Marginal CAC is the cost to acquire one more customer from an increase in spend, calculated as the extra spend divided by the extra customers it produced. Unlike blended CAC, which averages every customer ever acquired, marginal CAC isolates what the latest increment actually cost, which is what reveals when scaling stops being profitable.
Marginal vs blended CAC
Blended CAC averages your entire history, so it moves slowly and hides deterioration. Marginal CAC looks only at the delta. Take spend from $10,000 to $13,000, divide that extra $3,000 by the extra customers it brought, and you see what the increment truly cost, which can run well above the blended average while the headline number still looks fine.
Why it finds your ceiling
Scaling has two questions: how fast to move, and when to stop. A percentage rule answers pace; marginal CAC answers the ceiling. When the marginal cost stops clearing what a customer is worth to you, you have found the profitable limit, whatever percentage you used to get there. It is the number to watch when you scale a Google Ads budget, read alongside CAC and payback.