Churn Rate
The percentage of customers (or revenue) you lose over a given period — the leak in your bucket.
Churn rate measures how many customers stop paying you over a set stretch of time, usually a month or a year, shown as a percentage of where you started. If you began the month with 200 customers and 10 left, that's a 5% monthly churn rate. Some businesses track "revenue churn" instead, which weights each lost customer by how much they were paying.
Every business loses some customers — the question is how fast, and whether new ones are joining quicker than old ones leave. High churn quietly cancels out your marketing and sales effort, because you're refilling a bucket that keeps draining. Lowering it is often cheaper than chasing new customers just to make up the gap.
Why it matters
Churn decides whether the customers you work hard to win actually stack up over time or slip away as fast as you add them.