How to use Percentage Change Calculator
Percentage change expresses the difference relative to the original value. For a positive original value, a rise from 100 to 120 is a 20% increase. The same absolute drop from 120 to 100 is approximately a 16.67% decrease because the starting base is different.
A zero starting value has no defined percentage change. For a negative starting value this tool uses its absolute magnitude as the denominator, so a positive result consistently indicates an increase. State this convention when reporting such changes.
The formula
A worked example
Original value: 100 · New value: 120
Percentage change: 20%
For negative starting values, the denominator uses the absolute original value. Positive means increase; negative means decrease.
Questions about this calculation
Why are an increase and its reverse decrease different percentages?
Their starting values differ, so the denominator changes.
What happens from zero to 100?
The absolute change is 100, but a percentage change from a zero base is undefined.
Formula checks: 2026-10-08. Assumptions & corrections.