Summing a row of data based another row being filled
 

It seems like this answer should be so easy, but I just can't figure it out.

Here's my situation:

On one worksheet, '2009' I have a series of volumes Jan through December with totals at the end.

On a more recent worksheet, '2010' I have the same exact worksheet, and I'm filling it in as the months go by (currently in July). However, I want to add a column for Year-to-date comparison, and I don't want to have to go to the previous worksheet and add up the months I need.

So, is there a formula that would automatically add the correct months in '2009' based on the data for that month being entered in '2010', e.g. I fill in August in '2010' and on the '2009' worksheet it adds Jan-Aug. etc.

Thanks for your help and time. I just can't figure this one out.

Let's say your table looks like this:

2009 Worksheet:
A B C D E F
JAN FEB MAR APR MAY JUN
1 1 2 3 4 5 6

2010 Worksheet:
A B C D E F
JAN FEB MAR APR MAY JUN
1 10 9 8 7 6

For your Last Year's YTD:
=SUM(OFFSET('2009'!A1,0,0,1,COUNTA('2010'!A1:F1))

For your Current YTD:
=SUM('2010'!A1:F1)

NOTE: If a month has a null value, do not leave it as blank - instead, place a zero.

Wow--thanks. This works perfectly. You've saved me so much rework--and sanity. I appreciate your time.

Thanks for the feedback.