CPA (Cost per Acquisition).

CPA links advertising costs to a result. While CPM and CPC describe what attention costs, CPA describes what a measurable success costs. What counts as an 'acquisition' is defined by the company itself: a purchase in the shop, a quote request, a table reservation, a registration. That is why the CPA values of two accounts are only comparable if both count the same event.

Closely related is CPL, which describes the cost per lead. CPL is a special case of CPA for when the conversion is a contact. In e-commerce, campaigns are often steered by ROAS instead of CPA, because orders vary in value. Where every conversion is worth roughly the same, CPA is the simpler and more robust steering metric.

Google Ads offers a Target CPA bid strategy. You set a desired average value, and the automation tries to achieve as many conversions as possible at that price. This only works if conversion tracking counts the right events and there is enough data. A target CPA set too low throttles delivery; one set too high burns budget. If you change the target, do so in small steps.

The most important idea behind CPA is the ceiling: how much may a conversion cost at most for the business still to pay off? That number does not come from the ad account but from your costing — from order value, margin and the rate at which enquiries turn into jobs. Without this calculation, every CPA value is just a number without meaning.