Percent Difference Calculator

What this calculator does

Sometimes neither number is the right one to measure against, they are just two readings you want to compare on equal footing. Two measurements of the same thing, two suppliers' quotes, two lab results. Which is the baseline? Neither. So this measures the gap against their average, which is what makes it even-handed: swap the two values and the answer does not change.

Type value A and value B, and the tool gives you the percent difference between them.

Using the calculator

1. Type value A.
2. Type value B.
3. Press Calculate.

Both values must be above zero. The order does not matter: A and B are interchangeable here, and you will get the same answer either way.

The formula | |A − B| ÷ average × 100

The percent difference is:

(|A − B| ÷ ((A + B) ÷ 2)) × 100

Take the gap between the two values, ignoring which is bigger, then divide by their average, the midpoint between them, and turn it into a percentage. The vertical bars mean absolute value, so the gap is always counted as positive.

Why it divides by the average

The average in the denominator is the whole point. Because neither value is the "original" or the reference, it would be unfair to divide by one of them and not the other; the answer would depend on which you picked. Dividing by their midpoint instead treats them as equal partners. That is what makes the percent difference symmetric: swap A and B and the result does not change, which is exactly what you want when neither came first.

Difference, or change? They are not the same

This trips people up, so it is worth pinning down. A percent difference is symmetric and divides by the average, for comparing two values where neither is the baseline. A percent change is directional and divides by the starting value, for a before-and-after where one value clearly comes first. The quick test: if swapping the two numbers should change your answer, because one is "before" and one is "after", you want the percent change calculator. If swapping them should not change anything, because they are equal partners, you want percent difference. And if one of the two is a known correct or true value, neither of these fits: the percent error calculator, which divides by the true value, is the one to use.

A worked example | 70 and 120

Say the two values are 70 and 120.

1. The gap: |70 − 120| = 50.
2. The average: (70 + 120) ÷ 2 = 95.
3. As a percentage: (50 ÷ 95) × 100 ≈ 52.632%.

So the percent difference is about 52.63%. For contrast, the percent change from 70 to 120 is 71.43%, because that divides by the starting 70 rather than the average 95. Same two numbers, different question, different answer, which is why choosing the right one matters.

Questions people ask

What is the percent difference between 70 and 120?

About 52.63%. The gap of 50, divided by the average 95, times 100.

What is the percent difference formula?

(|A − B| ÷ ((A + B) ÷ 2)) × 100: the absolute gap divided by the average of the two values.

Why divide by the average and not one of the values?

Because neither value is the reference. Dividing by their midpoint treats them equally and makes the result the same whichever order you enter them.

How is this different from percent change?

Percent change has a clear before and after and divides by the starting value. Percent difference has no baseline and divides by the average, so swapping the two values does not change it.

Can it be negative?

No. It uses the absolute gap, so a percent difference is always positive. There is no increase or decrease, only how far apart.

References

A note on the idea behind it. The percent difference is the absolute gap between two values divided by their average, expressed as a percentage. Dividing by the average makes it symmetric, which distinguishes it from percent change, which divides by a baseline value, and from percent error, which divides by an accepted true value. For further reading, see Relative change and difference.

1. Percent difference, the absolute difference between two values divided by their average, a symmetric comparison with no baseline.
2. Percent change and percent error, which instead divide by a starting value or an accepted true value, for directional or reference-based comparisons.