=SUMPRODUCT(--(MOD(ROW(A18:A226)-ROW(A18)+0,13)=0),A18:A226)

Greg wrote:
> Is it possible to sum a column starting from row 18 and ending at row 226
> only summing every 13th row. For example, sum the following rows
> 18,31,44,57,70 ... 226? Please help!!
> Thanks in advance!!
>
>