Depends upon how the data is structured. It could be as simple as

=AVERAGE(A1:A12)

and copied down, if the data is accumulated per month in A1:An, but it may
be more complex.

Give us an example.

--
HTH

Bob Phillips

(remove nothere from email address if mailing direct)

"Jeff" wrote in message
...
I cannot remember how to calculate a 12 month rolling average for my
emissions. I believe I know the formula but need some reassuring.

Thanks.