NRR is what the customers of twelve months ago pay today, as a share of what they paid then, including upgrades and excluding new customers.
NRR = MRR today of the customers who paid 12 months ago ÷ their MRR 12 months ago
Above 100 % the existing base grows without a single new customer. The customers who left stay in the denominator: dropping them is the most common way to flatter the number.
Follow customers by identity. Total revenue now divided by total revenue then is not NRR, because it includes new customers.
sumwerk follows the customers who paid a year earlier, month by month, and prints the definition next to the number.
A restricted, read-only key is all it takes. The first import runs in a few minutes; every number you see opens into the customers and Stripe references behind it.