Percentage Difference Calculator

When neither number is the original

Percentage change needs a starting point. Percentage difference does not — it compares two values where neither came first, so it uses the midpoint of the two as its baseline.

|a − b| ÷ ((a + b) ÷ 2) × 100

Using the average is what makes the answer the same whichever order you put the numbers in. That is the whole point: if neither value is the reference, the measure should not depend on which you happen to name first.

The two are not interchangeable

Comparing 40 and 60:

• Percentage difference — 40 %, using the midpoint of 50.
• Percentage change from 40 to 60 — 50 %.
• Percentage change from 60 to 40 — 33 %.

Three defensible numbers for the same pair of values. Which one is right depends entirely on the question, and quoting the flattering one without saying which you used is a common way to mislead with true figures.

Which to use

Use change when time or sequence is involved: before and after, last year and this year, old price and new price. Use difference when comparing two measurements of the same kind: two suppliers' quotes, two instruments' readings, two branches' results.

Common uses

• Comparing measurements from two instruments or methods.
• Laboratory work, where percentage difference between duplicate samples is a standard quality check.
• Comparing quotes or offers where neither is the baseline.

The working, step by step

Questions people ask

What is the difference between percentage change and percentage difference?

Change has a starting point and divides by it. Difference has no starting point and divides by the average of the two numbers.

Why divide by the average?

So the answer is the same whichever number you name first. If neither is the reference, the measure should not depend on the order.

Which should I use?

Change when sequence matters — before and after. Difference when comparing two things of the same kind.

Nothing here is sent anywhere

The arithmetic happens in your browser as you type. The numbers you enter are not transmitted, logged or stored, because the page has no server behind it to receive them.