Definition
Percent change
A change compared with the original value and written as a percent.
What it means
Percent change asks, “How big was the change compared with where we started?” First find the difference, then divide by the original value, not the new one, and name the direction.
Remember it
Change over start, then × 100%.
Example
A collection grows from 50 cards to 65 cards. The change is 15, and 15 ÷ 50 = 0.30, so the percent change is a 30% increase.
Watch for
The usual formula is undefined when the original value is zero, because division by zero is not allowed.
Common mix-up
Percent change uses the original value as the reference. Dividing the change by the new value answers a different question.
Exact meaning
For a nonzero original value, signed percent change is (new − original) ÷ original × 100%. A positive result is an increase and a negative result is a decrease; some contexts state the positive magnitude with the direction in words.