Cost per acquisition (CPA): what it measures and how to use it

The short answer

Cost per acquisition (CPA) is what you paid in ads, on average, for one conversion. The formula is ad spend divided by conversions. Google Ads shows it as Cost / conv., and Meta shows it as cost per result. The number is only as useful as the conversions underneath it, because each platform decides what it counts. To know whether a CPA is good, compare it with what one order leaves after costs, using paid orders for the conversions.

The formula, and what each platform calls it

CPA = ad spend ÷ conversions

Google calls the column Cost / conv. and says it "tells you how much, on average, each of your conversions cost. It's calculated by dividing your total cost by the number in your 'Conversions' column" (Google Ads Help: conversion tracking data).

Meta calls it cost per result: "amount spent divided by results" (Meta help: Cost per result). A result is whatever the ad set was set up to get. On a sales campaign that's usually a purchase. On a traffic campaign it's a click.

The A can also stand for action. Google describes its bid strategy as "Target CPA (cost-per-action)" (Google Ads Help: Target CPA). Same formula. When you read a CPA, check which action it counts.

What's in the conversion count

Both platforms say part of the count can be estimated. Google's Conversions column "may include modeled conversions in cases where you aren't able to observe all conversions." Meta says that where results can't be measured directly, "statistical modeling may be used to account for some results."

Each platform also counts on its own. A sale that touched a Google ad and a Meta ad can show up as a conversion in both, while your payments show one order. So each platform's CPA is fine for comparing campaigns inside that platform. For what a sale cost you across all ads, divide total spend by paid orders.

CPA is not the same as CAC

CPA counts conversions. Customer acquisition cost (CAC) counts new customers. They drift apart in two ways.

A conversion might not be a customer at all: a sign-up, a lead, an add to cart. And a purchase from someone who has bought before counts as a conversion, but it didn't acquire anyone. If a lot of your ad-driven orders come from returning buyers, your CPA can look healthy while the cost of each new customer is higher.

When a client asks what a new customer costs, they're asking about CAC. Say which one you're reporting.

An illustrative example

Illustrative example. Made-up figures for one store in one month, not a benchmark.

A store spends €3,000 on Google Ads in a month. Google shows 60 conversions, so Cost / conv. is €50. For the same dates, 50 paid orders trace back to visits from those ads, so the cost per paid order is €60.

The average order is €120. Product, shipping, payment fees and expected returns come to €65 of that, so each order leaves €55 before ads.

At the platform CPA of €50, each order makes €5. At the paid-order CPA of €60, each order loses €5. Same campaign, same month, and the answer to "should we spend more here?" flips.

The most you can pay: break-even CPA

The €55 in that example is the store's break-even CPA. Pay more than that to get an order and the order loses money before rent, salaries or software.

That's how people in an r/ecommerce thread from March 2026 work it out (r/ecommerce thread). One reply spelled it out: "AOV minus product cost minus avg shipping minus avg discount minus payment processing fees minus estimated return cost = profit per order. that's your max CPA." Another said to "set your target cpa based on that number, not on revenue multiples."

The simplest reply in the thread was this: "take how much profit you made from sales from the ads and subtract how much you spent." CPA is that same check, worked out per order.

That's why a CPA benchmark from another store doesn't tell you much. Your own margin sets the limit.

Target CPA in Google Ads

Target CPA is also a Google Ads bid strategy. You "set your desired average cost per conversion", and Google sets bids to get as many conversions as it can at that average (Google Ads Help: Target CPA).

Three details from Google's help page are worth knowing:

- It's an average. "Some conversions may cost more than your target and some may cost less."

- Google's recommended target is "the average CPA from the last 30 days, adjusted for any conversion delays". That's your past CPA, not your break-even.

- Bidding only uses the conversion actions you include in the Conversions column. If that column counts more than paid orders, the target is aimed at more than paid orders too.

Google suggests judging performance over the last 30 days with at least 30 conversions. Set the target below your break-even CPA, worked out from paid orders.

A check you can run this month

1. Export last month's spend and conversions per campaign from each platform.

2. Work out each platform's CPA. The CPA calculator does the division.

3. For the same dates, find which campaigns the paid orders came from, and divide each campaign's spend by its paid orders.

4. Work out what one average order leaves after product, shipping, fees and returns. That's your break-even CPA.

5. Mark every campaign where the paid-order CPA is above it, whatever the platform CPA says.

Which CPA goes in your client reports today: the platform's, or the one from paid orders?