trying to find the percentage increase/decrease using a formula

[Modify_inc]

I'm trying to formulate a formula to calculate the percentage increase/decrease. I know how to calculate it manually but am not sure how to put it in a formula.

Ex:
G36 = a formula that calculates the sum of numbers, say 1000.
G37 = a formula that calculates the sum of numbers from a previous year, say 500.
I need H37 to calculate the percentage difference of those two.
Manually, it would be 1000 + 500 = 1500 -1000 / 1000 = .5

Using basic logic, not Excel logic, gives me this:

H37 = (g36+g37-g36)/g36 gives me .32

I feel I need to use something similar to this formula:

=(E12-(E12/3))/3 this is formula I use in my sheet that works using itself for another task, though it's only called twice while the other requires 3 instances.

Thanks
Mike

Update: Never mind, that formula I have does work. My totals were adjusted by mistake during testing, making my percentage look like it was wrong, when in fact it was correct for the given totals.

[FDibbins]

actually the basic math would be 1000/500 (or 500/1000 depending on what you are testing).
so the excel version of that would be =G36/G37 = .5

if you wanted to know this yr's contribution based on the last 2 years, then the formula would be

=g36/(g36+g37) =.66

[Modify_inc]

actually the basic math would be 1000/500 (or 500/1000 depending on what you are testing).
so the excel version of that would be =G36/G37 = .5

if you wanted to know this yr's contribution based on the last 2 years, then the formula would be

=g36/(g36+g37) =.66

Thanks for the reply, even though I figured it out; any additional advice is always welcomed!

[vlady]

=-(1-a2/a1)
=sum(a1-a2)/a1