Cost per Acquisition (CPA) is the average ad-platform-reported cost to drive a single conversion event in an ad account, computed as ad spend / conversions. “Conversion” here is whichever event the campaign is optimized for: a purchase, lead, install, add-to-cart, or signup. The same label covers cost per lead (CPL) and cost per install (CPI). Ad platforms report CPA per campaign, ad set, and pixel event.
CPA across an account’s campaigns will not equal CAC, for three independent reasons. First, when more than one platform claims credit for the same buyer, each platform’s CPA counts that conversion once, so summed CPA-derived spend double-counts customers; see attribution. Second, the CPA numerator is ad spend only, excluding creative production, agency fees, marketing salaries, and tooling — all costs CAC absorbs. Third, the CPA denominator counts conversion events, not unique net-new customers, so a returning buyer firing a purchase event still increments the count.
The audience split is the practical takeaway. Marketers tune CPA inside ad managers as a bid-strategy input (Google’s Target CPA, Meta’s cost-per-result goal); finance reads CAC out of business-level books. Reading a campaign-level CPA report as a customer acquisition cost overstates paid-acquisition efficiency and hides channel economics.