Everyday math
Compare two values using the percentage difference based on their average.
Enter your numbers to see the answer.
Percentage Difference Calculator guide
Percentage difference compares two values without treating either as the starting point. Instead of dividing by one of them, the calculator divides the gap by their average, so swapping the fields gives the same answer. It is the right measure when two lab groups report a result, two shops quote a price, or two runs of a test disagree and neither is “the original”. Contrast that with percentage increase or decrease, which need a before and an after, and with percentage error, which needs an accepted reference value. The result is always non-negative, since the calculator uses the absolute difference and the average of the absolute values. Because the denominator is the midpoint, the figure is always larger than the equivalent increase and smaller than the equivalent decrease between the same two numbers.
Percentage difference (%) = |first − second| ÷ ((|first| + |second|) ÷ 2) × 100.
Worked example with the defaults: first value 80, second value 100. The absolute difference is |80 − 100| = 20. The average of the two values is (80 + 100) ÷ 2 = 90. Divide and scale: 20 ÷ 90 × 100 = 22.22%, which the calculator displays as “22.22% difference” with a caption noting the average. For comparison, 80 → 100 is a 25% increase and 100 → 80 is a 20% decrease; the percentage difference sits between them at 22.22%.
Enter both values in the same unit; the unit cancels and the result is a percentage. Order does not matter, so put them in either field. Negative inputs are handled with absolute values, and the result is shown to two decimal places by default (22.22%), adjustable in the precision setting.
The result follows directly from the numbers you entered, so its accuracy is the accuracy of those inputs. The most common source of a surprising answer is a mismatch between the question you meant to ask and the one the calculator answered, so re-read the field labels if the figure looks wrong. The formula is shown above so you can check the arithmetic yourself. On this page the figure rests entirely on first value and second value, so start there if the percentage difference calculator returns something you did not expect.
Good to know: this is not the tool for “how much did it grow”. If one value came first in time, use percentage increase or decrease, which divide by the earlier value rather than the midpoint. If one value is a known standard, use percentage error. Reporting 22.22% as “a 22% increase” is wrong: the increase is 25%. Two values that are both zero give 0% by convention.
Results are planning estimates. Check the units, product coverage, carrier rules, or local rates before making a purchase or financial decision.
How it works
Compare two values using the percentage difference based on their average. This tool explains the calculation so you can adjust the assumptions to match your situation.
Worked example
With First value = 80 · Second value = 100 → 22.22 % difference (difference relative to the average of both values). Change an input above and this example updates with your numbers.
Common questions
22.22%. The gap is 20, the average of the two numbers is 90, and 20 ÷ 90 × 100 = 22.22%. The same two values give a 25% increase or a 20% decrease if one is treated as the starting point.
Percentage change divides by the original value and depends on direction: 80 → 100 is +25%. Percentage difference divides by the average of both values and ignores direction, giving 22.22% either way.
Use percentage difference when neither value is the accepted truth, such as two independent measurements. Use percentage error when one value is a known standard; 96 measured against an accepted 100 is a 4% error.
The formula uses the absolute value of the gap and the average of both numbers, and neither changes when the order is reversed. This symmetry is the reason to use it for peer comparisons.
Subtract one from the other and drop the sign, add the two numbers and halve them to get the average, then divide the gap by the average and multiply by 100. For 80 and 100 the steps are 20 ÷ 90 × 100 = 22.22%.