You could use a helper column and sumif function. Put a character in the
helper column in each row you want to include in the sum. If you put as s in
each applicable row of Col A, your formula would be =sumif
(a7:A500,"S",b7.b500)

"SusanK" wrote:

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.