How it's calculated, without odd formulas
Answers split into three groups: promoters (9 and 10), passives (7 and 8), detractors (0 to 6). The score is the percentage of promoters minus the percentage of detractors. Passives don't count towards it, but they stay in the denominator — which is why they «lower» the result without being counted.
An example with a hundred responses: 50 promoters, 30 passives, 20 detractors. That's 50% − 20% = 30. The result runs from −100 to +100, and it's written as a whole number with no percent sign: NPS 30, not 30%.
Why the scale is cut that way
Splitting at 6 and 8 looks arbitrary and partly is, but it comes from a repeated observation: people who give 7 or 8 behave like people with no opinion — they don't buy again more than average and they don't talk about you. That's why they don't count.
The cut at 6 surprises anyone used to school marks, where six is a pass. Here it's a detractor. That's deliberate: on a recommendation scale, «six» means they wouldn't recommend you.
The question that really matters is the second one
NPS on its own is a number with no instructions: knowing you're at 30 doesn't tell you what to do on Monday. The open question straight after — «what's the main reason for your score?» — is the one that produces work, and it must stay optional: forcing it collapses the response rate and produces one-word answers.
One trick that works: show a different question depending on the score, using conditional logic. Ask people who gave 9 or 10 what they'd tell a friend; ask people who gave 6 or less what went wrong. They're two questions because they're two different conversations.
When NPS isn't worth running
Below about a hundred responses the number swings too much to mean anything: three extra detractors out of fifty responses move the score by six points. Under that threshold, read the answers one by one and ignore the total.
And don't use it to measure a single interaction: «would you recommend the company» after a support call measures the company, not the call. For that you want a satisfaction question about the episode, which is a different thing.
NPS is promoters minus detractors as percentages, passives stay in the denominator — and without the open question underneath it's a number with no instructions.