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Cost Per Acquisition (CPA)
Cost Per Acquisition is total spend on a channel or campaign divided by the number of acquisitions it is credited with under the active attribution model. It expresses what one conversion cost that channel, not what the channel cost overall. A lower CPA means more efficient conversion generation, but only within campaigns using the same attribution logic.
Formula
Worked example
A mid-size retailer runs paid social for a month at a spend of €18,000. The attribution platform credits the channel with 450 completed purchases. CPA = €18,000 / 450 = €40 per acquisition. Against a target CPA of €35, the planner marks the channel as underperforming and reduces its share of next month's budget.
How it is used
Buyers set a target CPA per channel and use it to reallocate budget: channels beating target get scaled, channels missing it get optimized or paused. The recurring error is stacking CPA figures from channels running different attribution windows or models, so a 7-day click channel looks cheaper than a 30-day view-through channel when the two numbers never measured the same acquisitions.
The common mistake
CPA figures are only comparable across channels that share the same attribution window and model.