Payback Period

Whether the economics work could already be checked with a ratio. Divide lifetime value (LTV), the estimate of what one customer leaves behind, by the customer acquisition cost (CAC) it took to get them, and clearing 3x meant the advertising was worth scaling.

But companies with a healthy ratio kept running out of cash and stopping. Even at 4x, if that 4x trickles in over three years, none of it is available for this month's ad budget. The faster you acquire, the more the outflow leads and the inflow lags.

So a number that measures when the money comes back became necessary alongside how much comes back. The ratio answers whether the business works; this answers whether it can be pushed at the current speed.

Divide the acquisition cost by the profit one customer leaves per month.

1. Get the acquisition cost: ad spend plus marketing salaries, divided by the customers acquired in that period.
2. Get the monthly contribution profit: the price minus server costs, payment fees, and support costs. At $20 a month with an 80 percent gross margin, that's $16.
3. Divide. With an acquisition cost of $60, $60 over $16 gives 3.8 months.

Not dividing by revenue is the crux of this calculation. The same case divided by revenue reads 3 months, and the money actually available is less than that. Paying the platform and the servers happens before any of the price is yours.

Avoid computing it on averages too. Group the customers who arrived in the same month and find the month their cumulative profit passes the acquisition cost. On paper 3.8 months, but in a channel where half of them leave within three, the payback never actually happens.

Calculate it per channel. In the same product it's common to see four months for customers arriving from search and eleven for customers arriving from a feed. Collapse that difference into one average and where the budget should move stops being visible.

"LTV over CAC is 4x, but payback is 14 months. Cash won't hold at this speed."

Unit economics and cash flow are different questions.

"Divide by contribution profit, not revenue. Server costs and fees are missing."

Fix the denominator.

"Split by channel and search is four months while the feed is eleven."

Evidence for moving budget.

Pagedeck looked at this number first in the quarter it planned to scale advertising. Ravi reported five months on the blended average, and Grace asked to split it by channel. Search came back at four months, the feed at eleven, and the feed was consuming half the ad budget.

Halving the feed budget and moving it to search brought the overall figure down to 3.4 months. Slightly fewer customers arrived, and the same cash bought more months of advertising.