CPA formula and example
Divide campaign spend by the number of recorded conversions for the same campaign and period. If $1,200 in advertising produces 40 defined actions, the average CPA is $30. Google Ads describes cost per action in the same way: marketing cost divided by actions. An action may be a sale, registration, or lead, so state which one you are counting.
Match the numerator and denominator
Use spend and conversions from the same date range, attribution setting, and campaign scope. Do not combine spend from several campaigns with actions attributed to only one of them. Remove duplicate conversion events when the business definition calls for unique acquisitions. If the campaign produced no recorded conversions, CPA is undefined, not zero; investigate tracking and the observation window.
Actual CPA versus target CPA
This page calculates an observed average from results already recorded. A target CPA is a desired average for future bidding or planning. Google's guidance says actual CPA can differ from a target. See the target CPA calculator to estimate a break-even ceiling from revenue and pre-acquisition margin.
Decide whether the cost is worthwhile
A $30 cost per signup cannot be judged without knowing how many signups become customers and the contribution those customers generate. Compare cost per lead, cost per sale, repeat purchases, and the quality of each action. A low CPA for an easy but unqualified event can be less valuable than a higher CPA for a verified purchase.