How to calculate it
The question is “how likely are you to recommend us to a colleague?” Anyone who answers 9 or 10 is a promoter; 7 or 8, a passive; and 0 to 6, a detractor. NPS is the percentage of promoters minus the percentage of detractors, so it runs from −100 to +100.
An example
Out of 100 responses, 50 are promoters, 30 passives and 20 detractors. NPS is 50 − 20 = 30. Passives neither add nor subtract, but they count in the total.
NPS vs. CSAT
CSAT measures an interaction; NPS, the relationship. That is why NPS is asked every so often, not after every visit, and read by segment: by business line, customer type or region.
The usual mistake
Chasing the number instead of the detractors. An NPS that rises because you only ask happy customers is worthless. The value is in calling the people who score 0 to 6 and learning why, because they often signal churn months ahead.