One way to find the last row and count back.
=SUM(INDIRECT("a"&MATCH(999999,A:A)-11&":a"&MATCH(9999999,A:A)))

--
Don Guillett
Microsoft MVP Excel
SalesAid Software

"Michelle D" <Michelle wrote in message
...
I have a table of hours worked by temporary staff which I am constantly
adding new values to. I need to calculate average hours worked from the
last
12 weeks they worked at any given moment so I want a total of there hours
for
the last 12 weeks.

I have limited knowledge of excel - so at the moment - I add the latest
value - then recalculate the sum of those 12 cells - I would like a table
that automatically calculates the sum of the last 12 cells in a column -
whatever cells they are.

Hope you can help,

Michelle