Most business owners know how much they spend on ads. Few know how much each customer who walks through the door costs. That difference is what separates the people who invest from the people who gamble.
The name for that number is CAC: Customer Acquisition Cost. It answers a simple, uncomfortable question: to win one new customer, how much did you have to put on the table?
The math is simpler than it looks
The basic calculation is a division:
CAC = everything you spent to attract ÷ the number of customers it brought in
If you spent $3,000 on ads in a month and closed 20 customers, your CAC is $150. Each customer cost $150 to win.
It seems obvious, but notice what this math asks of you: knowing how much you spent and knowing how many customers came from it. It's the second number where almost everyone gets stuck.
The mistake that distorts everything
The most common mistake is mixing customers who came from the ad with customers who came from a referral, a returning client, or plain luck.
If you closed 20 customers in the month, but only 8 came from the campaign, your CAC isn't $150. It's $375. You just found out your marketing costs two and a half times more than you thought.
That's why the right question isn't "how many customers did I close?" but "how many customers did this spend bring in?"
CAC on its own tells you nothing
Here's the part a lot of people ignore: there's no good or bad CAC in the absolute. There's CAC compared to what the customer leaves with you.
A CAC of $400 is expensive for a pizzeria and cheap for a dental clinic. What changes is the ticket size, the margin, and how many times that customer comes back.
The calculation that really matters is this:
- Average ticket: how much the customer spends with you
- Margin: how much of that is left over
- Repeat rate: how many times they come back
If a customer leaves $1,200 in margin over the life of the relationship and costs $400 to win, the business pays for itself three times over. If they leave $300 and cost $400, every new sale is sinking your cash. And worse: the more you advertise, the faster it sinks.
ROAS: the other side of the coin
While CAC looks at the cost, ROAS looks at the return. It's how many dollars come back for every dollar invested.
ROAS = revenue generated ÷ spend
Spent $3,000 and made $12,000 from it? A ROAS of 4. Each dollar became four.
Except a high ROAS with a low margin is still a loss. A ROAS of 4 on a product with a 15% margin returns $1,800 of margin on $3,000 spent. You brought in good revenue and lost money. That's why the two numbers have to move together, always read against your real margin.
How to use this in practice
Finding your CAC isn't a spreadsheet exercise. It's what enables three concrete decisions:
- Knowing how much you can spend. If you know a customer leaves $900 in margin, you know how much you can pay for one without going broke.
- Knowing where to cut. Comparing CAC by channel makes it clear which campaign brings customers and which one only brings clicks.
- Knowing when to scale. If the CAC is healthy and stable, raising the budget stops being a risk and becomes a decision.
Without that number, increasing your investment is faith. With it, it's math.
Start with your own
If you've never calculated it, start with last month. Take what you spent, how many customers it brought in, and your ticket size. The result usually surprises people, almost always for the worse. And that's exactly where the opportunity is.
The math is right there, and it fits on a spreadsheet: divide what you spent in the period by the number of customers that spend brought in. The work isn't in the calculation, it's in knowing whether the number that comes out is good for your ticket and your sales cycle. If you want that read, the marketing diagnosis points to where your operation loses customers before the cost even enters the equation.
And if after that you want to understand why your number is where it is, Arya runs a diagnosis of your marketing and shows which pillar is holding back your growth.
