Cost Per Acquisition (CPA) is a marketing metric that measures the total cost of acquiring a specific conversion—such as a signup, a trial, or a purchase—within a specific campaign. Unlike CAC, which looks at the 'Whole Customer' and includes salaries and overhead, CPA is typically focused on 'Media Spend' only. It is calculated by dividing the total campaign cost by the number of conversions achieved. CPA is the ultimate benchmark for campaign-level efficiency; it tells you exactly what you are paying for an 'Action.' To remain profitable, your CPA must be lower than the 'Average Order Value' (for e-commerce) or the 'Initial Contract Value' (for SaaS). By monitoring CPA across different ad sets and platforms, growth teams can ruthlessly cut underperforming ads and double down on the creative and targeting strategies that deliver the lowest cost per result. It is the primary lever for scaling paid acquisition without blowing the budget.