Unit economics: CAC, LTV, and payback

Customer acquisition cost is total sales and marketing spend divided by the number of new customers that spend produced. The manipulation almost always happens in the numerator: leaving out sales salaries, agency fees, or the free trial infrastructure makes the number look far better than it is.

Blended CAC — all spend divided by all new customers, including the ones who arrived organically — is the most flattering and least useful version. Paid CAC, which divides paid spend by customers attributable to it, is what tells you whether you can buy growth. Track both and never quote only the first.

The common formula is average revenue per user times gross margin divided by monthly churn rate. Every term in it is uncertain, and the churn term sits in the denominator, so small errors there produce enormous swings in the result. At two percent monthly churn implied lifetime is roughly fifty months; at four percent it is twenty-five.

Two disciplines make the number defensible. Use gross margin, not revenue, so you are counting money the business keeps. And cap the horizon at something you can observe — a three-year LTV from a company that is eighteen months old is a forecast, not a measurement.

The three-to-one LTV-to-CAC rule of thumb is widely quoted and easy to game, because LTV is the softest number in the business. A company can hit the ratio comfortably while running out of cash, since the ratio contains no information about when the money arrives.

It also ignores scale. Early customers are cheap; the cost of the next thousand rises as you exhaust the easiest channels. A ratio measured on the first cohort rarely holds at ten times the spend, which is precisely the assumption a growth round is buying.

Payback period is how many months of gross profit it takes to recover the cost of acquiring a customer. It uses only observed data, and it maps directly onto cash: a twelve-month payback means every customer you buy today is a cash hole until next year.

Consumer subscription businesses generally want payback under six months. Business software commonly tolerates twelve to eighteen. Beyond twenty-four months you are running a financing operation as much as a product company, and each acceleration of growth makes the cash position worse before it makes it better.

In Garage to IPO your marketing allocation across organic, paid, and enterprise channels sets both the cost and the quality of the customers you acquire, while research and development spending raises product stability, which suppresses churn and lifts average revenue per user.

Because churn feeds revenue and revenue feeds valuation, a run that buys growth without fixing retention shows the same pathology as the real thing: impressive top-line numbers, a shrinking runway, and term sheets that get worse rather than better.