What is the percent change?

Answer 1

A percentage change is the change in any non-zero quantity, relative to its original value which is expressed as a percentage.

So, if a quantity changes from X to Y, the percentage change is 100*(Y - X)/X

or, equivalently, 100*(Y/X - 1)

Furthermore, although percentage changes can be used when X < 0, great care is required in their interpretation. For example, the change from -5 to +5 is quite different to the change from +3 to -3. But both are -200% changes.