"Dave" wrote:
if a1 is 250 and a2 is 500 what is the formula to show the percentage
difference between the 2 cells

You need to decide which "direction" that you want the percentage difference
to reflect: (1) how much more (or less) is a2 over a1 (a2 is 100% more than
a1); or (2) how much more (or less) is a1 over a2 (a1 is 50% less than a2).
Both are equally valid, depending on you want. The formulas a

(1) =a2/a1 - 1

(2) =a1/a2 - 1

FYI, these are the same as the other very common way to express these
formulas:

=(a2 - a1) / a1

=(a1 - a2) / a2

In all cases, beware of comparing with zero (i.e. a denominator of zero).
You cannot express the percentage difference between a "starting" value of
zero. Also beware of comparing two values with opposite signs (negative and
positive).