Try this Susan

=SUMPRODUCT((B7:B500)*(MOD(ROW(B7:B500)-ROW(B7),3)=0))

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Regards Ron de Bruin
http://www.rondebruin.nl


"SusanK" wrote in message ...
How do I add cell B7, B10, B13 (every 3 cells up to 500+)? Note: The other
cells such as B6 and B9 have other forumlas and info that is not to be
included). I did this manually but the formula has now gotten so long that I
get an error saying "formula too long". So I figured, there must be a
shorter formula that just adds every 3 (or whatever designated) cells
starting with one cell and ending with another.