I've been asked many times, how to calculate percentage change. So here is a handy calculator for your convenience, and a permalink to any changes you make below.

Variable	Definition	Value
Value 1
Value 2
% change, 1 → 2	(V2 - V1) / |V1|
% change, 2 → 1	(V1 - V2) / |V2|
% ratio, 1 : 2	V1 / V2
% ratio, 2 : 1	V2 / V1
% difference	|V1 - V2| / ((|V1| + |V2|)/2)

One needs to be careful when interpreting or phrasing the above results.
For example, if v1 and v2 are benchmark times, one can either say that:

• performance v1 to v2, has decreased by 100%
• performance v2 to v1, has increased by 50%
• performance of v1 is 200% that of v2
• performance of v2 is 50% that of v1

The percentage difference value can be used where there is no directionality implied, and one just wants the relative difference between two values.