Cost per action (CPA) — sometimes called cost per acquisition — is an advertising pricing and performance model in which the advertiser pays based on a specific completed action rather than for impressions or clicks. The action is defined by the campaign goal: a purchase, a sign-up, a lead form submission, an app install, a download, or any other conversion the advertiser values. CPA ties advertising cost directly to outcomes, making it one of the most accountable models in performance marketing.
 

The math is straightforward: CPA equals total ad spend divided by the number of actions (conversions) generated. If a campaign spends $5,000 and produces 250 conversions, the CPA is $20. This single number tells the advertiser exactly what each desired outcome costs, which is far more meaningful for business decisions than impression or click metrics that don't guarantee results.
 

CPA is attractive to advertisers because it shifts focus — and often risk — toward performance. In a pure CPA buying arrangement, the advertiser only pays when the action happens, transferring delivery risk to the publisher or network. More commonly in programmatic, advertisers buy on a CPM basis but optimize toward a target CPA, using the DSP's machine learning to bid more on impressions likely to convert and less on those that aren't. Either way, CPA is the north-star efficiency metric that determines whether spend is producing value.
 

For optimization, CPA connects the full funnel. To lower CPA, advertisers can improve any stage: sharpen audience targeting to reach higher-intent users, strengthen creative to lift engagement, refine landing pages to boost conversion rate, or adjust bids to win the right impressions at the right price. Because CPA reflects all of these factors at once, it's an excellent diagnostic — a rising CPA signals that something in the chain has degraded, while a falling CPA confirms efficiency gains.
 

CPA must be evaluated against the value of the action. A $30 CPA is excellent if each customer is worth $300 and terrible if they're worth $20. That's why sophisticated advertisers pair CPA with metrics like average order value, customer lifetime value, and return on ad spend (ROAS) to ensure the cost of acquisition makes economic sense.
 

Accurate CPA depends on reliable conversion tracking and protection from fraud, since fake conversions from bot traffic distort the metric and mislead optimization. When measured cleanly, CPA gives advertisers the clearest possible answer to the question that matters most: what does it actually cost to win a customer?