Finance.

The framework

1. 01

   Everything you spent to win them belongs on top.

   Fully loaded acquisition cost is every dollar spent winning customers in a period, divided by the customers that spending actually won. Every dollar means media, the salaries and commissions of everyone selling and marketing, tools, agency, events, content, and the cost of servicing the trials that didn't convert. Most businesses divide advertising spend by all new customers and land on a figure that's wrong in two directions at once: the top is missing most of the cost, and the bottom includes people who arrived by word of mouth and cost nothing. Both errors push the number the same way, which is why the CAC a founder quotes is routinely about half the real one.

2. 02

   Denominate in gross profit, monthly, per customer.

   The denominator isn't what a customer pays you each month. It's what survives after the cost of serving them: hosting, support, payment fees, delivery, the variable cost you met on Day 35. Divide loaded CAC by that and you have the months until the acquisition is square. Using revenue instead is the most common way this calculation gets understated, and the size of the understatement is exactly one over your margin. At eighty per cent it reads a fifth short, which is survivable. At thirty per cent it reads more than threefold short, which is the difference between a business that funds itself and one that quietly can't.

3. 03

   The payback period is a cash statement.

   Every customer you acquire is a loan you make. You pay the full cost on day one and collect it back in instalments. A twelve-month payback means each new customer leaves you out of pocket for a year, so double the rate of acquisition and the hole doubles with it. That's the uncomfortable geometry of subscription growth: the company doing better needs more cash rather than less, and the month the growth looks most impressive is the month the bank balance looks worst. This is Day 36's cash conversion cycle applied to a customer relationship instead of a shipment, and it has closed businesses whose product was working.

4. 04

   Shorten the months before you spend more of them.

   The ceiling is set by whoever funds the gap, not by a rule of thumb. If the cash is yours and there's no more of it, twelve months is reckless and four is the mandate; if a funded competitor is taking the market, a longer payback bought with somebody else's capital can be correct aggression. Then work the levers, and notice that the fastest one has nothing to do with cost. Ask for the year up front at a discount and payback collapses to the first invoice. A deposit, an onboarding fee, a monthly cohort moved to annual: each pulls cash forward without touching CAC at all.