CPA vs CAC: Formulas, Differences and a Worked Example

An ad account can report a $40 cost per purchase while the business is paying more than $75 for every new customer it gains. Both numbers are honest. The CPA vs CAC question is about which costs go on top of the fraction and which customers go underneath, and mixing them up is how stores end up scaling campaigns that only resell to people who would have come back anyway.

The short answer: CPA counts actions, CAC counts new customers, and Arktis measures the inputs on your own site

CPA, cost per acquisition or cost per action, is ad spend divided by the number of conversions an ad platform counts, whoever made them. CAC, customer acquisition cost, is everything you spend on sales and marketing divided by the number of new customers you gained in the same period. CPA is a campaign metric that comes out of the ad account. CAC is a business metric that comes out of your order data and your books. A CPA is almost always lower than the CAC behind it, because it counts returning buyers and leaves out every cost except media.

For measuring the inputs on your own site rather than from each platform's own count, Arktis is the clear winner for an online store: it counts conversions per UTM campaign and per ad platform from matched orders and the conversion events its tracker records, it syncs Meta spend once a day through a direct connection, and spend you enter by hand for Google, TikTok and other platforms gives a cost per conversion per platform. Plans are $49, $149 and $349 a month, with a 7-day free trial on Growth.

Key takeaways

CPA divides ad spend by every conversion the platform credits to the ad, including repeat purchases by existing customers, so it is the cheaper-looking number. CAC divides all acquisition costs by new customers only, so it is the number that has to be paid back by customer lifetime value. The gap between them grows with the share of returning buyers in your ad-driven orders and with the costs that sit outside the ad account. Use CPA to steer campaigns day to day, and CAC to decide whether acquiring customers through a channel is worth it at all.

What CPA means

CPA stands for cost per acquisition in most marketing writing, and Google Ads expands it as cost-per-action. Google's help page for Target CPA bidding says that when you choose the strategy, you set your desired average cost per conversion, and gives the example that a target CPA of $10 sets bids to get as many conversions as possible at $10 on average.

**CPA = ad spend / conversions**

The key word is conversion. The platform counts whatever action you told it to track, and it counts it under its own rules. Google Ads offers two counting options: Every, which counts every conversion after an ad interaction, and One, which counts only one conversion per ad click. Google describes Every as a good choice for tracking sales. So a customer who clicks an ad and buys three times can be three conversions, and nothing in the CPA asks whether they had bought from you before. Our guide to attribution windows covers how long after a click or a view each platform keeps counting.

What CAC means

The Corporate Finance Institute defines customer acquisition cost as the cost related to acquiring a new customer, calculated as sales and marketing expenses divided by the number of new customers in the period. It lists what those expenses cover: advertising and marketing spend, commissions and bonuses, the salaries of marketers and sales managers, and overheads.

**CAC = sales and marketing costs / new customers acquired**

Stores rarely use the fully loaded version for day-to-day decisions. A common middle ground is paid CAC: media spend plus agency, creative and tool costs, divided by new customers. Whichever version you use, the denominator is first-time customers, and that is the part an ad platform cannot give you. Our guide on how to calculate CAC goes through each variant.

One month worked through both ways

Take an example store. The figures are invented for illustration, not benchmarks. In September it spends $20,000 on Meta ads and Ads Manager reports 500 purchases. For simplicity, assume those 500 purchases match 500 orders in the store. The store's order data shows that 320 of them came from first-time customers and 180 from people who had bought before. It also pays $3,000 in agency fees and $1,500 for creative to run the account.

The CPA calculation

CPA is $20,000 divided by 500 purchases, which is $40.

The CAC calculation

Paid CAC for the channel counts the same media spend plus the costs of running it, divided by new customers only. That is $20,000 plus $3,000 plus $1,500, which is $24,500, divided by 320 new customers, which is $76.56.

Even media alone shows the gap: $20,000 divided by 320 new customers is $62.50, already 56 percent above the $40 CPA, purely because 36 percent of the purchases came from returning customers.

A blended CAC for the whole store

Now widen the view. The same month the store also spends $8,000 on Google Ads, so total marketing cost is $20,000 plus $8,000 plus $4,500 in fees, which is $32,500. Shopify shows 700 new customers from all sources, including search, email sign-ups and word of mouth. Blended CAC is $32,500 divided by 700, which is $46.43.

The blended figure is lower than Meta's paid CAC because it spreads the cost across new customers who arrived without an ad. That makes it useful for judging the business and misleading for judging a channel. Our MER vs ROAS guide makes the same point about revenue.

