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Glossary · Returns and budgets

What is marginal CAC?

Marginal CAC is the cost of acquiring the next customer in a channel at its current spend level. It rises as the channel saturates, and it is the figure to compare against customer lifetime value when deciding how much to spend.

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Reviewed November 2026

By the Odins team · Updated November 2026 · About 3 minutes

Definition

Customer acquisition cost is spend divided by the customers it brought in. Average CAC is that ratio over a period. Marginal CAC is what the next customer would cost at today's spend level in a channel. On a channel with diminishing returns the next customer is dearer than the average one, so marginal CAC is higher than average CAC, and the gap widens the further along the curve the channel is.

How it works

A marketing mix model estimates a response curve per channel in whatever outcome the business counts: orders, applications, new members, sign-ups. The slope of that curve at today's spend, inverted, is the marginal CAC. A team that steers day to day on a per-channel CAC threshold is steering on an average; the curve says that many customers in the channel were cheaper than the threshold and the next ones are dearer.

How much to spend, decided on the margin
Total marketing budgetReturn on the next unit spent (marginal)Return the business requiresTodayRight-sizedEvery unit between today and the crossingstill clears the required returnBeyond the crossing, the next unitfalls short of the required returnThe budget question, answered on the margin: spend until the next unit no longer clears the return the business requires.

In practice

The useful comparison is marginal CAC against what a customer is worth. State the long-term value of a customer per channel, and hold both the average and the marginal CAC up against it: a channel is worth scaling while its marginal CAC is below that value, and worth trimming when it is above. That replaces a flat threshold with a decision rule that follows the curve.

For the total budget, the same rule sets the level: find the spend where the last customer still costs less than the business requires. For subscription and lending businesses this is usually the sharper question than ROAS, and the Odins model can be set up to target the outcome the business actually counts.

Related terms

  • Marginal ROAS: What the next unit of spend in a channel earns at today's level. The number a budget change actually asks for.
  • ROAS: Return on ad spend: revenue per unit of spend. Useful, as long as you know which ROAS you are looking at.
  • Saturation curve: The response curve per channel: how sales respond to spend, and where the next unit stops paying.
  • Budget optimization: Finding the split and the total where the next unit works hardest, under the constraints the business has.

Go one level deeper: Set the right marketing budget. MMM for banks, fintech and long sales cycles. Or go back to all terms in the glossary.

FAQ

Frequently asked questions.

Why is marginal CAC higher than average CAC?

Because the easiest customers are reached first. The average mixes the cheap early customers with the expensive recent ones; the marginal figure is the price of the next one.

Can the model target customers instead of revenue?

Yes. The modeled outcome can be orders, applications, new members or revenue, decided per market with the customer. The marginal figures follow the outcome chosen.

How does marginal CAC relate to customer lifetime value?

Lifetime value sets the ceiling: a channel is worth scaling while its marginal CAC is below the value of the customers it brings in. That turns a CAC threshold into a rule that follows the curve.

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