Plain definition
Customer churn is the share or count of customers from a starting cohort who stop being customers during a defined period. The denominator should be the customers at the start of the period; customers acquired later belong in new-business reporting, not in the retention denominator.
Churn needs an explicit definition of “left”: cancelled, failed payment, inactive after a grace period, or another rule. Mixing these states hides whether the commercial issue is product value, billing, onboarding, or voluntary cancellation.
Illustrative example
Suppose a company starts a quarter with 200 customers and 12 of those customers cancel. Customer churn is 6%. Adding 30 new customers does not reduce the cohort churn rate; it changes the ending customer count.
A segment with 6% churn can have a different revenue effect depending on which customers left, so customer churn should be read with revenue churn and cohort evidence.
| Customers who cancelled from starting cohort | |
|---|---|
| Starting customers | |
| Convert to percentage | |
| Customer churn |
Use matched periods and cohorts; annualising one unusual month can turn a temporary event into a misleading headline.
Why it matters
Customer churn shows whether the company can keep its customer relationships. New sales can conceal a weakening base until acquisition cost and service capacity become painful.
Practical consequence
Freeze the starting cohort, document the exit rule, and track voluntary, involuntary, and product-driven losses separately where possible. Review churn by acquisition cohort, plan, tenure, and segment, then connect the largest driver to an owner and a concrete retention experiment.