Churn and What It Does to Your Runway
Churn gets discussed as a customer satisfaction metric, which undersells it. What churn actually does is set a hard ceiling on how large your business can ever get at your current rate of sales — and you can calculate exactly where that ceiling sits with one division.
That number is worth knowing before you plan a hire, model a funding round, or convince yourself that growth will solve a retention problem. It usually won't.
The ceiling
Every month you add some new recurring revenue and lose a percentage of what you already had. Those two forces meet at a point where the revenue you're losing exactly equals the revenue you're adding, and your MRR stops growing no matter how well sales are doing.
The formula is unglamorous:
Ceiling MRR = new MRR per month ÷ monthly churn rate
Suppose you're adding £5,000 of new MRR a month. Here's where you top out:
| Monthly churn | Ceiling MRR | Ceiling ARR |
|---|---|---|
| 2% | £250,000 | £3.0m |
| 3% | £166,667 | £2.0m |
| 5% | £100,000 | £1.2m |
Same sales performance in all three rows. Going from 5% churn to 2% churn is worth £1.8m of ARR without selling a single extra deal. That is a much larger lever than most founders realise they're holding, and it's why "we'll grow out of it" is generally the wrong instinct.
Small percentages, compounded
Monthly churn figures sound harmless in isolation. Compound them over a year and they stop sounding harmless.
| Monthly churn | Still there after 12 months | Average customer lifetime |
|---|---|---|
| 1% | 88.6% | 100 months |
| 2% | 78.5% | 50 months |
| 3% | 69.4% | 33 months |
| 5% | 54.0% | 20 months |
| 7% | 41.9% | 14 months |
At 5% a month you replace your entire customer base roughly every twenty months. Every acquisition cost you paid gets paid again, permanently, just to stand still. If your payback period on acquisition is longer than that average lifetime, you are losing money on each customer and growing faster makes it worse rather than better.
Two numbers, not one
Customer churn and revenue churn are different measurements and the gap between them is informative.
Customer churn
The proportion of accounts that leave. Treats a £20/month hobbyist and a £2,000/month enterprise account as equally significant, which for cash purposes they very much are not.
Revenue churn
The proportion of MRR that leaves. This is the one that hits your bank account, and it also captures downgrades — a customer halving their plan is invisible to customer churn but is very real revenue churn.
If revenue churn runs meaningfully higher than customer churn, your larger accounts are the ones leaving, and that's a different problem with a different fix than losing a tail of small accounts. If it's lower, you're losing small customers while keeping big ones, which is uncomfortable but survivable.
There's a third possibility worth aiming at. If your existing customers expand — upgrades, seats, usage — by more than your losses, net revenue churn goes negative and your MRR grows even in a month where you sell nothing at all. That removes the ceiling entirely, and it's the single strongest thing a subscription business can demonstrate to an investor.
The bit that catches people out
Early on, churn barely shows up. If you signed twenty customers in the last three months, almost nobody has had time to leave yet, and your churn rate looks superb because the denominator is full of accounts that are too new to have cancelled.
The real number surfaces six to twelve months later, typically at the exact moment you've hired against the growth curve you extrapolated from the good early figures. This is a common way for a company with genuinely rising revenue to run out of money.
Approaching the ceiling is also gradual rather than sudden, which makes it easy to misread. At 3% monthly churn you reach half your ceiling in about 23 months and 90% of it in about 76 months. Growth doesn't stop, it just decelerates, and month by month that's almost impossible to distinguish from a temporary sales dip.
What to do with this
Work out your ceiling. Divide your monthly new MRR by your monthly revenue churn rate. If that number is below what you need the business to be worth, no amount of sales effort fixes it and retention work is now your growth strategy.
Then put honest revenue projections into the runway calculator — projections with churn subtracted, not gross new sales. And check the break-even calculator against your ceiling: if your ceiling MRR sits below your monthly costs, the current business model cannot reach profitability at any point in the future. Better to know that now than after another two hires.
One practical note: measure cohorts, not aggregates. Group customers by the month they joined and track each group over time. It's more work than a single blended figure and it's the only way to see whether the product is actually getting stickier or whether new sales are just masking the same leak.