Percentage Difference Calculator
Compare two numbers without picking a 'before' and 'after'. Enter any two positive values and get the percentage difference — the gap divided by the average of the two — alongside both percent-change readings for contrast.
Example: with Value A 40 · Value B 60 → Percentage difference: 40%.
- Absolute difference20
- Percent change A to B+50% (from 40 to 60)
- Percent change B to A-33.33% (from 60 to 40)
Computed by the calculator below using its default values. Change any input to see your own numbers.
Percentage difference = |A − B| ÷ ((A + B)/2) × 100. It treats both numbers equally — unlike percent change, which measures against whichever value you call the start.
Percentage difference vs. percent change
Percentage difference answers 'how far apart are these two numbers, relative to their typical size?' It divides the gap |A − B| by the average of the two values, so it is symmetric: comparing 40 to 60 gives exactly the same answer as comparing 60 to 40. That makes it the right tool when neither number is a baseline — two vendors' quotes, two cities' rents, two lab measurements of the same thing.
Percent change divides the same gap by one specific value — the starting one — so direction matters. Going from 40 to 60 is a +50% change, but going from 60 to 40 is only −33.33%. Neither of those is wrong; they just answer a different question than the symmetric 40% difference. News copy routinely blurs these, which is how the same pair of numbers gets reported three different ways.
How it’s calculated
Percentage difference = |A − B| ÷ ((A + B)/2) × 100, i.e. the absolute difference divided by the arithmetic mean of the two values. Percent change A→B = (B − A) ÷ A × 100; B→A swaps the base. Results are computed exactly and displayed rounded to 2 decimals.
The percentage-difference convention is meant for two positive quantities; if the average is zero or negative, the ratio loses meaning and this tool says so instead of printing a number.
Same numbers, three readings
| A | B | % difference | % change A→B | % change B→A |
|---|---|---|---|---|
| 40 | 60 | 40% | +50% | −33.33% |
| 100 | 110 | 9.52% | +10% | −9.09% |
| 25 | 75 | 100% | +200% | −66.67% |
| 10 | 190 | 180% | +1,800% | −94.74% |
Computed with |A−B|/((A+B)/2) and (new−old)/old; rounded to 2 decimals.
Common mistakes
- Using percentage difference when one value really is a baseline — a price that rose from $40 to $60 went up 50%, not 40%.
- Averaging the two percent changes and expecting the percentage difference — +50% and −33.33% do not average to 40%.
- Feeding in a negative and a positive value: the average can hit zero and the ratio becomes meaningless.
- Confusing percentage difference with percentage-point difference — 5% vs 7% is 2 points but a 33.33% difference.
Frequently asked questions
What is the percentage difference formula?
Percentage difference = |A − B| divided by the average of A and B, times 100. For 40 and 60: 20 ÷ 50 × 100 = 40%.
How is percentage difference different from percent change?
Percent change divides by the starting value, so it depends on direction: 40→60 is +50% but 60→40 is −33.33%. Percentage difference divides by the average of the two, so it is 40% either way. Use difference when neither number is a baseline; use change when one clearly comes first.
Can percentage difference be more than 100%?
Yes. Once the larger value is more than three times the smaller, the difference passes 100%, and it approaches (but never reaches) 200% as the smaller value shrinks toward zero.
Does it matter which number I enter as A?
No — the formula uses the absolute difference and the average, both of which ignore order. That symmetry is the whole point of the measure.
Why does my result say not defined?
The denominator is the average of your two values. If that average is zero or negative — which happens with negative inputs — dividing by it produces a number with no sensible interpretation, so the calculator declines rather than mislead.