These two terms get used interchangeably in casual conversation, but they use different formulas and answer different questions. Mixing them up is one of the most common percentage mistakes people make.
Percent Change: Use When You Have a Clear Before and After
Percent change compares an original value to a new value. It always divides by the original value, since that is the fixed starting point.
Example: a product's price rises from 50 dollars to 65 dollars. Percent change = (65-50)/50 x 100 = 30% increase.
Percent Difference: Use When Neither Value Is the Starting Point
Percent difference compares two independent values where there is no clear original or before-and-after relationship. It divides by the average of the two values instead.
Example: two scales measure the same object at 40 grams and 60 grams. Neither reading is the true starting value, so percent difference = |40-60|/50 x 100 = 40%.
Why the Choice Matters
Using percent change when you should use percent difference (or the reverse) produces a different number for the exact same pair of values, and can make a comparison look more or less dramatic than it actually is. If one value is clearly the original and the other is what it became, use percent change. If you are comparing two independent measurements or estimates, use percent difference.
Our Percentage Calculator includes both as separate, clearly labeled tools, so you never have to guess which formula applies.
Frequently Asked Questions
- Can percent difference be negative?
- No, because it uses an absolute value in the numerator, percent difference is always zero or positive. Percent change can be negative, which indicates a decrease.
- Which one should I use for exam scores before and after studying?
- Percent change, since there is a clear original score (before) and new score (after).