Why is the percentage difference between 80 and 100 not 20%?

Because percentage difference divides the gap (20) by the *average* of the two numbers (90). 20 / 90 = 22.2%. If you want to know the change from an old value of 80 to a new value of 100, use the Percentage Change Calculator.

When should I use percentage difference?

Use it when comparing two values of the same type where neither one is obviously the "starting" or "correct" value, such as comparing the heights of two people, or the grades of two friends.

Why do science teachers ask for this?

In lab experiments, percentage difference is used to assess the precision of two repeated experimental measurements to see how closely they agree with each other.

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Percentage Difference Calculator

Find the absolute percentage difference between two numbers, measuring how far apart they are relative to their combined average.