CAC payback period: how to calculate it and what a good one is

The CAC payback period is the number of months a new customer takes to pay back, in gross margin, what it cost to acquire them. If acquiring a customer costs €480 and they leave €40 of margin a month, payback is 12 months. For those 12 months, that customer is cash you have fronted.

The LTV/CAC ratio tells you whether a customer is profitable by the end of their lifetime. Payback tells you how long it takes to get there, and that is what limits how hard you can push acquisition before you run out of cash.

The CAC payback formula

CAC payback (months) = CAC ÷ monthly gross margin per customer.

• CAC: everything you spend on acquisition in a period (media, agency, tools and, in B2B, the sales cost) divided by the new customers of that period. If you need the basics first, start with what CAC is and how to calculate it.
• Monthly gross margin: what the customer pays each month minus what it costs to serve them (product, shipping, payment fees, returns, support). Use revenue instead of margin and payback comes out shorter than it really is.

A subscription example in round numbers: a €50 monthly plan, 80% gross margin (€40 a month) and a CAC of €480. 480 ÷ 40 = 12 months.

Why the simple formula falls short: churn

The division above assumes every customer keeps paying until their CAC is recovered. That never happens. Part of the cohort leaves early, and the margin from the customers who stay has to cover what the leavers cost as well.

Same example with 5% monthly churn. Per customer acquired, the cohort's cumulative margin is:

• After 12 months: €368. Still short of €480.
• After 17 months: €466.
• After 18 months: €482. This is where CAC is recovered.

The formula said 12 months and the cohort says 18: six more months of fronted cash for every customer. That is why the payback worth deciding on is the cohort one: add up the real margin of the customers acquired in a month, month by month, and see when the total passes what it cost to acquire them.

Payback and LTV/CAC: the same maths seen through time

With a stable monthly margin, the two metrics are tied by a simple relationship:

LTV/CAC = average customer lifetime ÷ payback.

If a customer stays 36 months on average and payback is 12, the ratio is 3:1. If payback stretches to 18 months with the same lifetime, the ratio drops to 2:1 without any change in price.

That gives the working rule: payback should stay below a third of the average customer lifetime (the same as a 3:1 LTV/CAC) and below the number of months of cash you can finance. A business can have a healthy ratio and still run out of cash if it grows fast on a two-year payback.

In ecommerce without subscriptions

Without a monthly fee, payback is measured on the cohort's orders. A hypothetical example: €60 average order, 35% margin after shipping and returns (€21 per order) and a CAC of €45.

• First order: €21 of margin. €24 to go.
• Cumulative orders per customer at 6 months: 1.8. Margin: €37.80.
• At 12 months: 2.3 orders. Margin: €48.30. CAC recovered between month 6 and month 12.

If first-order margin already covers CAC, payback is immediate and every repeat order is profit. That is the case where you can scale with the most confidence, and it shows up first in your first-purchase minimum profitable ROAS. If it does not, the business depends on repeat purchases, and cohort payback is the number that tells you how long you are financing each customer.

In B2B, payback starts the day you spend

In B2B with a long sales cycle, count from the spend, not from the signature. Example: a €2,000 monthly contract at 60% margin (€1,200 a month) and a CAC of €9,000 including media and the sales team.

• Accounting payback: 9,000 ÷ 1,200 = 7.5 months from signature.
• If the sales cycle lasts 3 months, the money goes out 3 months before the first invoice: cash payback is 10.5 months.

The gap matters when you decide how much to raise spend: double the budget today and your cash feels it for the sales cycle and the payback combined. It is the same logic behind why a cheap CPL raises your CAC in B2B, and the B2B calculator gives you the CAC you can afford with your own rates.

The mistakes that shorten payback on paper

• Revenue instead of margin. In the subscription example, 480 ÷ 50 gives 9.6 months instead of 12. The error always favours the optimist.
• CAC without all its costs. Leaving out the agency, the tools or the sales team makes CAC cheaper and cuts payback in the same proportion.
• Returning customers in the denominator. Divide spend by every conversion instead of new customers only and CAC comes out low. It is the difference we separate in CAC vs CPA.
• No churn. The simple formula gives the best case. The cohort gives the real one.
• One payback for every channel. Branded search pays back in weeks because it captures demand that already existed; prospecting on Meta or YouTube takes longer. Blended together, the fast channel hides the slow one.

How we use it to set what to pay per customer

At NML, payback goes into the maths before the campaign target does. The sequence:

1. We work out monthly margin per customer from back-office or CRM data, by cohort.
2. We set the months of payback the business can finance with its cash and that stay below a third of the average lifetime.
3. Maximum CAC = monthly margin × acceptable payback months. With €40 of margin and 9 months of payback, maximum CAC is €360.
4. We translate that maximum CAC into the bid: target CPA for acquisition or a first-purchase target ROAS in ecommerce, and we track the real payback of every new cohort.

To do it today with your own business: take the customers you acquired 12 months ago, add up their margin month by month and mark the month the total passes what it cost to acquire them. That is your real payback, and it is almost always longer than the formula's.

Frequently asked questions