Blended versus paid CAC
Tell blended, paid and fully loaded CAC apart, and decide which one goes to the board, to a channel owner and to an investor.
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The same month of the invoicing app produces two CACs, and one is 40% lower than the other. Paid CAC divides paid spend by the customers paid media acquired; blended CAC divides the same spend by every new customer, including the ones who arrived without a paid click. Both are arithmetically right, and each answers a different question. Reporting the wrong one to the wrong person is how a company doubles its spend on a number that was flattering it.
One month, two CACs
In the month the funnel lesson followed, $50,000 of paid search and paid social produced 96 paying customers. Another 64 started paying without a paid click: they found a post in search results, heard from a colleague, or typed the name. That is 160 new customers in all.
paid CAC = $50,000 ÷ 96 = $521
blended CAC = $50,000 ÷ 160 = $313
The organic 64 cost no media, so they pull the average down. Run both through the payback formula from CAC, LTV and payback, 800 × (1 − 0.95ⁿ), and they give different answers to the question every board asks. At $313 payback is month 10, inside the course's 12-month target. At $521 it is month 21, nine months outside it.
The mistake: scaling spend on the blended figure
The founder reads the dashboard: CAC $313, payback in ten months, well inside the target. The obvious move is to double paid spend to $100,000 a month.
The next $50,000 buys paid customers. The 64 organic customers arrive whether spend doubles or stops, so none of them is for sale. At best the second $50,000 buys another 96 customers at $521. More likely it buys fewer, because it reaches many of the people the first $50,000 already reached, and the customers it adds cost more than $521 each. Doubling spend on a $313 dashboard buys customers that pay back in month 21 or later.
The dashboard will not show it. After the doubling, blended CAC is $100,000 ÷ (192 + 64) = $391 even in the best case, still close to the target, while every customer the extra money bought cost $521 or more. The figure that moves too slowly to warn anyone is the one the decision was made on.
The number for a spend decision is the paid CAC, and better still the marginal paid CAC: what the last increment bought. If cutting spend from $50,000 to $40,000 loses only 6 of the 96 paid customers, the last $10,000 was buying customers at $1,667 each, more than twice the $800 lifetime margin, and it should stay cut.
Why a rising organic share flatters the blended figure
Organic customers grow for reasons that have nothing to do with the paid program: a content library that compounds, word of mouth, a referral credit, a brand people have started to search for. Hold paid at $50,000 and 96 customers, and let organic grow:
Now let paid get worse at the same time. Next year the same $50,000 buys 80 customers, so paid CAC rises to $625 and payback to month 30. Organic reaches 200. Blended CAC is $50,000 ÷ 280 = $179: down 43% on the year, while paid got 20% more expensive. A blended trend line reports the content team's success as the paid team's efficiency, and the board approves more paid spend on the strength of it.
The organic count has its own error, in the other direction. Some customers labelled organic saw an ad first and later typed the name, and the last click goes to the search result. If so, paid causes more customers than the 96 it is credited with, and its true CAC is below $521. No dashboard can say how many. Switching paid off in a few regions for a month and counting what happens to organic sign-ups there can; when an A/B test is the wrong instrument sets up that kind of holdout, and the five decisions explains why attribution allocates credit without measuring cause.
Fully loaded: the cost neither figure counts
Both figures count media only. The company also spends $36,000 a month on the people and tools that run acquisition: a performance marketer, a content writer, an agency retainer and the analytics tools. Organic customers are not free; they cost the writer and the content budget.
fully loaded CAC = (media + team and tools) ÷ all new customers
= ($50,000 + $36,000) ÷ 160 = $538
At $538 payback is month 22. It sits close to the paid figure by coincidence; it answers a different question, which is what the whole company spends to win a customer. It is also the figure an investor computes without asking, by dividing the sales and marketing line of the P&L by new customers.
Which number goes to whom
| Reader | The decision they make | The number they need | Here |
|---|---|---|---|
| Channel owner | where the next $10,000 in their channel goes | paid CAC of their channel, media only, and the marginal CAC of the last increment | $521 on average; the marginal figure comes from a budget test |
| Board | how much paid spend to approve | paid CAC and its payback month, with blended beside it and the organic share named | $521, month 21; blended $313 at 40% organic |
| Investor | whether the whole engine is efficient enough to fund | fully loaded CAC and its payback, with paid CAC and the organic share as trends | $538, month 22 |
The channel owner needs the number their own decisions move. Blended CAC would credit them with customers they did not buy, and fully loaded would charge them for a content writer they do not manage.
The board approves a spend, and spend buys paid customers, so the headline is paid CAC as a payback month. Blended goes beside it with the organic share stated, because a rising share is good news about content and no news about paid.
The investor will rebuild the fully loaded figure from the P&L in diligence. Give it to them first, with its payback month. A founder who presents $313 to someone about to compute $538 has spent credibility that the rest of the deck needed.
In practice
To the board: "Paid CAC is $521 and pays back in month 21 against a 12-month target. Blended is $313 because 40% of new customers arrive without a paid click; that share is rising, which is the content program working." Then ask for the spend decision on the paid number, and for the holdout that would show whether organic depends on paid.
To the channel owner: report and reward on marginal paid CAC for their own channel, measured on each increment of budget, never on the blended trend.
The mistake that costs money is approving paid spend on the blended CAC. The next $50,000 buys paid customers at $521 or more, not at $313, and the blended figure rises too slowly to warn anyone.
Check yourself
1. Next quarter organic customers double to 128 while paid stays at $50,000 for 96. Blended CAC falls to $223, and the head of paid asks for a bonus on "CAC down 29%". Do you pay it?
No. Paid CAC is $50,000 ÷ 96 = $521, unchanged, and payback is still month 21. The fall came from 64 more organic customers, so the credit belongs to whoever runs content and referral. Pay the paid team on their marginal paid CAC.
2. An investor's analyst divides $86,000 a month of sales and marketing spend by 160 new customers. You had shown $313. What do they compute, and what should the slide have said?
$538, which pays back in month 22, not month 10. The slide should have shown $538 fully loaded with its payback month, and paid ($521) and blended ($313) beside it with the 40% organic share, so the analyst's figure confirms the deck instead of contradicting it.
3. The paid team trims spend from $50,000 to $45,000 for a month as a test, and paid customers fall from 96 to 91. Do you restore the $5,000?
No. The last $5,000 bought 5 customers, $1,000 each, against $800 of lifetime margin, so it never pays back at any horizon, even though the average paid CAC of $495 looks better than before. Keep the cut, try the next $5,000 down the same way, and offer the saving to the channel with the lowest marginal CAC.