Net Dollar Retention
Net dollar retention measures how much recurring revenue you keep and grow from your existing customers over a year, ignoring any new ones. You take a cohort's revenue at the start of the period, add expansion from upgrades and add-ons, then subtract downgrades and churn, and express the result as a percentage of where you started. Above 100 percent means your existing base grew on its own; 120 percent means it grew by a fifth without a single new sale.
This is one of the most telling metrics in SaaS because it reveals whether your product compounds. A business with strong net dollar retention can grow even if acquisition slows, because the customers it already has keep spending more. For a lean founder it points to where the cheapest growth lives: expanding accounts you already serve costs far less than winning new ones. A figure below 100 percent means churn is outrunning expansion, and no amount of new acquisition fixes that for long.