If you're up for some stimulating reading, we have a whole article on the subject - the tl;dr version is that NPS is a proven methodology that's been around for almost 20 years backed by industry research - the score is calculated as follows:
Total % of "Promoter" customers (9s & 10s) - Total % of "Detractor" customers (0-6s.)

Bigger picture, NPS is intended to provide a way of indicating over time your customers value and to inform important business decisions around customer experience, product launches, and process updates.

So, though we know from personal experience that those 6s sting, think of them as an indication you are really close to bringing a dissatisfied customer back into the fold.

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