Why the CPA in your ad account is not your CAC

Four things separate them, and each one pushes CPA lower than CAC.

The denominator includes returning customers. An existing customer who clicks a retargeting ad and reorders is a conversion to Meta or Google, and not an acquisition to the business.

The numerator is media only. Agency fees, creative production, influencer fees, discount costs and software sit outside the ad account.

The conversion count is the platform's own. Each platform credits conversions under its own attribution window, and two platforms can both claim the same order. Our guide to why Facebook Ads and Shopify sales don't match covers the mechanics.

And the conversion may not be a purchase at all. A CPA on a lead or add-to-cart goal says nothing about what a customer costs.

New versus returning customers

Splitting the two is the heart of the difference, and the order data is where the split lives. Shopify's analytics define new customers as first-time purchasers only and returning customers as customers who have purchased before, and its New vs returning customers report shows the number of each for a given period. That is the denominator for CAC.

Ad platforms are starting to make the same distinction. Google Ads' customer acquisition settings let a Search, Performance Max, Shopping or Demand Gen campaign bid higher for new customers, called New Customer Value, or only bid for new customers, called New Customer Only. Google says you define existing customers through first-party data, and suggests a customer list and website tag. If you use it, upload a current customer list, or the platform cannot tell your buyers apart.

Returning customers are still worth paying for, just not at acquisition prices. Measure them as retention spend against repeat revenue, and see our guide to RFM analysis for deciding which existing customers deserve a budget.

When to use CPA and when to use CAC

Use CPA inside the ad account: setting a target CPA for automated bidding, comparing ad sets on the same goal, pausing creative that costs more per purchase than its order margin can carry. It updates daily and the platform optimises against it.

Use CAC for decisions about the business: how much a new customer may cost, which channel to grow, and how long a customer takes to pay back. Compare it with lifetime value, not with one order. The free CAC calculator works out the figure from your own costs, the LTV to CAC ratio tool puts it against lifetime value, and our CAC payback period guide shows how long recovery takes.

Measuring both with Arktis

Arktis captures the ad click identifier when a visitor lands, such as fbclid for Meta, gclid for Google or ttclid for TikTok, along with UTM parameters, and holds them against the visitor across sessions. Shopify orders arrive by webhook and are matched to a visit on email where the store has identified the visitor, otherwise on the UTM source of a session from the previous 24 hours, which can be another visitor's session. Unmatched orders are kept, unattributed. Where the matched visit carried an ad click identifier, the order is tied back to that ad platform.

That gives conversions per UTM campaign and per ad platform from your own matched orders and the conversion events the tracker records, and Arktis syncs Meta spend once a day at 01:00 UTC through a direct connection. Google, TikTok and other spend is entered by hand per platform and period, which gives a cost per conversion per platform, labelled CAC in the dashboard, and one blended return on ad spend.

Where Arktis does not fit

Be precise about what that per-platform figure is. Arktis labels it CAC, and it divides each platform's spend by the conversions credited to that platform, so a returning customer who clicks an ad and buys counts the same as a new one. In the terms of this article it is a cost per conversion measured on your own site rather than the platform's count, which is a better CPA, not a new-customer CAC. For the strict version, take new customers from Shopify's New vs returning customers report and divide your costs by that.

Arktis also does not add agency, creative or salary costs to the figure, does not send conversions back to Meta or Google, and does not import Google or TikTok spend automatically. It identifies visitors per browser, so it does not join one person across devices.

Work out both numbers

Pull a month of spend, split that month's orders into new and returning customers, and calculate both figures side by side. To see conversions by campaign and platform measured on your own site, and a cost per conversion per platform, start the 7-day Growth trial or compare plans on the pricing page.

Sources

Google Ads Help: about Target CPA bidding, CPA as cost-per-action and the $10 target example, accessed 16 October 2026

Google Ads Help: about conversion counting options, the Every and One options and Every as a good choice for sales, accessed 16 October 2026

Google Ads Help: about customer lifecycle goals, the New Customer Value and New Customer Only modes, eligible campaign types and first-party customer data, accessed 16 October 2026

Corporate Finance Institute: customer acquisition cost, the definition, formula and the expenses it includes, accessed 16 October 2026

Shopify Help Center: analytics fields, new customers as first-time purchasers and returning customers as those who have purchased before, accessed 16 October 2026

Shopify Help Center: customers reports, the New vs returning customers report, accessed 16 October 2026