Why it matters
It tells you whether growth pays for itself. Compare it with what a customer is worth (lifetime value, or LTV). If a customer pays you $50 a month for 20 months, that's $1,000 of revenue; a CAC of $1,500 loses money on every sale.
How to calculate it honestly:
- Include ad spend, tools, freelancers, agency fees and the people's time you can reasonably attribute.
- Use the same period for spend and new customers.
- Look at it by channel when you can, and blended for the whole business.
Founders often track a simpler number first, like cost per trial or cost per sign-up, and then follow those through to paying customers.
How Cosmo handles it
Cosmo reports cost per sign-up and cost per customer from your ad accounts, Stripe and GA4, in the morning brief and the Monday report ("9 sign-ups at $16 each"). When a channel's cost climbs past the line you set, he proposes the cut as an approval card.