Sum every 12th cell works until 12th iteration ... why?

It was suggested to me to use this series for the sums I needed to get (total streamflow for every month over many many years)

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But the one that represents November in my set (code below) ...

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... is the one that will not work. I get a "0" result.

You can see an example of the data setup in the attached screenshot.

Example4.jpg

this part

MOD(ROW($U$2:$U$100),12)=12))

change to

MOD(ROW($U$2:$U$100),12)=0))

on highlited color from 12 change to 0 (null)

ahhh late from sktneer...........

Thanks guys! I finally have it working! The pros of this forum rock!

Another method that you could use, I'm guessing from your screen cap that 'All Jan' is in V18.

Formula:

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Then just drag down, no need to change the formula for each month.