Guide
Calculating NPS and CSAT, and how far to trust the number
Net Promoter Score is one of the most widely used customer survey measures. It asks one question, usually “How likely are you to recommend us to a friend or colleague?”, on a scale from 0 to 10. The calculation is simple, but surveys are small and noisy, so the number should come with an idea of how far it can be trusted. This calculator gives you both, and also covers CSAT, the satisfaction score.
The NPS method
Respondents who score 9 or 10 are promoters, 7 or 8 are passives and 0 to 6 are detractors. NPS is the percentage of promoters minus the percentage of detractors, so it ranges from −100, if everyone is a detractor, to +100, if everyone is a promoter. Passives count in the total but do not add to or subtract from the score. You can enter counts of each group or paste the individual scores, separated by spaces, commas or new lines.
The range around the score
A survey covers a sample of customers, so the true score could be higher or lower. For a score built from the difference of two shares in one sample, the standard error is the square root of (p + d − (p − d)²) ÷ n, where p and d are the promoter and detractor shares and n the number of responses. The tool multiplies it by 1.96 and by 100 for a 95% range in NPS points. With few responses the range is wide, and a warning appears below 30 responses saying a change of a few points is noise.
CSAT
Switch to “CSAT” to enter the number of responses for each score from 1 to 5. The CSAT is the share of 4s and 5s, the top-two-box measure. The tool also shows the average score, the distribution as a table with counts and shares, and a 95% range for the percentage using the usual formula for a proportion. A CSAT definition varies between companies, so use the same one over time.
Worked examples
For 48 promoters, 30 passives and 22 detractors, the NPS is 48% minus 22%, which is 26.0 from 100 responses. The standard error is the square root of (0.48 + 0.22 − 0.26²) ÷ 100, about 0.0795, so the 95% range is ±15.6 points, from 10.4 to 41.6. For the list of scores 10 9 9 8 7 10 6 3 9 10 8 9 5 10 9 7 8 9 10 4, there are 11 promoters, 5 passives and 4 detractors in 20 responses, so NPS is 35.0, with a warning that the range is wide (about ±34.7 points). For the CSAT sample, 4, 6, 15, 40 and 35 responses for scores 1 to 5 give 75 satisfied out of 100, a CSAT of 75.0%, an average score of 3.96 and a range of 66.5% to 83.5%. You can download the breakdown as a CSV file.
Getting useful results
Survey a random, representative sample, and ask at consistent moments. Read the comments as well as the number, since they explain why. Track the score over time and by segment, and look at whether a change is larger than the range. Close the loop with detractors, and note that response bias, such as only angry or delighted customers answering, is not corrected by any formula.
Limits
The range assumes a random sample and uses a normal approximation, so it is rough for very small samples or extreme scores. It does not correct bias in who responds. NPS and CSAT are measures of opinion, and results depend on wording, timing and culture. Nothing is uploaded.
Related tools
- Startup Idea Validator combines evidence from customers with unit economics.
- OKR Planner & Scorer can track an NPS target as a key result.
- Email ROI Calculator values the campaigns that often deliver surveys.
- Break-Even Calculator shows the sales that keep the business viable while satisfaction improves.
Checked against the current version of the NPS & CSAT Calculator on .