Percentage Difference Calculator

How to use the percentage difference calculator

1. Type the first number in Value A and the second in Value B. Decimals can be written with a point or a comma.
2. Read the Percentage difference straight away; it updates as you type.
3. Next to it you will find the Absolute difference (the gap between the two numbers), the Change from A to B and the Change from B to A.
4. Look at Step by step to see the full calculation, which you can copy into a report or a lab notebook.
5. Use the ⇄ button to swap A and B, and Clear to start over.

The percentage difference formula

The percentage difference measures how far apart two values are, relative to their size, when neither of them is a natural reference. The standard formula divides the gap by the average of the two values:

Percentage difference = |A − B| / ((|A| + |B|) / 2) × 100

Example with A = 120 and B = 150:

1. |120 − 150| = 30
2. (120 + 150) / 2 = 135
3. 30 / 135 ≈ 0.222222
4. 0.222222 × 100 ≈ 22.22 %

Because the formula uses the average, the result is symmetric: comparing 120 with 150 or 150 with 120 gives the same 22.22 %. This tool uses the absolute values in the average so that the result stays meaningful when one of the values is negative; for two positive numbers it is exactly the usual formula.

Percentage difference or percentage change?

The two are often confused, and they answer different questions.

• Percentage change has a direction. It compares a new value to an old one and divides by the old value: from 120 to 150 is +25 %, but from 150 to 120 is −20 %. Use it for prices over time, growth, before/after measurements.
• Percentage difference has no direction. It compares two values of equal status, such as two measurements of the same quantity, two shops’ prices, or the results of two labs. The gap is divided by their average.

That is why this tool shows both: the main result is the percentage difference, and the two percentage changes are listed so you can pick the figure that fits your situation.

Typical uses

• Science and engineering: comparing two measurements or two methods. A small percentage difference suggests the results agree.
• Shopping: comparing the same product in two stores without deciding which price is the “normal” one.
• Data checks: spotting large gaps between two sources, two versions of a report, or two sensors.

Limits

The percentage difference is not defined when both values are 0, since the average would be zero; the tool then shows a message. When the values have opposite signs, such as −4 and 4, the result reaches its maximum of 200 %. Results are computed with exact fractions, and anything longer than six decimals is rounded and marked with “≈”.