The two numbers that decide a subscription business
LTV (lifetime value) is the total gross profit a customer generates before they leave: monthly revenue × gross margin × average lifespan. CAC (customer acquisition cost) is what you spend on sales and marketing to win one customer: total spend ÷ new customers. Together they answer the only question that matters for growth: does each customer bring in more than they cost to acquire?
The 3:1 rule
The widely-cited benchmark is an LTV:CAC ratio around 3:1 to 5:1. Below 3, you're spending too much relative to what customers are worth — growth burns cash. Above 5, counterintuitively, you may be under-investing: you could likely spend more to acquire customers faster and still profit. Exactly 1:1 means each customer just barely pays back their acquisition cost, leaving nothing for overhead or profit.
Payback period: the cash-flow reality
A great ratio can still strain cash if it takes too long to recover CAC. The CAC payback period — months of gross profit needed to earn back acquisition cost — should ideally be under 12 months for a healthy startup. A 3:1 ratio with a 3-month payback is a money machine; the same ratio with a 30-month payback can bankrupt you before the value arrives, because you fund acquisition today and collect slowly.
Improving the ratio
- Raise LTV: reduce churn (longer lifespan is the biggest lever), increase prices, or expand revenue per customer through upsells.
- Lower CAC: improve conversion, lean on organic and referral channels, and target higher-intent audiences.
- Watch churn above all — because LTV multiplies by lifespan, cutting churn compounds through the whole model.
These unit economics are what separate a business that grows profitably from one that simply buys revenue at a loss. Get the ratio and payback right, and scaling makes you money; get them wrong, and scaling accelerates the losses.