Churn Rate
Also called: customer churn, revenue churn, attrition rate
Churn is the rate at which customers or revenue leave. Logo churn counts customers; revenue churn weights them by value, and the two can point in opposite directions when small accounts leave and large ones expand. Monthly churn of 5% sounds survivable and means losing nearly half your customers in a year, which is why it's the first number a subscription investor asks for.
Formula
Churn rate = Customers lost in period ÷ Customers at start