CPA (cost per acquisition)
Cost per acquisition connects ad spend to results: what it cost to get one sale, sign-up or lead. It only becomes useful once you know how much one of those is worth to you.
What CPA means
CPA is ad spend divided by the number of conversions it brought in. A conversion is whatever you decided to count: a purchase, a free trial, a form filled in, an app install. Because that choice changes from account to account, always say what the A stands for when you report it. Cost per purchase and cost per lead can be very different numbers in the same campaign.
Ad platforms also use CPA as a bidding goal. When you set a target CPA in Google Ads or a cost cap in Meta, you are telling the algorithm how much you are willing to pay for one conversion, and it will look for people it expects to convert at that price.
The formula
CPA = ad spend ÷ conversions
CPA = cost per click ÷ conversion rate
Break-even CPA = profit per order before ads = price − product cost − shipping and fees
A worked example
Back to the bakery from the CPM and CTR pages. Its second ad brought 900 clicks for $300. The bakery sells a $20 weekend box online, and 27 of those visitors ordered one, a conversion rate of 3%. CPA is 300 ÷ 27, about $11.11. The bakery and its numbers are made up for this example.
Each box costs $8 to bake and $4 to pack and deliver, which leaves $8 before advertising. That $8 is the break-even CPA. At about $11.11 per order the bakery loses roughly $3 on every box it sells through ads, even though its CTR improved. Unless those customers come back and order again, it needs a lower CPA or a bigger order.
What moves CPA
CPA is the result of every step before it. CPM sets the price of attention, CTR decides how many people reach your page, and conversion rate decides how many of them act. You can lower CPA at any of those steps, and the cheapest fix is often on the landing page or in the offer, not in the ad account.
Three mistakes to watch for
Judging CPA without margin. A $30 CPA is cheap for a $300 order with a healthy margin and ruinous for a $25 one. Compare CPA with your break-even CPA, not with other campaigns.
Forgetting repeat purchases. If customers usually come back, the first order does not have to pay for the whole acquisition. That is where lifetime value and payback period come in.
Counting the wrong conversion. If the campaign optimises for add-to-cart or a page view, the reported CPA will look low while real sales stay flat.