You also asked where to find cola/inflation rates.
The OP asked for __both__. The OP wrote: "I'd need some explanation
of the __formula__ also if you could?", emphasis added for the reading-
impaired. And yes, the OP also wrote: "Also, where do you get cola/
inflation rates per year since 1939?" Besides, we often address the
non-Excel part of questions in these newsgroups.
Anyway, moving beyond this childish sniping.....
Bernard answer the second question best, I think: google for
"inflation rates" without quotes, and you will find sites that provide
calculators and/or tables of rates. Caveat emptor: these are US CPI
rates, for the most part, which may or may not be related to wage
inflation rates. (Probably not, IMHO.) It is the best you can do.
However, following the details, you might find specific tables of wage
inflation rates, ideally broken down by industry, but probably not.
(I used to have a link to the US govt website that is the primary
source of all this data. I believe it did have different inflation
rate tables. But, sigh, it seems that I have lost track of the link.)
As for a formula, well, that really depends on how you decide to
represent the data. Suppose you have a table of annual rates
(percentage increases) in A1:A68 for 1939 through 2006. (It might be
a little early to find 2007 rates.) The following array formula
(commit with ctrl+shift+Enter) computes the average annual rate:
=geomean(1+A1:A68)-1
Caveat emptor: GEOMEAN sometimes fails for a large number of periods
and/or widely varying fractions. I don't think either would be a
problem for this application.
If you have the average annual rate in B1, the following would compute
a 1939 price in 2008 dollars:
=fv(B1, 2008-1939, 0, -0.25)
HTH.