I've looked at enough subscription business models to know that churn is the variable most early-stage founders underweight — and the one that ultimately determines whether the economics work.
Churn is the rate at which customers stop paying. In subscription businesses it is the most important metric that does not appear in the revenue line — because the revenue line shows what you have gained, and churn quietly shows what you are losing. A business that does not measure churn accurately is optimising the wrong number.
Churn Basics
Monthly churn rate is the percentage of customers (or revenue) that is lost each month. If you begin a month with 400 customers and end with 388, you lost 12 customers. Monthly churn rate = 12 / 400 = 3%.
Customer churn and revenue churn are related but different. Customer churn measures the number of accounts lost. Revenue churn measures the monthly recurring revenue (MRR) lost from cancelled accounts. If the customers who churn tend to be higher-value accounts, revenue churn will be higher than customer churn. If churning customers are smaller accounts, revenue churn will be lower.
Net revenue churn accounts for both losses and expansion revenue — upgrades, additional seats, upsells to existing customers. A business with 4% gross revenue churn but 6% expansion revenue has negative net churn — meaning it is growing revenue from its existing customer base even as it loses some customers. Negative net churn is a powerful growth dynamic that compounds over time.
The Churn Impact Calculator converts your churn rate into its annual revenue impact, showing how much MRR is lost, what the implied customer lifetime is, and what the LTV implications are at different churn levels.
Revenue Impact
The relationship between monthly churn rate and customer lifetime is the inverse of the churn rate. At 5% monthly churn, average customer lifetime is 1 / 0.05 = 20 months. At 2%, it is 50 months. At 1%, 100 months — over eight years.
The revenue impact of small churn differences is larger than intuition suggests. Consider a SaaS business with 600 customers each paying £100/month (£60,000 MRR):
- At 2% monthly churn: 12 customers lost per month = £1,200 MRR lost monthly. Annual customer lifetime = 50 months. LTV at 70% gross margin = £3,500.
- At 4% monthly churn: 24 customers lost per month = £2,400 MRR lost monthly. Annual customer lifetime = 25 months. LTV = £1,750.
- At 6% monthly churn: 36 customers lost per month = £3,600 MRR lost monthly. Annual customer lifetime = 16.7 months. LTV = £1,167.
Moving from 4% to 2% monthly churn doubles LTV. The same improvement doubles the sustainable CAC, meaning the business can spend twice as much on acquisition per customer while maintaining the same ratio. This is why churn reduction frequently unlocks growth capacity more effectively than any marketing investment — it simultaneously increases LTV and justifies higher acquisition spend.
Growth Limits
High churn creates a growth ceiling that becomes increasingly hard to escape. To grow a customer base at high churn rates, new acquisition must not just add to the base — it must replace the lost customers before any net growth occurs.
At 5% monthly churn on a base of 600 customers, you lose 30 customers per month. To grow the base by 5% per month (30 customers of net growth), you need to acquire 60 customers every month — half of all acquisition just replaces losses. At 10% monthly churn, acquiring 60 customers per month produces zero net growth. Every acquisition pound is spent running to stand still.
This dynamic is why high-churn businesses feel perpetually busy and perpetually stressed — enormous acquisition effort produces modest or no visible growth. The leaky bucket metaphor is accurate: pouring water in faster does not help as much as reducing the size of the holes.
The compounding effect works in reverse too. A business reducing monthly churn from 5% to 3% does not just slow the losses — it changes the trajectory. At the same acquisition rate, net growth accelerates, the customer base grows faster, and every subsequent month's churn loss is smaller as a proportion of the growing base. The improvement compounds in the same way that the original problem did.