Unit economics: CAC, LTV, and payback

Customer acquisition cost is total sales and marketing spend in a period divided by the number of new customers acquired in that period. The word doing the work is total. Advertising is the obvious component, but salaries for the sales and marketing team, commissions, tooling, events and content production all belong in the numerator.

Founders routinely quote a blended figure that includes customers who arrived organically, which flatters the number badly. The useful metric is paid CAC — spend divided by customers who came from that spend — reported next to the organic count, not merged into it.

The standard formula is average revenue per account multiplied by gross margin, divided by monthly churn rate. A customer paying $100 a month at 80% gross margin with 2% monthly churn is worth $100 × 0.8 / 0.02 = $4,000. Move churn to 5% and the same customer is worth $1,600.

That sensitivity is the whole story. Nothing else in the formula moves lifetime value the way churn does, which is why retention work usually beats acquisition work on return per hour spent. It is also why lifetime value quoted without the churn assumption behind it should be treated as decoration.

The ratio everyone quotes is LTV divided by CAC, with three-to-one treated as the threshold of a healthy business. It is a reasonable check, but it hides the constraint that actually bites: a lifetime value spread over four years does not pay this quarter's payroll.

Payback period — CAC divided by monthly gross profit per customer — tells you how many months of that customer's payments it takes to recover what you spent to win them. Under twelve months, growth largely funds itself. Beyond eighteen, every new customer widens the cash hole before it narrows it, and growth has to be financed by investors.

In products where customers grow their spend over time — more seats, more usage, upgraded plans — the correct measure is net revenue retention: revenue from a cohort this year divided by revenue from the same cohort last year, including expansion, contraction and cancellations.

Above one hundred percent, a cohort grows even with no new customers at all, and a longer payback period becomes tolerable because the customer is worth more every year. Below one hundred percent, acquisition is running on a treadmill, and no amount of top-of-funnel spending fixes it.

Using revenue instead of gross profit in the lifetime value calculation inflates the answer by whatever your cost of service is — often twenty to fifty percent. Excluding salaries from acquisition cost inflates it again. Both errors are common enough that an unfamiliar LTV/CAC ratio should be recalculated from raw inputs before it is trusted.

The subtler mistake is averaging across segments that behave nothing alike. Enterprise customers with long sales cycles and near-zero churn averaged together with self-serve customers who cancel in month three produce a composite customer who does not exist. Segment first, then calculate.

Garage to IPO tracks acquisition cost per user against the marketing channels you fund. Organic growth is slow but effectively free, paid acquisition is fast and expensive, and enterprise sales is slowest, costliest and stickiest — a direct trade between payback period and durability.

Product quality feeds churn, churn feeds lifetime value, and lifetime value determines whether your marketing budget compounds or drains the account. A run that pours money into paid channels while ignoring product stability will show impressive user growth and a valuation that refuses to move.