Percentage Difference Calculator

The Calculate menu offers only Percentage difference between X and Y, so you can go straight to the numbers.

1. Enter the first number in First value (X). It begins at 80.
2. Enter the second number in Second value (Y). It begins at 120.
3. Read the large result. For the starting values it says Percentage difference 40.00% between 80 and 120.
4. Use the small table for context: Absolute difference is 40, Average of the two is 100, and As a percentage of the smaller is 50.00%.
5. Read the formula line, which shows the sum with your values filled in.

Swap the two boxes and the answer does not change. That symmetry is the whole point of this measure.

Type negative numbers or decimals freely. The boxes accept any value, so you can compare 0.5 with 0.6 or 12.75 with 14.2, and the result is rounded to two decimal places for display.

The formula is |X - Y| / ((X + Y) / 2) x 100. The gap between the numbers is divided by their average, so neither value gets to be the base. Percent change is different: it divides by the old value and has a direction. The same pair of numbers gives three answers depending on what you ask:

Use percentage difference when two things sit side by side and neither came first, such as two suppliers, two measurements or two groups. Use percent change when one value follows from the other in time. The percent change calculator handles that case, and it is the one to reach for with before and after data.

Two quotes for the same job come in at 1,200 and 1,500. The gap is 300 and the average is 1,350, so the percentage difference is 22.22 percent. Measured against the cheaper quote alone, the gap is 25.00 percent, which the table shows on the last line.

Two readings from an experiment are 9.0 and 11.0. The gap is 2, the average is 10, and the percentage difference is 20.00 percent. Two classes average 65 and 60 on the same test, an 8.00 percent difference. If you want a single central figure for several values, the average calculator gives mean, median and spread.

Percentages of this kind only make sense on scales with a true zero, such as money, length and weight. Do not use them on temperatures in degrees Celsius or Fahrenheit. A move from 10 to 20 degrees Celsius is not a doubling of warmth, because zero is an arbitrary point on those scales.

If the two values add up to zero, the average is zero and the tool warns that they cannot add up to zero. If one value is zero and the other is positive, the percentage difference is always 200 percent, for example 0 and 10, and the line comparing against the smaller number shows a dash because it cannot divide by zero. Negative values are accepted, but the result is rarely meaningful.

This is also not percent error. To compare a measured value against an accepted one, divide by the accepted value, which you can do by entering the accepted value as the old value in the percent change calculator and ignoring the sign. The family of preset pages begins at the general percentage calculator.

The numbers are processed on your device and shown to two decimal places. The tool describes the size of a gap, not whether it matters in practice.

One more habit helps: always say what the percentage is relative to. A difference of 20 percent between two salaries sounds large, yet it may be a small amount of money, while the same percentage between two house prices is a very large one. The Absolute difference line in the table keeps the real size in view next to the percent.