What is CPA (Cost Per Acquisition)?

CPA (cost per acquisition) is how much you spend on average to get one conversion, typically one customer or order. It is the metric most directly tied to whether your ads are profitable.

Formula: CPA = Total Spend / Total Conversions

Why it matters

CPA rolls the whole funnel into one number: it equals CPC divided by conversion rate, so it worsens whenever clicks get pricier or the page converts worse. Its usefulness comes from comparison against what a customer is worth — an acceptable CPA depends on your margin and average order value, and on whether customers repeat-purchase. Because conversions are rarer events than clicks, CPA needs meaningfully more spend than CTR before it stabilizes.

Reading the signals

• Good: CPA comfortably below your gross margin per order, holding as spend scales
• Bad: CPA fine on retargeting but blowing out on cold traffic — the creative isn't converting strangers
• Bad: judging CPA off a handful of conversions — small samples swing wildly

What to do about it

Since CPA is downstream of both click cost and conversion rate, the highest-leverage fix is usually a stronger creative angle rather than bid surgery. Testing many angles at once — for example generating a batch of ad variations inside Claude via Polaris — finds the concepts that pull CPA down before you scale. See the full ecommerce ads glossary, or put it into practice with the Polaris AI UGC ad generator and the free ROAS calculator.