Customer acquisition cost: the formula, a worked example and the mistakes

Customer acquisition cost is the total cost of sales and marketing in a period divided by the number of new customers acquired in that period. The formula is simple and the arguments are all about what goes into the numerator. Include everything spent to win new customers: media, agency fees, salaries and commissions of the people doing that work, content production, and the tools they use. Exclude the cost of serving customers you already have. Then read CAC against two other numbers, payback period and lifetime value, because CAC on its own says nothing about whether the business works.

CAC = (sales costs + marketing costs in the period)
      -------------------------------------------
          new customers acquired in the period

What goes in, and what stays out

Include	Exclude
Advertising and media spend	Customer support and account management
Agency and freelance fees	Product development
Salaries of marketing and new business sales staff	Salaries of staff serving existing customers
Sales commission on new business	Commission on renewals and upsells
Content, creative and event production	General overheads such as rent and finance
Marketing and sales tools	Discounts given to retain existing customers

Two rules make the number trustworthy. Write down your inclusion list once and keep it the same every quarter, because a CAC that improves because the definition changed is worse than no CAC at all. And count new customers, not total customers.

A worked example

The figures below are invented purely to show the arithmetic. Substitute your own.

• Quarterly sales and marketing cost: 60,000
• New customers in the quarter: 120
• Blended CAC = 60,000 / 120 = 500 per customer

Blended CAC answers the board's question. Channel level CAC answers the marketer's. Splitting the same invented quarter:

Route	Cost	New customers	CAC
Paid advertising	24,000	30	800
Organic and content	21,000	60	350
Partners and referral	6,000	20	300
Outbound sales	9,000	10	900
Blended	60,000	120	500

The blended figure hid a three times difference between the cheapest and dearest route. That gap is where the decisions are.

Reading it against payback and lifetime value

Continuing the same invented example, suppose each customer pays 100 a month and your gross margin is 80 per cent, so each contributes 80 a month.

• Payback period = CAC / monthly contribution = 500 / 80 = 6.25 months. This is the number a cash constrained business should watch most closely, because it says how long your money is tied up.
• Lifetime value, with monthly churn at 2 per cent, gives an average life of 1 / 0.02 = 50 months, so LTV = 80 x 50 = 4,000.
• LTV to CAC = 4,000 / 500 = 8 to 1. Use gross margin rather than revenue in this calculation, or the ratio flatters you by the size of your cost of delivery.

A high ratio with a long payback can still starve a company of cash, and a short payback with a thin ratio means you are buying customers who leave. Both numbers, every quarter.

The mistakes that produce a wrong number

• Leaving salaries out. Ad spend divided by customers is cost per acquisition for that channel, not CAC for the business.
• Ignoring the lag. If your sales cycle runs three months, this quarter's customers came from last quarter's spend. For long cycles, compare cohorts by the month the enquiry arrived rather than the month the deal closed.
• Mixing motions. Self serve sign ups and salesperson led deals have different economics. Report them separately.
• Counting unqualified sign ups as customers. Pick one definition of a customer, usually first payment, and hold it.
• Judging a new channel on its first month, when the learning cost is highest and the volume lowest.
• Chasing an industry benchmark. Your payback target and your margin decide what good looks like, and your own trend over four quarters is the comparison that means something.

How to bring it down

1. Fix conversion before buying more traffic. A clearer pricing page, a shorter form or a faster callback lowers CAC across every channel at once.
2. Speed up the first response. In most businesses the firm that replies first wins a disproportionate share of enquiries that are already paid for.
3. Narrow the targeting. Cutting the segments that convert worst raises cost per click and lowers cost per customer.
4. Build the routes with the lowest CAC. Referral and partner programmes usually sit at the bottom of the table and are the least systematically worked.
5. Raise retention and margin. They do not lower CAC, but they raise what you can afford to pay, which is the more useful lever.

This page covers calculating and reading CAC. For the programmes that generate the acquisitions in the first place, demand generation covers the build order, and a marketing plan sets the budget those numbers come from.

The customer acquisition cost questions worth an answer

What is a good customer acquisition cost?

One that pays back inside the period your cash position allows and sits comfortably below the gross profit a customer delivers over their life. Those two tests are specific to your business, which is why borrowed benchmarks mislead.

What is the difference between CAC and CPA?

Cost per acquisition usually measures the cost of a single defined action in a channel, such as a lead or a sign up. CAC measures the whole cost of winning a paying customer across the business, including people and tools.

Should salaries be included in CAC?

Yes, for the people whose job is winning new customers, including the share of a part time person's time. Leaving them out is the most common reason a CAC figure looks healthy while the company does not.

How often should CAC be calculated?

Monthly for channel level decisions and quarterly for the blended number, with the same inclusion list every time. For long sales cycles, report it by enquiry cohort rather than by closing month.

How do I handle a long sales cycle?

Tag each new customer with the month their first enquiry arrived, then divide that month's spend by the customers it eventually produced. It reports later than a simple monthly division, and it tells you the truth.

A line in the budget that is still forming

The CAC formula adds up what you spend to win a customer. A new cost sits next to the ads and the salaries: the effort of becoming the company an assistant names when a buyer asks.

It is small today, mostly clearer pages and better third party mentions, but it feeds customers who arrive pre sold and often cost less to close. Track it as its own row. Count how many of your last twenty customers say they asked an assistant first, and compare their payback with the rest. The other half, how often you are named at all, is what AI Knows Us measures.