Short answer: CPA (cost per acquisition) is what you pay for any conversion you have defined as a goal — a lead, a registration, a cart, a purchase. CAC (customer acquisition cost) is what you pay to acquire a net-new customer who actually pays. CPA tracks whatever event you configured and counts only channel spend. CAC counts first-time buyers and typically includes all acquisition costs, not just media.
In a business with no repeat purchase and a single possible conversion, the two numbers are almost identical. In any other case they diverge enough that using one in place of the other drives the opposite decision.
The two formulas
CPA = advertising spend ÷ number of conversions
The conversion is whatever you have configured. That is where the trap lives: two accounts with the same CPA of €25 are not comparable if one counts purchases and the other counts contact-form submissions.
CAC = total acquisition cost ÷ new customers
Total cost includes media, agency fees, tools, and in the strictest version the sales team. New customers: those buying for the first time. An order from a customer you already had does not count in the denominator.
Why CPA misleads in accounts with repeat purchase
A store spends €20,000 in a month and records 500 purchases. CPA = €40. Looks healthy.
Cross-referencing the orders against the customer history reveals that 180 of those 500 purchases came from people who had already bought before. Real new customers: 320.
CAC = 20,000 ÷ 320 = €62.50, 56% above the reported CPA.
The consequences of watching €40 instead of €62.50:
- You scale assuming a margin that doesn't exist. You raise budget against an acquisition cost that is a third more expensive than you believe.
- Retargeting looks like a genius. That is where returning customers concentrate, so its CPA is the best in the account while its contribution of new customers can be marginal.
- The saturation curve shows up late. When a channel starts to exhaust itself, the first thing that rises is the cost of acquiring new customers; the blended CPA masks it because repeat purchase holds it down.
How to separate new customers from returning ones
Three methods, from least to most reliable:
- New-customer segment in the ecommerce platform. Shopify and most platforms include it natively. Fast, and good enough for the monthly read.
- Send the signal in the conversion event. Adding a new-or-returning parameter to the purchase event lets you see the split inside the platform dashboard and, in Google Ads, activate new-customer acquisition bidding, which adjusts the real value of each type.
- Cross orders against the customer base in the data warehouse. This is what we do in the accounts we manage: the source of truth is the back office, with the first order from each email marking the cut. It also lets you calculate CAC by cohort and see whether it is getting more expensive month over month.
Whatever the method, CAC must be read against customer lifetime value, not against average order value. That is the LTV/CAC read.
The other failure mode of CPA: counting conversions that don't exist
Before comparing CPA with CAC, confirm that the denominator of your CPA is not inflated. The two cases we find most often in lead-generation audits:
- Duplicate conversions from configuration. A conversion counted once per click instead of once per session multiplies reported leads. We covered a real case in counting types in Google Ads for lead gen.
- Micro-conversions mixed in with the real ones. If the conversions column combines "purchase", "add to cart" and "view contact page", the CPA is a meaningless average and the algorithm is optimising toward whatever is cheapest to get.
A CPA calculated on inflated conversions makes CAC look disproportionate when the problem is in the measurement, not in the acquisition.
Which one to track and when
- CPA to operate the account. Compare campaigns and creatives against each other daily or weekly. It is what the platform knows how to optimise and it responds quickly.
- CAC to make budget and business decisions. Monthly, measured against customer lifetime value. It is the number that tells you whether the business can grow by buying customers.
- New-customer CAC by channel, in trend. The earliest signal of channel saturation — far ahead of any move in blended ROAS or overall CPA.
Frequently asked questions
Are CAC and CPA the same thing?
No. CPA is the cost of any conversion defined as a goal, including conversions from customers you already had and conversions that are not sales. CAC is the cost of acquiring a net-new paying customer. They only coincide in businesses with no repeat purchase where the only measured conversion is the purchase itself.
Does CAC include the agency fee?
In the most widely used definition, yes: CAC adds up all acquisition costs — media, fees, tools, creative production, and even the sales team in models with assisted selling. What matters is fixing a definition and holding to it, because comparing a media-only CAC with one that includes overhead is meaningless.
How is new-customer CAC calculated?
Divide the total acquisition cost for the period by the number of customers who made their first-ever purchase in that period. The "first purchase" cut comes from the back office by cross-referencing each order against the customer history — not from the advertising platform's dashboard.
What is an acceptable CAC?
One that is clearly below the margin that customer generates over their lifetime. The standard reference is an LTV/CAC ratio of at least 3:1 in businesses with repeat purchase or subscription; in single-purchase models, CAC must stay below the gross margin of the order.
Do you know what a net-new customer actually costs you?
The viability audit separates new customers from returning ones and gives you the real CAC by channel. Free, no strings attached.