How much a new customer can cost: your cash sets a limit too

The maximum CAC has two ceilings

The usual formulas derive the allowable CAC from customer value and divide it by a factor. I consider that incomplete. A new customer can cost at most the contribution margin they bring back within the calculation period, and at most what you can pre-finance until then. The lower of the two limits applies. A high customer value is of little use if you wait twelve months for the money and run out of it after seven.

CAC itself is quick to calculate: sales and marketing expenses divided by the number of customers won. For a ceiling I would define it strictly, so including agency and tool costs, and counting new customers from the shop system. The platform's new customer label is no substitute for that. Deduct first-order vouchers from the contribution margin.

Why the customer value rule of thumb is wrong in both directions

The common basic formula is order value times purchase frequency times customer lifespan. The result is revenue. On top of that comes the frequently quoted rule of thumb of 3:1 to 5:1 between customer value and CAC, which is a practitioner's rule, not a research finding. An invented worked example with round numbers shows what this does.

Suppose a shop makes €100 of net revenue on the first purchase at a 30% contribution margin, and its cohorts develop like this per new customer:

Calculated on revenue, 3:1 allows a CAC of around €83. That is more than the €75 of contribution margin the customer brings in over two whole years. Calculated on contribution margin, the same rule allows €25, less than the first purchase alone earns. The shop would turn away customers who pay for themselves immediately.

Then there is the missing time axis. The same ratio with a payback of three months and with one of nine months describes two different businesses: one finances itself, the other needs capital.

The earnings limit: only contribution margin your cohorts have proven

A rigorous calculation uses margin, retention rate and the cost of capital as the discount rate. For day-to-day retail this means: count contribution margin after all variable costs, so cost of goods, shipping, payment fees, packaging and returns. Exactly which items belong in it is covered under the costs that matter for advertising decisions.

In the example, the earnings limit is the right-hand column of the table. At a CAC of €75 and 24 months, however, you earn nothing, and fixed costs and profit are still uncovered. The figure therefore marks where the loss-making business begins. It only becomes a target once you add your profit mark-up.

There is a simple formula for the time until the cost is recovered: CAC divided by the monthly contribution margin per customer. Because in a shop the first purchase earns back a large part straight away, I prefer this version: (CAC minus first-purchase contribution margin) divided by the monthly contribution margin from repeat purchases.

The cash limit: how much capital your pace of acquisition ties up

This is the second step the usual calculations leave out. Every new customer whose CAC exceeds the first-purchase contribution margin is a small loan to yourself, and a new cohort joins every month. In the same example, with a constant 200 new customers a month:

The last column adds up the outstanding amounts of all running cohorts. As an estimate, the short formula is enough: new customers per month times pre-financing times months until recovered, divided by two. It assumes even repayments and, over long periods, comes out up to a quarter above the table, because in this example more flows back early on. For a ceiling, that is the right side to err on.

The jump from €48 to €75 CAC multiplies the capital tied up by eight. On paper the sums work in both cases, but in the bank account these are two different companies. Faster growth also increases the share of customers who have not yet paid back. Good months therefore make the situation tighter.

Interest is rarely the problem. If you finance the €32,400 at an assumed 10%, that costs €3,240 a year, or around €1.35 per customer across 2,400 new customers. The earnings limit barely moves. The question is whether you have the money at all, or can get it.

What the table does not yet include

In the DACH region (Germany, Austria and Switzerland), other items add to the customer loan.

Payment on invoice: according to EHI, its share of German e-commerce revenue in 2025 was 26.1%. With this method the money only arrives after dispatch. With a quarter of orders paid on invoice and an assumed 30 days until payment, around €5,000 is outstanding in the example.

Withdrawal: in Germany the period is 14 days, and the refund is due no later than 14 days after that, including standard shipping. Part of the first-purchase revenue is therefore not secure until up to four weeks after delivery, even if it is already in your account. Plan a reserve for it.

Stock: the contribution margin deducts the cost of goods, but you paid for them in advance. With an assumed €45 cost of goods on the first purchase and two months from purchasing to payment, the pace ties up another €18,000. That turns the €32,400 at a CAC of €60 into roughly €50,000 to €55,000.

Same customer value, different maximum CAC

Take two retailers with exactly the figures above. After stock and outstanding invoices, retailer A can still spare €15,000 for pre-financing customers, retailer B €40,000. A ends up at a CAC of around €51, while B can carry €60 and would still have a reserve. The customer value is identical, and yet what is sensible for B would be wrong for A.

If A wants to double the pace to 400 new customers a month, the maximum CAC drops to €45, even if stock and outstanding invoices were financed some other way. At the same time, a faster pace is more likely to push CAC up, because additional reach usually gets more expensive. The limit falls while the actual figure rises.

This also answers the question of which period to use for customer value: it is the shorter of two spans. One is the age of the cohorts for which you have real data. The other is the time your cash lasts at the planned pace. Twelve or 24 months as a default assumption has no place in this calculation.

How to set the figure

1. Pull cumulative contribution margin per new customer from real cohorts, broken down by month.
2. Set the amount you can tie up permanently without putting stock purchasing and salaries at risk. Deduct stock and outstanding invoices first.
3. Run the table for your planned pace of acquisition and read off the highest CAC that fits within that amount.
4. Compare it with the earnings limit, after your profit mark-up. The lower figure is your CAC limit.
5. Check whether the measured new customers really come from the advertising. Otherwise the true CAC is higher than the measured one.

The new customer value in Google Ads is a financing decision

According to Google, the new customer value you set is added to the purchase value and feeds into reporting and Smart Bidding. As a starting point Google suggests twice the value of a regular customer, and itself describes this guidance as directional and business specific (as of November 2026). Alongside the fixed amount, Google is rolling out percentage multipliers, so the uplift then grows with the order value. By my calculation, the system then allows more per new customer on large orders, and the capital tied up rises with it.

For you this means: raising the value allows the system to spend more per new customer, and in doing so increases the capital tied up. I would only set it once the resulting CAC works in the table at your pace. The implementation is covered under the new customer goal in Google Ads.

Without repeat purchases, the first purchase is the limit

For product ranges without repeat purchases the question of time disappears, and the limit is the first purchase. The head of a wallet brand describes it this way for his business: with no customer lifetime value, the very first purchase had to be profitable.

With ample equity or investor money, the cash limit moves far upwards, and the earnings limit including the cost of capital decides. Annual subscriptions paid in advance also turn the calculation around: the money comes early, and the risk then lies in the service you still owe. And if you advertise below break-even on purpose, for example to enter a market, make it a fixed amount with an end date so that the exception does not become permanent.