Manni,

You could use an array formula (entered using Ctrl-Shift-Enter) like

=SUM(IF(COLUMN(B4:AE4)=LARGE(IF(B4:AE4<"",COLUMN (B4:AE4)),12),B4:AE4))

This formula will sum 12 non-blank values ending with the last filled cell
of row 4. (If there are other blanks to the left of the last filled cell,
they will be ignored, and a cell further to the left will be summed in.)

If you just want the last 12 columns - including blanks - as defined by the
last filled cell, then use (array entered)

=SUM(IF(COLUMN(B4:AE4)=MAX(IF(B4:AE4<"",COLUMN(B 4:AE4)))-11,B4:AE4))

HTH,
Bernie
MS Excel MVP



"manni923" wrote in message
...
Hi - I want to count based on a rolling data range and I can't seem to
figure
out how to do it.

My data looks something like this:

Jan09 Feb09 Mar09 Apr09................ Jan10 Feb10 Mar10 Apr10
15 25 1 4 7 15
9 28

I want to count the numbers based on 12 months of data. So if Apr10 has a
number in it, I want to count from Apr10 all the way back 11 months
through
May09.

If Apr10 was blank then I would like the formula to pick up from Mar10
(assuming it has a number in it) all the way back through Apr09

Is this possible?