A percentage change compares a difference with the value you started from. The denominator matters as much as the difference.
The formula
Change (%) = (later − earlier) ÷ earlier × 100. A change from 80 to 100 is 25%, while a change from 100 to 80 is −20%. The differences have equal magnitude but different bases.
A worked example
Enter 240 as the earlier value and 300 as the later value. The difference is 60. Dividing 60 by 240 gives 0.25; multiplying by 100 gives 25%. Keep both values in the same currency or unit.
When it is undefined
Starting from zero makes division by zero unavoidable. The tool reports an error rather than an infinite percentage. Negative starting values are excluded because their interpretation can be ambiguous. A later value of zero is valid and means a 100% decrease.
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