The calculation is total sales and marketing spend in a period divided by the number of new customers won in that period. The honest version includes more than ad spend: content, tools, agency fees, commissions, and the portion of your own time spent selling. Founders routinely quote a CAC that only counts advertising, then wonder why the business feels tighter than the spreadsheet says.
CAC only means something next to two other numbers: what a customer is worth over their lifetime, and how long it takes to earn the acquisition cost back. A $400 CAC is superb for a $6,000 annual contract and fatal for a $15 one-off product. Payback period matters just as much — a CAC recovered in two months lets you reinvest six times a year; the same CAC recovered in fourteen months quietly consumes your runway even while revenue climbs. Track CAC per channel, not just in aggregate, or a single expensive channel will hide inside a healthy average.
A concrete example: last quarter you spent $9,000 — $6,000 on ads, $2,000 on a freelance writer, $1,000 on tools — and signed 30 customers. CAC is $300. Your product is $60 a month with a 65% gross margin, so you earn about $39 a month per customer. Payback is roughly eight months. That is workable if customers stay two years and dangerous if they leave after nine months.