The formula
Percent change compares the difference between a new value and an original value with the original value: (new − original) ÷ original × 100. A positive result is an increase; a negative result is a decrease.
Increase example
A score rises from 60 to 75. The change is 15. Divide 15 by the original 60 to get 0.25, then multiply by 100. The score increased by 25%.
Decrease example
A value falls from 80 to 60. The change is −20. Divide −20 by 80 to get −0.25, so the result is a 25% decrease.
Starting at zero
The standard formula divides by the original value. If the original value is zero, percent change is undefined because division by zero is not valid. Report the absolute change or explain the situation instead of forcing a percentage.
Why results can exceed 100%
A value rising from 10 to 30 increases by 20, which is twice the original 10. The percent increase is therefore 200%. A result above 100% is not automatically an error.
Use the original value in the denominator
A common mistake is dividing by the new value instead of the original value. Percent change asks how large the change is relative to where you started, so the starting value belongs in the denominator. Write “old → new” beside the numbers before calculating if the direction is easy to confuse.
Related resources
Reviewed: October 2026. CurioLift explanations are educational; when a school, instructor or required style guide uses different rules, those rules take priority.