I searched for this on the net, couldn't find it, so wrote a simple formula
myself.

(The net promoter score is a number assigned to a one question survey:
"Would you recommend us to a friend or colleague?"
There's been a great deal of research on the use of this number)

I needed to calculate the NPS for several of my surveys.
Here's what I came up with:

You can paste the formula below in cell A21 to get the NPS of A1:A20
=100*(COUNTIF(A$1:A20,"8")-COUNTIF(A$1:A20,"<7"))/COUNT(A$1:A20)

Once you have it in a spreadsheet, just copy that cell and past it to the
bottom of ANY list of individual survey results (regardless of the number of
results) to get
the Net Promoter Score (NPS).

If you make improvements, please let me know via email or by posting to this
thread. If you use it, please let me know via email - I'm a proponent of NPS.

Thanks,
Joan Roco
--
World Class Project Management, Instructional Design and Facilitation