Revenue growth can hide a serious problem: a SaaS business can add new customers every month and still be quietly dying if it's losing existing customers just as fast, or faster. Churn rate is the metric that exposes that, it measures retention directly, separate from however impressive new-customer acquisition looks on its own.
The formula and a worked example
Monthly churn rate = customers lost during the month ÷ customers at the start of the month × 100. Start the month with 1,000 customers, lose 30 by the end, and churn rate is 30 ÷ 1,000 = 3%. That sounds small, but compounded monthly, a 3% churn rate means you'd lose about 30% of a static customer base over a year even with zero new cancellations added mid-year, since each month's loss compounds on a shrinking base. The Customer Churn Rate Calculator handles this calculation and shows the annualized impact directly.
The flip side of churn is retention, and the Customer Retention Rate Calculator reports the inverse view (the percentage you kept, not the percentage you lost), which some teams find more intuitive to track on a dashboard even though it's mathematically the same underlying data.
What counts as a good rate
For B2B SaaS, monthly churn under 1% (roughly 10-12% annually) is generally considered healthy, 2-3% monthly is a warning sign worth investigating, and above 5% monthly usually means the product isn't delivering enough value to justify its price for a meaningful share of customers. B2C subscription products tolerate higher churn since acquisition costs and price points are typically lower, but the same underlying math applies: even a 5% monthly churn rate mathematically caps how large a stable subscriber base can ever get, no matter how much you spend on acquisition.
Why churn matters more than the growth rate it's paired with
A business adding 100 customers a month while losing 90 looks identical on a "customers gained" chart to a business adding 20 while losing 10, but the first business is running twice the acquisition spend just to stand still. Churn is also directly linked to two other numbers worth tracking together: it determines how long an average customer stays, which feeds straight into lifetime value, and it's the single biggest lever in MRR growth, since reducing churn compounds the same way growth does, just in the opposite direction.