CAC, CPA, ROAS and MER: which one to watch
All four answer different questions, and picking the wrong one costs money. The CPA-versus-CAC confusion, a worked example, and how to calculate your CAC ceiling.
Both of these can be said in the same meeting without either being false: “the campaign is running at 2.7 ROAS” and “we are losing money on every new customer”. It happens more often than it should, and almost always for the same reason: the wrong metric is being watched for that type of business.
The four that get confused are CPL, CAC, ROAS and MER. They do not compete: they answer different questions, and which one to watch depends on how you make money.
The confusion that costs the most
It is CPA versus CAC, and it sneaks into almost every report. CPA is the cost of the action somebody decided to call a conversion: usually a form. CAC is the cost of getting someone who actually paid.
Between the two sits your close rate, and that rate is rarely anywhere near 100%. If one in eight leads closes, CAC is eight times CPL. Presenting CPL as the cost of a customer is not lying: it is dividing by the wrong number.
The four metrics, in one line each
| Metric | What it divides | What it answers |
|---|---|---|
| CPL / CPA | Spend ÷ actions obtained | What it costs me to get someone to raise their hand |
| CAC | Spend ÷ customers who paid | What a real customer costs me |
| ROAS | Attributed revenue ÷ channel spend | What this channel returned, according to that channel |
| MER | Total revenue ÷ total spend | Whether the whole business improves while I spend |
Look at the third column: four different questions. ROAS is the only one that depends on a third party’s opinion, because “attributed revenue” is decided by the platform’s model. We wrote about that gap separately: why platform ROAS does not match your revenue.
A worked example (invented numbers)
Note: the figures below are invented to illustrate the mechanics. They come from no account, no client and no market average. They are here to show how the four metrics move together, not as a benchmark for what anything “should” cost.
| Figure | Value |
|---|---|
| Ad spend for the month | 3,000,000 |
| Leads | 150 |
| CPL | 20,000 |
| Customers closed | 18 (12% of leads) |
| CAC | 166,667 |
| Average ticket | 450,000 |
| Revenue from new customers | 8,100,000 |
| ROAS on that revenue | 2.70 |
| Total business revenue | 14,500,000 |
| MER | 4.83 |
At a glance everything looks fine: a 2.70 ROAS and a 4.83 MER are numbers nobody would argue with in a meeting. Until margin shows up.
Assume a 40% gross margin. Each customer then leaves 180,000 in contribution against a CAC of 166,667. That is 13,333 per customer left over. The business is not losing money, but it is far closer to the line than a 2.70 ROAS suggested — and any dip in close rate crosses it.
And here is the interesting part: the cheapest lever is not lowering CPL. At the same spend, raising the close rate from 12% to 15% puts CAC at 133,333, and the leftover contribution goes from 13,333 to 46,667. That does not get fixed in the ad platform: it gets fixed in response speed, lead qualification and what happens after the form.
Which one to watch, by business type
- Direct online sale, one-off purchase. ROAS at campaign level to operate, MER at business level to set budget. CAC matters less because the sale happens at the moment of the click.
- Lead generation with a human sale. CAC against margin, always. CPL is for comparing channels with each other, never for deciding whether it is worth it.
- Subscription or recurring revenue. CAC against lifetime value, not against the first sale. A CAC that loses money in month one can be excellent by month six.
- High-repeat business. MER above everything. ROAS only sees the first purchase and will systematically understate what advertising is generating.
The cross-cutting mistake is using whichever number the platform shows by default — almost always ROAS — regardless of which of these four cases you are in.
The number nobody calculates: your CAC ceiling
Before arguing whether a CAC is high or low, you need to know what it is being compared against. The ceiling is calculated without looking at any platform:
- Take your contribution margin per customer, not the ticket.
- If there is repeat purchase, multiply by the purchases a customer makes over a horizon you can actually sustain.
- Subtract what you need left as profit.
- What remains is the most you can pay for a customer.
That number is what turns “CAC went up” into a decision. Without it, any CAC looks high when the month is weak and low when it is strong.
FAQ
- Are CPA and CAC the same thing? No, and that confusion is expensive. CPA is the cost of the action you defined as a conversion — usually a lead. CAC is the cost of acquiring a paying customer. If only a fraction of leads buy, the two numbers are separated by exactly that fraction.
- What is a good ROAS? There is no universal answer, because ROAS ignores margin. A ROAS of 2 is excellent for a business with a 70% margin and a loss for one with 30%. Anyone quoting a target ROAS without asking about your margin is guessing.
- Why look at MER if I already have ROAS? Because ROAS is an attribution answer and MER is not. MER divides total revenue by total spend, so no platform has an opinion in it. It will not tell you which channel worked, but it will tell you whether the whole business is improving while you spend.
- Which one do I report to management? MER and CAC against your CAC ceiling. Those two say whether the business is growing and whether each customer is worth what it cost. ROAS and CPA are operating instruments for whoever runs the campaigns.