference between their readings quantifies the level of agreement. A small percentage difference suggests the two measurements are consistent and the method is reproducible; a large one signals that something — equipment, technique, or conditions — is causing a discrepancy worth investigating. Because it is symmetric, percentage difference is the honest choice here: neither measurement is assumed to be the "true" value, so neither is privileged as the denominator. This is precisely why it differs from percentage error, which is used only when one value is an accepted standard. Reporting the percentage difference between repeated trials is a routine part of writing up experimental results in physics, chemistry, biology, and engineering.
You can always work out a percentage difference without a calculator by following four short steps. First, subtract one value from the other and take the absolute value, so the result is positive regardless of order. Second, add the two values together and divide by two to get their average. Third, divide the absolute difference from step one by the average from step two. Fourth, multiply by 100 to turn the ratio into a percentage. For the numbers 40 and 60: the absolute difference is 20, the average is 50, dividing gives 0.4, and multiplying by 100 gives a 40% difference. Working through it by hand a few times builds the intuition that percentage difference grows both when the gap between the numbers widens and when the numbers themselves get smaller relative to that gap — the same absolute difference of 20 represents a much bigger percentage difference between 10 and 30 than between 1000 and 1020.
A few errors crop up regularly with percentage difference. The most common is dividing by one of the two values instead of their average, which produces percentage change, not percentage difference, and breaks the symmetry. Another is using percentage difference when one value really is a reference or starting point — in that case percentage change or percentage error is the correct measure. People also sometimes forget the absolute value and report a negative percentage difference, which is not meaningful, since the measure has no direction. Finally, watch out for the edge cases the formula cannot handle: when the two values are equal and opposite their average is zero and the result is undefined, and when the numbers have very different magnitudes the percentage difference can balloon past 100% in a way that may mislead a casual reader. Keeping the formula — absolute difference over average, times 100 — firmly in mind steers you clear of all of these traps.
Take the absolute difference between the two numbers, divide it by their average, and multiply by 100: % difference = 100 × |V₁ − V₂| / ((V₁ + V₂)/2). For 70 and 85 the result is about 19.355%.
Percentage change divides by the original value and has a direction (increase or decrease). Percentage difference divides by the average of the two values and is always positive and symmetric.
No. Percentage difference uses the absolute difference, so swapping V₁ and V₂ gives exactly the same result.
Yes. When the two numbers are far apart, the percentage difference can exceed 100% — for example, 1 and 100 differ by about 196%.
No. Because the formula uses the absolute value of the difference, the result is always zero or positive.
Avoid it when one value is a clear baseline (use percentage change), when comparing a measurement to a true value (use percentage error), or when the two numbers have opposite signs or sum to zero.
This Percentage Difference Calculator is provided for general informational and educational purposes. It computes the standard percentage difference using the absolute difference divided by the average of the two values. For comparisons involving a defined reference value, consider the percentage change or percentage error calculators instead.