Percentage difference calculator

Percentage difference compares two values of equal standing with their average:

percentage difference = |a − b| / ((a + b) / 2) × 100

The top of the fraction is the absolute difference, so the result is always positive, and swapping the two numbers changes nothing. That is the key difference from percent change, where the old value is the reference and the answer depends on which direction you go.

If the two values add up to zero (for example −5 and 5), their average is zero and the percentage difference is undefined. The calculator tells you so rather than printing a misleading number.

Two contractors quote for the same kitchen remodel: $80 per hour and $100 per hour.

1. Absolute difference: |80 − 100| = 20.
2. Average: (80 + 100) / 2 = 90.
3. Percentage difference: 20 / 90 × 100 = 22.22%.

If you used percent change instead, you'd get two different answers: from 80 to 100 is +25%, but from 100 to 80 is −20%. Neither quote is the "old" one, so the percentage difference, which sits between those two figures, is the fairer way to describe how far apart they are. If you want to say how much more expensive the second contractor is than the first, that is a percent change question, and 25% is the right answer.

Two students measure the same sample's mass and get 4.2 g and 4.6 g.

• Absolute difference: 0.4 g.
• Average: 4.4 g.
• Percentage difference: 0.4 / 4.4 × 100 = 9.09%.

A difference that large between two careful measurements of the same thing suggests something is off: an uncalibrated balance, a wet sample or a reading error. If one of the two results came from a certified reference, you'd compare against it with percent error instead, because then one value is the accepted truth.

The formulas look alike but answer different questions. Ask yourself: is there a natural starting point, or a known correct value? If there's neither, use percentage difference.

• It isn't a percentage of the larger number. Between 80 and 100 the percentage difference is 22.22%, not 20%. 20% is what you get by dividing by 100, which treats 100 as the reference.
• It tops out at 200%. When one value is zero and the other positive, the difference is exactly twice the average, so the result is 200% whatever the other number is. Near that limit the measure tells you little, and an absolute difference or a ratio is more useful.
• Watch out for opposite signs. If one value is negative and the other positive, the average can be tiny or zero, and the result explodes. Percentage difference is meant for quantities that are both positive, like prices, lengths and masses.
• Don't mix it up with percent change in reports. If a headline says sales were "22% different", readers will assume it means a 22% increase or decrease. Say which measure you used.

• Shopping and quotes: comparing the same product across stores or the same job across contractors, when you simply want to know how far apart the prices are.
• Science classes and labs: checking how well two trials or two groups' results agree with each other. Many lab manuals ask for it alongside percent error.
• Engineering and quality control: comparing readings from two instruments, or two parts that should be identical.
• Statistics and surveys: comparing two groups' averages when neither is a baseline, such as two regions or two schools.

In finance and economics, where most comparisons are over time, percent change is far more common. When in doubt, the calculator shows both, so you can see how much the choice matters for your numbers.

Results are shown with up to four decimal places; for a lab report or a summary, one or two decimals is usually enough. Every step of the working is shown, so you can check the arithmetic or copy it into an assignment. The values you enter are stored in the page link, and the Share button copies a link that opens this exact calculation.

To calculate it for a whole column of pairs in Excel or Google Sheets, with the two values in A2 and B2:

=ABS(A2-B2)/AVERAGE(A2,B2)

Format the cell as a percentage, or multiply by 100. Fill the formula down for every row. For percent change instead, use =(B2-A2)/ABS(A2), with the old value in A2. Spreadsheets won't warn you when the average is zero; they show a #DIV/0! error, which is the same case the calculator flags.