Every growth decision eventually reduces to one question: is a customer worth more than it cost to acquire them? The LTV:CAC ratio answers it directly, lifetime value divided by customer acquisition cost. Get it right and you can spend confidently to grow; get it wrong and you’re buying customers at a loss.
The two numbers
CAC (customer acquisition cost) is what it costs, on average, to win one customer:
CAC = total sales & marketing spend ÷ new customers acquiredLTV (customer lifetime value) is the profit a customer generates over their whole relationship with you:
LTV ≈ avg revenue per customer × gross margin × avg lifetimewhere average lifetime ≈ 1 ÷ churn rate. The single most common mistake is using raw revenue instead of gross margin: that overstates LTV by ignoring the cost of actually serving the customer. Always margin-adjust.
What’s a good ratio?
The famous heuristic is 3:1: each customer worth about three times their acquisition cost. As a sanity check it’s useful:
- Below ~1:1: you lose money on every customer. Unsustainable unless you’re deliberately buying market share with a plan to fix economics later.
- Around 3:1: the commonly-cited healthy zone, with profitable acquisition and room to invest.
- Well above ~5:1, often a sign of under-spending. You could likely grow faster by acquiring more, even at a lower ratio.
But 3:1 is a rule of thumb, not a law. It depends on your margins, how long customers actually stay, and, crucially, how long your cash is tied up before it comes back.
Why payback period matters as much
A great LTV:CAC ratio can still strangle a business if the “lifetime” takes years to play out. CAC payback period (how many months of margin it takes to recover acquisition cost) measures the cash reality. Two companies can both hit 3:1, but the one that recovers CAC in 6 months can reinvest far faster than the one that takes 24. Many software teams target payback within roughly a year.
Calculate yours
Use the free CAC & payback calculator to get acquisition cost, the LTV:CAC ratio, and payback period, and the customer lifetime value calculator to build LTV from ARPU, margin, and churn. The inputs only become trustworthy when they’re grounded in real behavior - retention cohorts for churn, conversion for the funnel feeding CAC, which is exactly what product analytics gives you.
The bottom line
Margin-adjust your LTV, treat 3:1 as a compass rather than a target, and watch payback period alongside the ratio. Healthy unit economics aren’t one magic number: they’re a worthwhile customer, won at a sane cost, paid back fast enough to fund the next one.