Summing every third cell in a range

[Ajtoo]

Is it possible to sum every third cell in a column?
- where the number of entries is variable, and
- ignoring cells with text in them

[Tom Schreiner]

Yes

Are you familiar with macros???

[Mark W.]

If A2:A11 contains...

{10;20;30;4;5;"dog";12;11;7;6}

...the array formula...

{=SUM(IF(MOD(ROW(A2:A11)-ROW(A2)+1,3),,A2:A11))}

...produces the value 37.

Note: Array formulas must be entered using the Control+Shift+Enter key combination. For more on array formulas see the Excel Help topic for "About array formulas and how to enter them".

[Dave Patton]

An array formula to sum every 3rd.
Enter it with Ctrl-Shift-Enter.

For ease of use:

The first amount to be summed should be in a row that is divisible by the Nth factor.
Example Row 9/3

One can insert or delete row to accommodate this.

=SUM(IF(MOD(ROW(A1:A17),3)=0,A1:A17))

or more generic
Nth 3 or alternative
rD range of numbers
=IF(Nth=0,0,SUM(IF(MOD(ROW(rD),Nth)=0,rD,0)))