What this calculator does
Percentage difference between two numbers measures how far apart they are relative to their average, and it comes out the same regardless of which number is listed first. This calculator finds it with the standard symmetric formula: the absolute difference between the two numbers, divided by their average, times 100.
This is a genuinely different calculation from percentage increase, even though the two get mixed up constantly. Percentage increase is directional: it treats one number as the starting value and asks how much bigger the other one is relative to that specific starting point, so swapping which number is "first" changes the answer. Percentage difference between two numbers has no starting point at all; it just measures the gap between two values against their shared average, which is why it gives the same result either way round.
The formula
Take the absolute difference between the two numbers (so the order they are entered in does not matter), then divide by the average of the two numbers, then multiply by 100 to express it as a percentage. Because the average, not either individual number, is used as the base, the result treats both numbers symmetrically rather than favouring one as a reference point.
| Term | Meaning |
|---|---|
| a, b | The two numbers being compared. Order does not affect the result. |
| |a − b| | The absolute difference between the two numbers, always taken as a positive value. |
| (a + b) / 2 | The average of the two numbers, used as the base of the percentage rather than either number individually. |
The inputs explained
| Field | What to enter |
|---|---|
| First number | The first number to compare. |
| Second number | The second number to compare. Swapping this with the first number gives the same percentage difference. |
When to use it
Comparing two measurements or estimates
When two values, such as two lab readings or two independent estimates of the same quantity, need to be compared without treating either one as the "correct" baseline, percentage difference gives a fair, symmetric comparison.
Checking how far apart two competing figures are
Two prices, two survey results or two forecasts can be compared for how far apart they sit relative to their average, which is often more informative than an absolute gap alone.
Distinguishing from a percentage increase question
If the question is really "how much bigger is the new value than the old one", that is a directional percentage increase, not a percentage difference. Using this calculator for that question would give a different, non-directional answer to what is actually being asked.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How does percentage difference compare with percentage increase for the same two numbers?
Percentage difference for a range of second numbers, holding the first number at 80.
| Second number | Absolute difference | Average of the two numbers |
|---|---|---|
| 80 | 0 | 80.000 |
| 90 | 10.000 | 85.000 |
| 100 | 20.000 | 90.000 |
| 120 | 40.000 | 100.000 |
| 150 | 70.000 | 115.000 |
| 200 | 120.000 | 140.000 |
Questions
How is percentage difference different from percentage increase?
Percentage increase is directional and uses one specific value as the starting point: (new − old) ÷ old × 100. Percentage difference is symmetric and uses the average of both numbers as the base: |a − b| ÷ ((a + b) ÷ 2) × 100. Swapping which number comes first changes a percentage increase but never changes a percentage difference.
Does it matter which number I enter first?
No. Because the formula uses the absolute difference and the average of both numbers, entering 80 and 100 gives exactly the same percentage difference as entering 100 and 80.
What if I actually want to know how much one value increased from another?
That is a directional question, best answered with the percentage increase calculator, which uses a defined starting value rather than an average.
Can percentage difference be used when one number is negative?
The formula works as long as the two numbers do not average to exactly zero, since that would make the base zero and the result undefined. With a negative and a positive number that average to a nonzero value, the calculation still runs, though the result is less commonly meaningful in that case.
For a directional comparison from a defined starting value, see the percentage increase calculator.