Percentage Difference Calculator
Enter two values and get the percentage difference — a symmetric comparison that treats neither number as the reference, so swapping the inputs never changes the answer.
Two values in, one percentage out
Enter both values and the calculator returns the percentage difference: the absolute gap between them divided by their average, times 100.
Not the same as change
Percentage difference is symmetric. If you have a clear before-and-after value, you want percentage change instead.
What is a percentage difference calculator?
A fair, symmetric comparison of two numbers
A percentage difference calculator measures how far apart two values are, relative to their average, and reports it as a percentage. Unlike percentage change, it treats both numbers as equal partners: there is no "old" value and no "new" value, just two figures of the same standing. That makes it the right tool when neither number is the obvious reference — two lab readings of the same quantity, prices at two shops, or the outputs of two methods. Because it divides by the average of the two values rather than by one of them, swapping the inputs leaves the result unchanged.
Enter your two values to get the percentage difference instantly — the order you type them in does not matter.
One formula: divide the absolute gap by the average, then turn it into a percentage.
%diff = |a − b| ÷ ((a + b) ÷ 2) × 100The bars around a − b mean absolute value, so the gap is always positive no matter which value is larger. The denominator (a + b) ÷ 2 is simply the average of the two numbers. Because both the gap and the average ignore the order of the inputs, the whole expression is symmetric — comparing 40 with 60 gives the same result as comparing 60 with 40.
Suppose you are comparing the values 40 and 60.
Find the absolute gap
The absolute gap |40 − 60| = 20 — the distance between the two values, always taken as a positive number.
Find their average
(40 + 60) ÷ 2 = 50 — the midpoint the difference is measured against.
Divide and convert to a percentage
20 ÷ 50 × 100 = 40 — the two values differ by 40 % relative to their average.
The single percentage tells you how far apart two numbers sit relative to their own average, and the key insight is that it is symmetric: there is no reference value, so swapping the inputs never changes the answer. This is exactly what you want when neither figure is privileged — two thermometer readings, two price quotes, two survey results. A result of 0 % means the values are identical; small percentages mean they are close; and the measure can climb toward 200 % when the two values are far apart in size. That symmetry is also why percentage difference reads lower than percentage change for the same pair: 40 and 60 differ by 40 % here, because the gap of 20 is measured against the average of 50, whereas going from 40 up to 60 is a 50 % change measured against the starting value of 40. If you actually have a clear before-and-after — a price that rose, a population that grew — reach for percentage change instead, because direction matters there. Use percentage difference whenever both numbers stand on equal footing and you simply want a fair measure of how far apart they are.
The formula is exact, but a couple of cases deserve attention.
Undefined when the average is zero, and not a substitute for change
When the two values are equal in size but opposite in sign — for example 5 and −5 — their average is zero, the formula divides by zero, and the result is undefined; the calculator returns no value rather than an infinite one. Percentage difference is also not the same as percentage change: it is symmetric and has no direction, so it cannot tell you whether a value rose or fell. If you need that, use a percentage change calculator instead.