CPA (Cost Per Acquisition)

Cost per acquisition (CPA) is total ad spend divided by attributable customer acquisitions. An advertiser spending $10,000 to drive 200 sign-ups has a CPA of $50. CPA is the bottom-of-funnel ROI metric for performance campaigns; it captures the actual revenue-generating outcome rather than intermediate signals like clicks or impressions.

CPA depends entirely on the attribution model. A last-click model credits CPA to the channel of the final click before conversion; a multi-touch model splits credit across all upper- and mid-funnel touches. Different models can produce CPA estimates that differ by an order of magnitude for the same campaign.

For DOOH and CTV — neither of which is click-driven — CPA requires a cross-device attribution stack. The standard approach: pixel-match a screen exposure to a household via mobile-location data, then track that household's conversion behavior. Magna's 2025 DOOH attribution benchmark report shows median DOOH CPA tracking ~30% better than display in similar verticals when the attribution model accounts for view-through (not just click-through).

Compare to ROAS, which inverts the same arithmetic — both are valid framings of campaign ROI. Performance-oriented advertisers prefer CPA when they can name a discrete conversion event; ROAS is more flexible when conversion is multi-step or revenue is variable.