Free tool · Meta and Google Ads
CAC calculator: from CPM and CTR to cost per purchase
This CAC calculator starts from three numbers you'll find in any ad account, CPM, CTR and conversion rate, and tells you what each order costs. It also shows the ROAS that follows and which lever is worth pulling to bring the cost down.
Cost per purchase (CPA)
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What each order costs you in ads. Compare it with your break-even CPA.
Cost per click (CPC)
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ROAS
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Orders per day
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What moves your cost per purchase
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How do you get from CPM to cost per purchase?
Every order that comes from ads goes through three filters: people see the ad, some of them click, some of the clickers buy. Each step has a number you already have in your reports.
- CPM: what you pay for a thousand impressions.
- CTR: how many people out of a hundred click after seeing the ad.
- Conversion rate: how many visits out of a hundred turn into an order.
CPC = CPM ÷ (1,000 × CTR)
Cost per purchase (CPA) = CPC ÷ conversion rate
With the calculator's starting numbers: a thousand impressions cost $12, 1.2% click, so you get 12 visits at $1.00 each. 2.5% of those visits buy: 0.3 orders per thousand impressions. Cost per purchase is 12 ÷ 0.3 = $40. With a $70 average order, ROAS is 1.75.
Which lever lowers cost per purchase the most?
The three levers multiply. Raising CTR or conversion rate by 20% cuts cost per purchase by about 17%; a CPM 20% lower cuts it by 20%. Here's the same example, one lever at a time:
| Scenario | CPM | CTR | Conversion | Cost per purchase |
|---|---|---|---|---|
| Starting point | $12.00 | 1.2% | 2.5% | $40.00 |
| Higher CTR | $12.00 | 1.7% | 2.5% | $28.24 |
| Higher conversion | $12.00 | 1.2% | 3.0% | $33.33 |
| Holiday CPM (+20%) | $14.40 | 1.2% | 2.5% | $48.00 |
CPM is mostly set by the market: auctions, seasons, competition. CTR and conversion rate are on you. Stronger creative lifts CTR; a clear product page, social proof and a fast checkout lift conversion.
CPA vs CAC: what's the difference?
The calculator gives you your campaigns' cost per purchase, or CPA: ad spend divided by orders. Customer acquisition cost is stricter: it puts all marketing spend on top and only new customers at the bottom. If part of your orders come from existing customers, your real CAC is higher than your CPA.
CAC = total marketing spend ÷ new customers
To know whether your cost per purchase is sustainable, compare it with your break-even CPA from the break-even ROAS calculator. Every definition is in the ecommerce KPIs glossary.
How to use the result
- Below your break-even CPA: your campaigns pay for themselves on the first order.
- Above break-even: look at site conversion first, it's the lever you control most.
- Low CTR: it's usually the creative, before it's the audience.
- Use at least a week of data: with few orders, conversion rate swings a lot.
To put your campaign numbers in order, start from the Meta Ads strategy for ecommerce or get in touch: it's what I do as a Meta Ads consultant.
FAQ
CAC: frequently asked questions
How do you calculate CAC?
Divide your marketing spend by the new customers you acquired in the same period. If you spend $11,000 in a month and win 500 new customers, your CAC is $22.
What's the difference between CPA and CAC?
CPA is your campaigns' cost per purchase: ad spend divided by orders. CAC uses all marketing spend and only counts new customers, so it's usually higher.
Which CTR should I use on Meta?
Link CTR, the clicks that take people to your site. CTR (all) also counts reactions, comments and profile clicks, and would make your cost per purchase look too good.
Why is Meta's cost per purchase different from the calculator?
Meta counts conversions with its own attribution rules, including some sales after an ad view. The calculator uses the conversion rate you measure on your site.
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