How NPS is calculated
Net Promoter Score comes from a single question: how likely are you to recommend us to a friend or colleague, on a scale of 0 to 10? People who answer 9 or 10 are promoters, 7 or 8 are passives, and 0 to 6 are detractors. The score is the percentage of promoters minus the percentage of detractors. Passives count toward the total but not either side.
With 60 promoters, 25 passives and 15 detractors out of 100 responses, NPS is 60 − 15 = +45. A score can be negative: more detractors than promoters.
The margin of error
A survey is a sample, and NPS swings a lot on small ones. Treating each response as +1 (promoter), 0 (passive) or −1 (detractor), the 95% margin of error is 1.96 × √((p + d − (p − d)²) ÷ n) × 100, where p and d are the promoter and detractor shares and n is the number of responses. The +45 above, from 100 responses, is ±14.5: anything from about +31 to +60 is consistent with it. Before you celebrate a 10-point rise, check that it is bigger than the margin of error on both surveys.
Using the number
NPS is most useful compared with itself over time and broken down by customer segment, and least useful compared across industries, where typical scores differ widely. Read the comments detractors leave: the score tells you how many unhappy customers you have, and the comments tell you why.