Formula question

[Guest]

I have a spreadsheet with about 50 rows that gradually will be filled up with
numbers. How can I make formula that will count and ad up only the 12 last
rows at any given time? The position of the last 12 rows will here obviously
change over time, since more numbers will be added frequently.

Thanks very much for any help.

 

[Guest]

For example, to add up the last twelve items in column A:

=SUM(OFFSET(A2,MATCH(2,1/(A2:A65536<>0))-1,,-12))

this is an array formula must be entered with CNTRL-SHIFT-ENTER