Churn rate definition
Churn rate is the rate at which a SaaS business loses customers or revenue over a defined period. A monthly churn rate of 2% means the business loses 2% of its customer base (or ARR) every month. Compounded over a year, 2% monthly churn produces roughly 21% annual attrition — a significant drag on growth.
Churn has two distinct flavours: logo churn (the number of customers cancelling) and revenue churn (the ARR lost from cancellations and downgrades). These metrics can diverge substantially. A business that loses many small customers but retains its large enterprise accounts may have high logo churn but low revenue churn. Both metrics matter, but revenue churn is more directly connected to ARR and NRR outcomes.
How churn is calculated
Logo (customer) churn rate: Customers who cancelled in period ÷ Customers at start of period × 100.
Gross Revenue Churn rate: ARR lost from cancellations and downgrades in period ÷ Opening ARR × 100. This is the "gross" figure — it does not net off expansion revenue.
Net Revenue Retention (NRR) is the inverse complement: NRR = 100% − Gross Revenue Churn% + Expansion Revenue%. When NRR exceeds 100%, expansion is outrunning churn.
Churn is typically measured monthly and then annualised. Note that monthly rates do not simply multiply by 12 to give annual rates — a 2% monthly churn is a 21.5% annual churn rate (because you're losing 2% of a compounding smaller base each month).
Why churn is the most important SaaS metric to manage
The compounding nature of SaaS revenue means that churn has an outsized long-run impact compared to its in-period P&L effect. A SaaS business with £10m ARR and 20% annual churn will, all else equal, never grow beyond £50m ARR no matter how much it spends on new customer acquisition — because it is constantly refilling a leaky bucket. The same business at 5% annual churn has a theoretical ARR ceiling 20 times higher for the same new ARR rate.
For this reason, retention is usually a better investment than acquisition at every stage of a SaaS business. Reducing churn by 5 percentage points typically creates more enterprise value than the same investment in new logo acquisition. CFOs who understand this dynamic focus disproportionate analytical attention on the drivers of churn — product fit, customer success investment, pricing structure, and contract terms — rather than purely on top-of-funnel growth.
Cohort analysis and churn
The most rigorous way to understand churn is through cohort analysis: tracking the ARR and logo retention of each group of customers acquired in the same period, month by month, to build a survival curve. Cohort analysis reveals whether churn is stable, improving, or worsening over time, and whether it differs by acquisition channel, customer segment, or product type.
A business where early cohorts churn heavily but recent cohorts retain well has likely improved its product-market fit — a positive signal for investors. A business where churn is consistent or worsening across cohorts has a product or go-to-market problem that no amount of new customer acquisition will fix. Building and maintaining cohort models is a core FP&A capability for SaaS businesses at any scale.
