If your values are in column N, with the last at the bottom of the column then:

=AVERAGE(OFFSET(N1,MAX(0,COUNTA(N:N)-12),0,12,1))

If your most recent data is always inserted at the top of column N, then:

=AVERAGE(N1:N12)
--
Gary''s Student - gsnu200768


"Ian" wrote:

I have a spreadsheet recording monthly sales over several years.

I can easily sum these and find the average sales per month but what I would
like to be able to do is find the sum or average of just the last twelve.
Again this is easy if I am prepared to edit the average formula each time I
enter a new monthly figure but I should like to automate this.