How to calculate customer acquisition cost (and blended CAC)
The customer acquisition cost formula is one line. The hard part is what goes into it, and a CAC built on the wrong inputs will tell you a channel is working when it is not. Here is how to calculate CAC for an online store, how blended CAC differs from what the ad platforms report, and how to tell whether a customer is worth what they cost.
CAC is only as good as the purchase count under it. A Mimetic scan checks whether your store's purchase tracking counts each order exactly once in GA4, Google Ads and Meta, on desktop and mobile, and shows the evidence for anything it finds.
The customer acquisition cost formula
CAC = acquisition spend in a period ÷ new customers acquired in the same period
If you spent $12,000 on marketing in September and 240 people bought from you for the first time in September, your CAC is $50.
That is the whole formula. The work is deciding what counts on each side, and that is where most stores' CAC goes wrong.
What goes into the spend
Everything you pay to win a first order:
- Ad spend on every platform: Meta, Google, TikTok, Pinterest and the rest
- Agency, freelancer and media buyer fees for acquisition work
- Ad creative: shoots, UGC creators, editing
- Affiliate, influencer and referral payouts
- Tools you use only for acquisition
Leave out what serves customers you already have, like retention email and customer service. Those belong in lifetime value, not CAC.
What counts as a new customer
- First-time customers, not orders. One person who orders twice in a month is one customer, and a returning customer was not acquired this month at all. Shopify's analytics split sales into first-time and returning customers; start from the first-time number.
- Net of refunds and cancellations. A refunded first order did not produce a customer.
- No test orders. A new store's first few orders are often the owner checking that checkout works. They are not customers, and on a small store they can make CAC look half what it is.
Blended CAC vs what the ad platforms report
Each ad platform reports its own cost per purchase, based on the purchases it claims. Those claims overlap. A shopper who saw a Meta ad, later clicked a Google ad and then bought is often counted by both. Platform purchases also include returning customers, and Meta can count purchases from people who only viewed an ad.
Add up what the platforms claim and you get more purchases than your store recorded. An example:
| Purchases claimed | Spend | Cost per purchase | |
|---|---|---|---|
| Meta | 180 | $8,000 | $44 |
| 120 | $4,000 | $33 | |
| Both platforms | 300 | $12,000 | $40 |
| Your store: first-time customers | 210 | $12,000 | $57 |
(Example numbers, not a benchmark.)
That last row is blended CAC: total acquisition spend divided by every new customer your store recorded, from every channel, including organic and word of mouth. It is the number your bank account agrees with. Use platform numbers to compare things inside one platform (this ad set against that one), and blended CAC to decide whether to spend more.
Blended CAC has one blind spot. Strong organic sales or word of mouth can hide an expensive paid channel. So track paid CAC next to it: paid media spend divided by new customers whose first visit came from paid media, taken from your store's own order and UTM data rather than the platforms' claims. If blended CAC holds steady while paid CAC climbs, your organic customers are paying for your ads.
Pick a period and keep it
Spend and purchases do not always land in the same month. For a product people think about for a while, a click in late September can become a customer in October. Use a window long enough to absorb that: a calendar month or a rolling 30 days for most stores, 90 days for longer buying cycles. Then use the same window every time, or you are comparing different things.
LTV to CAC, and payback
CAC on its own does not tell you whether a customer is worth acquiring. For that you need what they are worth.
Contribution margin per order = order value minus product cost, shipping, payment fees and discounts.
Payback asks whether the first order covers what it cost to get it. With a $50 CAC and an $80 first order at a 50% contribution margin, the first order earns $40. You are $10 short until that customer orders again.
LTV (lifetime value) = contribution margin per order × orders per customer over a set period, usually 12 months. At $40 an order and 1.6 orders a year, LTV is $64, and LTV to CAC = $64 ÷ $50 = 1.3.
The 3:1 LTV to CAC target you will see quoted comes from subscription software, where customers pay every month. For a store where most customers order once or twice, first-order payback is the more useful test from week to week: if the first order covers CAC, every repeat order is profit.
When CAC jumps and nothing changed
Check the inputs before the campaigns:
- Did the purchase event stop firing, or start firing twice? Across 936 sites our scanner audited, one in five had duplicate tracking corrupting its counts, and 37% had no working analytics at all.
- Did a theme or app update break checkout or add to cart on mobile?
- Are returning customers, refunds or test orders being counted as new customers?
- Did the time window change between the two numbers you are comparing?
If the inputs are sound and CAC is still too high, how to lower customer acquisition cost is the order we work in.
Mimetic checks your purchase tracking, checkout and product pages on desktop and mobile, and shows the evidence for each problem it finds. Related: how to lower customer acquisition cost, conversion tracking setup, GA4 not matching Shopify.