The Net Promoter Score is a measure of customer satisfaction and customer loyalty. It is based on a single question: How likely is it that you would recommend [Company X or product Y or service Z] to a friend or colleague?
This question is then scored against a 10 point scale (0=Not at all likely, 10 Extremely likely). Responses between 0-7 are classified as detractors and responses between 9 and 10 as promoters.
Net Promoter Score = % of Promoters - % of Detractors
This indicator is included in the book: Key Performance Indicators - the 75+ measures every manager needs to know, which contains an in-depth description of this KPI, as well as practical advice on data collection, calculations, target setting, and actual usage.