Cost Per Acquisition (CPA): Formula, Cost Basis, and Limits
Cost per acquisition divides selected marketing costs by a defined acquisition event. The number is only comparable when the cost basis and the conversion denominator are explicit.
What is cost per acquisition?
Cost per acquisition (CPA) is selected marketing cost divided by the number of defined acquisitions in the same period. Google Ads defines cost per action as marketing cost divided by actions, and its glossary defines CPA as total cost divided by total conversions. Those conversions are whatever the account is set to count. They are not automatically new customers, and they are not automatically incremental.
CPA helps compare efficiency only when the acquisition event and included costs match. Cost Per Acquisition (CPA) also connects with conversion rate and lifetime value (LTV).
Formula and worked example
Google’s formula:
CPA = marketing cost / number of actions
Hypothetical month:
| Input | Value |
|---|---|
| Media cost | $12,000 |
| Platform-counted conversions | 80 |
| Platform-average CPA | $12,000 / 80 = $150 |
If only 20 of those 80 conversions become paying customers, customer CPA for the same media is $12,000 / 20 = $600. The $150 figure was a cheap lead or cheap conversion CPA. It was not the cost of a customer.
If a holdout shows that 8 of those 20 customers would have arrived anyway, incremental CPA for the extra 12 customers is $12,000 / 12 = $1,000. That illustration is labeled hypothetical. The arithmetic is what changes when the denominator gets stricter.
Platform CPA, full CAC, marginal CPA, and incremental CPA
| Approach | Numerator | Denominator | Use | Main limit |
|---|---|---|---|---|
| Ad-platform average CPA | Costs the platform reports, usually media in that account | Conversions the platform counts | In-channel bidding and pacing | Omits other costs and non-platform conversions |
| Full customer acquisition cost | Media plus the other acquisition costs you choose to include, such as fees, creative, and sales | New customers | Unit economics | Longer delay; definition must be written down |
| Marginal CPA | Added cost for the next increment of spend | Added conversions in that increment | Whether the next dollar is efficient | Google’s glossary: cost divided by increase in conversions |
| Incremental CPA | Cost of the tactic in a test | Conversions the test attributes to the tactic versus holdout | Causal efficiency | Needs a valid test, not only a platform report |
Google’s Ads glossary defines marginal CPA as the cost of additional conversions: cost divided by the increase in conversions. That is not the same as incrementality. A budget increase can raise platform conversions that would have occurred from brand demand anyway.
Cheap leads are not profitable customers
Prevent a low lead CPA from masquerading as efficient acquisition:
- Name the event: click, lead, qualified lead, sale, or new customer.
- Include comparable costs. A platform CPA that omits agency fees and creative is not comparable with a fully loaded CAC.
- Connect leads to close rate and lifetime value using the same customer definition, with the LTV horizon named.
- Reject a “winning” CPA whose denominator is an event the business would not willingly buy again.
Our view at Anderson Collaborative: We refuse to optimize an undefined acquisition. The denominator must represent an outcome the business would willingly buy again.
Anderson Collaborative’s paid search capabilities cover campaign structure and measurement when CPA findings need account work.
Frequently Asked Questions
What is cost per acquisition?
Cost per acquisition divides selected marketing costs by the number of defined acquisitions or conversions in the same period. Google Ads also calls this cost per action: marketing cost divided by actions.
What is the difference among platform CPA, full CAC, marginal CPA, and incremental CPA?
Platform-average CPA uses costs and conversions the ad platform reports. Full customer acquisition cost uses new customers and the other acquisition costs you choose to include. Marginal CPA is added cost divided by added conversions for the next increment of spend. Incremental CPA uses conversions a test shows would not have happened otherwise.
Which costs should CPA include?
Include the costs that match the decision. A platform CPA usually counts media in that account and omits other costs. Full CAC can include media plus fees, creative, and sales when you write that definition down. A platform CPA that omits agency fees and creative is not comparable with a fully loaded CAC.
Why can a low lead CPA still be unprofitable?
A low lead CPA can look efficient while few of those conversions become paying customers. If $12,000 buys 80 platform conversions, platform CPA is $150. If only 20 become paying customers, customer CPA is $600. A cheap lead CPA can hide poor close rates and still fail against lifetime value.
Sources
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