Formula
CPA = ad spend / conversions.
Meta calls this metric cost per result. Its help page defines it as "amount spent divided by results". A result is whatever the ad set was set to optimize for: a purchase, a lead, a link click or an app install. Google calls it average CPA, "calculated by dividing the total cost of conversions by the total number of conversions". Google's example: two conversions that cost $2.00 and $4.00 give an average CPA of $3.00.
Worked example
You spend $500 on a lead campaign and it brings 25 leads. CPA = 500 / 25 = $20 per lead. The same $500 on a sales campaign brings 10 purchases. CPA = 500 / 10 = $50 per purchase.
Whether $50 is good depends on the order. If each order leaves $80 of margin after product and shipping costs, $30 remains after the ad. If the margin is $40, the campaign loses $10 per order.
Why it matters to a media buyer
CPA is the number lead-generation and subscription businesses manage to, because they know what a lead or a customer is worth. The ceiling is your break-even CPA: price minus every per-order cost. Above that line each conversion loses money, whatever the ROAS looks like.
Two cautions. First, a Meta result is only as good as the optimization event behind it. A $2 cost per result on a link-click ad set and a $40 cost per result on a purchase ad set are not comparable. Compare CPA between ad sets with the same event. Second, CPA is unstable while an ad set is in the learning phase. Meta says ad sets "usually have a higher CPA" during that period, so judge it after about 50 results.
In Adsap
The performance dashboard reports CPA for Meta and Google Ads next to spend and ROAS, from yesterday's numbers. The break-even ROAS calculator also returns your maximum CPA from price, cost of goods and other per-order costs. The recommendations engine flags a campaign that spends with no conversions. Automation rules can pause an ad set when its CPA passes a threshold you set, with the guardrails described in the rules guide.
Frequently asked questions
Yes, when the result is the conversion you care about. Cost per result is amount spent divided by results, and the result follows the ad set's optimization event. If the ad set optimizes for link clicks, cost per result is a cost per click, not a cost per acquisition.
Anything under your break-even CPA, which is the order value minus every per-order cost. For a lead, work back from the share of leads that become customers and what a customer is worth. Industry averages hide the margin, so use your own numbers.
A significant edit to an ad set puts it back into Meta's learning phase, and performance is less stable during that period. Meta recommends waiting for about 50 results in a week before judging the new CPA.
Sources
- Meta Business Help Center, Cost per result: "amount spent divided by results". https://www.facebook.com/business/help/762109693832964
- Google Ads Help, Average CPA: "dividing the total cost of conversions by the total number of conversions", with the $2.00 and $4.00 example. https://support.google.com/google-ads/answer/6396841
- Meta Business Help Center, About the learning phase: higher CPA during learning, about 50 results in a week. https://www.facebook.com/business/help/112167992830700
- Adsap, break-even ROAS calculator (maximum CPA).