The maths
CPA = CPC ÷ conversion rate
Max affordable CPA = your contribution margin
The second line explains why CPA moves when you have changed nothing about your targeting. Cost per acquisition is cost per click divided by conversion rate, so a landing page change or a checkout problem shows up as a CPA movement even though the media is behaving identically.
The third line is the only benchmark that matters. Your maximum affordable CPA is your contribution margin in currency — what remains from an order after every variable cost. Anything above that ceiling loses money on every conversion, no matter how efficient the campaign looks.
Reading it properly
Before trusting a CPA, check what is being counted. I regularly find accounts where the conversion action includes newsletter sign-ups alongside purchases, or where a single lead fires two events and halves the apparent cost. A CPA that improved suddenly and inexplicably is usually a tracking change rather than a marketing achievement.
CPA and CAC are related but not interchangeable. CPA typically counts media cost against conversions in the ad platform. Customer acquisition cost counts all sales and marketing cost against genuinely new customers. CAC is the harder number and the more honest one; CPA is the one you can optimise against daily.
Common mistakes
- Counting several conversion actions of unequal value together. A blended CPA across newsletter sign-ups and purchases is not a number anyone can act on.
- Double counting. Duplicate tags, or a browser event firing alongside a server event, halve your reported CPA and flatter everything downstream.
- Ignoring the lag. In considered-purchase categories the conversion often arrives days after the click, so recent CPA always looks worse than it will settle at.
- Setting a target CPA nobody derived. The ceiling comes from contribution margin. Inherited targets are guesses wearing a suit.
Embed this calculator
Free to use on your own site, for your team or your students. The link inside is the only attribution required, and I will not add tracking to it later.
Questions people actually ask
What is the difference between CPA and CAC?
CPA measures media cost per conversion inside the ad platform. CAC measures total sales and marketing cost per genuinely new customer, including salaries, tools and agency fees, and excluding conversions from existing customers. CAC is the business metric; CPA is the operating metric.
What is a good CPA?
Any CPA below your contribution margin per order. That is a figure you can calculate exactly rather than benchmark, and it is specific to your business. The break-even ROAS calculator on this site produces it for you.
Why did my CPA jump after a website change?
Because CPA is cost per click divided by conversion rate. If the site change reduced conversion rate, CPA rises immediately even though nothing in the media account moved. Compare conversion rate before and after the change first.
Should I use target CPA bidding?
It works well once the campaign has consistent conversion volume and clean tracking. With sparse or unreliable conversion data the algorithm optimises towards noise. Fix measurement before handing over control of bidding.
Does CPA include the cost of my time or my agency fee?
Not as normally reported — platform CPA counts media cost only. If you want the number that reflects reality, add agency fees, tooling and internal cost, at which point you are calculating CAC rather than CPA.
Go deeper: How I run performance marketing · Measurement & analytics · Field notes