Percent Difference Calculator
Find the percent difference between two values with the working shown — the symmetric measure that treats neither number as the “original” — plus the percent change in both directions and the gap in percentage points.
How it works
Percent difference = |a − b| ÷ ((|a| + |b|) ÷ 2) × 100 · against the larger value: |a − b| ÷ max(|a|, |b|) × 100 · against the smaller value: |a − b| ÷ min(|a|, |b|) × 100 · percent change a → b = (b − a) ÷ |a| × 100
Percent difference compares two values when neither is a reference “before” figure. It divides the gap by the average of the two magnitudes, so swapping a and b gives the same answer. Percent change is different: it divides by a specific starting value, so the answer changes if you reverse the order. Use percent difference to compare two independent measurements, such as two lab readings of the same quantity or two suppliers’ quotes. Alongside the percent difference, the calculator shows the percent change in both directions and the first value as a percent of the second, so you can see why the three figures differ. Under More options you can measure the gap against the larger or smaller value instead of the average, as some textbooks and industries do. You can also mark the values as percentages to get the gap in percentage points, and fix the rounding.
Worked example
For 40 and 60, the difference is |40 − 60| = 20 and the average is (40 + 60) ÷ 2 = 50. The percent difference is 20 ÷ 50 × 100 = 40%. Read as a change, 40 → 60 is +50% while 60 → 40 is −33.33%, and 40 is 66.67% of 60. Measured against the larger value the gap is 20 ÷ 60 = 33.33%, and against the smaller value it is 20 ÷ 40 = 50%. If the values were interest rates of 4% and 6%, they would be 2 percentage points apart.
Sources: Percentage difference — Wikipedia
Frequently asked questions
How is percent difference different from percent change?
Percent change divides the gap by the original value, so it depends on which number comes first: going 40 → 60 is +50%, but 60 → 40 is −33.33%. Percent difference divides by the average of both, so it is symmetric — 40% either way. The calculator shows all three figures.
When should I use percent difference?
Use it when the two values are on equal footing and neither is the natural starting point. Two experimental measurements, two estimates or two samples are typical cases. If one value is clearly the baseline, use percent change. If one value is the accepted correct answer, use percent error.
Can percent difference be negative?
No. It uses the absolute value of the gap, so it is always zero or positive. A value of 0% means the two numbers are identical. The direction is shown separately by the percent change figures.
Can percent difference be more than 100%?
Yes, against the average it reaches 100% when one value is three times the other (1 and 3 average 2 and differ by 2). It approaches 200% as one value shrinks towards zero. Large results are a sign that percent difference is not the most useful comparison for those numbers.
Why do some sources divide by the larger or smaller value?
Without a natural reference, any choice is a convention. The average is the most common and the one most textbooks call “percent difference”. Dividing by the larger value tells you how far the smaller one falls short, and dividing by the smaller one tells you how much bigger the larger one is. Choose the base under More options.
What about comparing two percentages?
Comparing 4% with 6% can mean two different things. They are 2 percentage points apart, but their percent difference is 2 ÷ 5 × 100 = 40%. Switch on “The values are percentages” to see both.
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This calculator is for educational purposes. Double-check important results before acting on them.