CAC Payback for Ecommerce: The Formula and How to Read It by Cohort

CAC payback is the number of months it takes a customer's contribution margin to cover what you paid to acquire them. The formula is customer acquisition cost divided by monthly contribution margin per customer. Read it by acquisition channel and by monthly cohort, not as one blended number, and you'll know which customers are actually making you money.

This guide covers the formula, how to calculate it by cohort, and how one fast-growing DTC brand uses it to run the business.

What is CAC payback?

CAC payback answers one question: how long until a new customer has paid back what it cost to get them?

Until that point, every new customer is a loan. After it, they're profit. The shorter the payback, the faster you can reinvest in growth without running out of cash.

ROAS tells you what came back on the first order. CAC payback tells you what came back over the customer's life, after costs. For any brand with repeat purchase or subscription revenue, that's the number that decides whether growth is healthy.

The CAC payback formula

CAC payback (months) = CAC ÷ monthly contribution margin per customer

• CAC: total acquisition spend for a channel or period, divided by the new customers it brought in.
• Monthly contribution margin per customer: revenue per customer per month, minus product cost, shipping, fulfilment, payment fees and discounts. Not revenue. Margin.

A worked example, with illustrative numbers only:

• You spend $30,000 on one channel in March and acquire 600 customers. CAC is $50.
• That cohort generates $20 of contribution margin per customer per month.
• Payback is $50 ÷ $20 = 2.5 months.

If the same cohort only produced $10 a month, payback would be 5 months. Same CAC, twice as long to get your money back.

Why one blended number hides the answer

A blended payback period averages your best and worst channels together. It can look fine while one channel quietly loses money.

Jim Phillips, founder and CEO of Graymatter, a DTC drink mix brand where about 95% of buyers subscribe, looks at it the other way round. On Checked Out Episode 9 he described the questions his team asks: "what's it cost to acquire a customer? What does it cost to acquire a customer by each channel that we're acquiring them in?"

Then the other side of the coin: "how long are we keeping them? And in particular, how much contribution margin... are we getting on a monthly basis?"

Two numbers, read together, by channel and by cohort. That's the whole method.

How to calculate CAC payback by cohort

1. Group customers by acquisition month and channel. March paid social is one cohort. March email signups are another.
2. Calculate CAC for each cohort. Spend for that channel and month, divided by new customers from it.
3. Track contribution margin month by month. For each cohort, add up the margin it generates in month one, month two, month three and onwards. Count repeat orders and subscription renewals, minus costs.
4. Find the break-even month. The first month where cumulative contribution margin per customer is greater than CAC. That's your payback period for that cohort.
5. Compare cohorts. Which channels pay back fastest? Which months? Did a new offer shorten or lengthen payback?

Cumulative is the key word. A cohort that earns $15 in month one and $20 a month after that pays back a $50 CAC during month three ($15 + $20 + $20 = $55).

What "break even by month two or three" looks like

Jim's team knows each cohort's break-even point early: "by month two or month three, depending on... what the cost was and what they purchased... when do we break even and when do we start actually making money on this cohort?"

Two things stand out.

It depends on what the customer bought first. Different entry products carry different margins, so payback isn't one number even within a channel.

It's checked constantly. Jim described an update from his media buyer every two hours: "Here's what our current CAC is on the day. Here's what it is on the month. Here's what our spend is compared to goal."

That's Graymatter's business, not a benchmark. Your payback will depend on your margins, your repeat rate and how much cash you can tie up. The point is the habit: know the number for every cohort, early.

How payback changes your decisions

Once you can see payback by cohort, the arguments get shorter.

• Spend: scale channels that pay back fastest. Cap the ones that don't, even if their ROAS looks good on day one.
• Offers: a deeper first-order discount raises effective CAC. Payback tells you whether the extra customers are worth it.
• Creative: let results decide, not taste. Jim's team calls it "data over ego". As he put it, you can look at an ad and think "this ad's gonna crush... and then you put it out there and it does nothing. And then you'll see an ad that looks what you think is terrible... and it just ends up doing so well."

CAC payback vs LTV to CAC ratio

They answer different questions.

• LTV to CAC asks how much a customer is worth over their whole life compared to what they cost. It's a measure of value.
• CAC payback asks how long it takes to get the acquisition cost back. It's a measure of time and cash.

A brand can have a strong LTV to CAC ratio and still struggle if payback takes 12 months and cash runs out at month six. Track both. More on the ratio in LTV to CAC ratio.

Common mistakes

• Using revenue instead of contribution margin. Revenue makes payback look faster than it is.
• Using blended CAC. It hides the channels that don't pay back.
• Ignoring discounts and free shipping. They're part of the cost of that customer.
• Checking once a quarter. By then the spend is gone. Cohorts tell you within a month or two which way they're heading.

Frequently asked questions

How do I calculate CAC payback for an ecommerce brand?

Divide customer acquisition cost by the monthly contribution margin each customer generates. For accuracy, calculate it per acquisition channel and per monthly cohort, using cumulative margin.

When does a customer cohort break even?

In the first month where its cumulative contribution margin per customer is greater than its CAC. For a subscription-heavy brand like Graymatter, the founder says that's around month two or three, depending on cost and first product.

Should I track CAC by channel or blended?

By channel. Blended CAC averages good and bad channels together and hides where money is being lost.

What's a good CAC payback period?

It depends on your margins, repeat rate and cash position. Shorter is safer: the faster a cohort pays back, the sooner that money can fund the next one.

How to track CAC and payback in Peel

Peel pulls ad spend from your connected Facebook Ads, Google Ads and TikTok Ads accounts and matches it to your Shopify revenue, so CAC and payback come from one place instead of three spreadsheets.

1. Connect your ad accounts. Spend in Peel depends on which ad accounts are connected, so connect every paid channel you run.
2. Check CAC. Peel calculates CAC as total ad spend from connected platforms divided by new customers. You can count a new customer from their first order (CAC Order) or from sign-up (CAC Sign-up).
3. Open the Payback Period report. It plots ad spend against the revenue from the orders those ads drove, day by day after the click (D1, D2, D3 and on), so you can see when spend is earned back.
4. Split it by channel. Filter by channel (Google, Facebook or TikTok paid), UTM source or UTM campaign, and choose the attribution model (first touch, last touch or linear).
5. Group by month. Grouping by month lines each month's spend up with what it earned back, the same way the cohort method above does.

One difference to keep in mind: Peel's Payback Period report measures revenue (gross sales minus discounts and returns, plus tax and shipping). To read it the way this guide does, on contribution margin, take product, shipping and fulfilment costs off those revenue figures.

See payback by cohort

Peel turns your Shopify data into cohort views of acquisition cost, contribution margin and LTV, so you can see when each cohort pays back. Start with cohort analysis, customer acquisition cost and contribution margin.

Watch the full conversation with Jim Phillips on Checked Out Episode 9.