Customer Acquisition Cost
Date: 2026-08-16
What it costs to get one new customer. Two definitions circulate — blended and paid — they differ by a large factor, and quoting the wrong one flatters everything downstream.
What it is
Customer acquisition cost (CAC) is total acquisition spend divided by the number of new customers it produced.
The two versions, on the running model:
BLENDED CAC
all marketing spend £225,000
÷ ALL new customers 10,000
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£22.50
PAID CAC
paid media spend only £180,000
÷ new customers from paid 4,500
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£40.00
Nearly double. Blended CAC divides paid spend across customers who arrived organically, by word of mouth, or through direct search — none of whom the spend acquired.
Which to use
| Question | Use |
|---|---|
| Is the business viable overall? | Blended |
| Should we spend more on this channel? | Paid, by channel |
| What’s our marketing efficiency? | Blended, tracked over time |
| Can we afford this campaign? | Paid, marginal — see below |
The dangerous combination is blended CAC with paid decisions. It makes every channel look affordable, because organic customers are subsidising the maths. The reverse — paid CAC in a board pack — makes a healthy business look broken.
Say which one you’re quoting, every time.
Marginal CAC is the one for decisions
Average CAC tells you what you’ve paid. Marginal CAC tells you what the next customer costs, and it’s almost always higher — the cheapest, highest-intent audiences are bought first.
spend new customers marginal CAC
£100k 3,000 £33
£140k 4,000 £40 ← the extra £40k bought 1,000
£180k 4,500 £80 ← the extra £40k bought only 500
Average CAC at £180k is £40. Marginal CAC is £80 — and the decision “should we spend another £40,000” is answered by £80, not £40.
In plain terms: the average includes the cheap customers you already had. The question is always whether the next one is worth it, and the next one is dearer than the last.
Diminishing returns is why a channel that looks efficient on average can be unprofitable at the margin, and it’s the single most useful thing to understand about paid budgets.
What belongs in the numerator
- Media spend — yes
- Agency fees and creative production — yes, they’re part of the cost of acquiring
- Affiliate commission and referral incentives — yes
- Discounts offered to first-time buyers — yes. A £10 welcome code is £10 of acquisition cost, and excluding it understates CAC systematically
- In-house salaries — usually excluded for channel decisions, included in a full blended figure
- Retention and lifecycle marketing — no. That’s spend against existing customers, and mixing it in inflates CAC while hiding retention economics
That fourth point is the one most often missed, and it flatters CAC most in businesses that lean on welcome offers.
What it’s compared against
CAC on its own means nothing. It’s only ever read against:
- Customer Lifetime Value — is the customer worth more than they cost — LTV to CAC Ratio
- Payback Period — how long until you get the money back, which is the cash-flow constraint the ratio hides
- First-order Contribution Margin — on the model above, a £15 contribution against a £22.50 CAC means every new customer loses £7.50 on their first order. Whether that’s fine depends entirely on whether they come back
Where it’s measured badly
- Attribution. Which customers a channel acquired is an attribution question, with all its problems — Attribution Models, Walled Garden Reporting
- New versus returning. Paying to reacquire existing customers inflates apparent new-customer volume — New vs Returning Customer Acquisition
- Lag. Spend in one period acquires customers in the next, so monthly CAC is noisy on long consideration cycles
- Incrementality. CAC assumes the spend caused the acquisition. Much of it didn’t — Incrementality Testing