Payback period
The payback period is the time it takes for a customer's margin to repay the cost of acquiring them. While LTV/CAC says how many times the investment returns, payback says when. For cash flow that is the more important question: a company with a long payback can have great economics on paper and still run out of money while scaling.
In short
| Formula | CAC / monthly margin per customer, more precisely the cohort curve of cumulative margin |
| What it says | How long a customer takes to repay their own acquisition |
| Why it matters | It sets how much growth a company can afford from its own cash |
| Rough bands | E-commerce within 6 months is good, SaaS within 12, beyond 18 only with strong funding |
How it is calculated
Formula and inputs
Simplified formula: payback in months = CAC / monthly contribution margin per customer. CAC is fully loaded (media, creative, agency, a share of the team); margin is revenue net of VAT minus variable costs, never revenue. The formula assumes the same margin every month, which holds for subscriptions: at €20 of margin per month and a CAC of €120, payback is 120 / 20 = 6 months.
In e-commerce, margin does not arrive evenly: the first order brings the most; later orders are irregular and thin out over time. The precise calculation therefore works with a cohort: for customers acquired in the same month, add up the contribution margin from all their orders month by month, divide by the cohort size and accumulate. Payback is the month in which cumulative margin per customer first reaches CAC.
Worked example
Worked example: the January cohort has 500 customers and acquisition cost €60,000, so CAC = 60,000 / 500 = €120. The first order brings an average of €40 of contribution margin per customer. Repeat purchases then bring an average of €18, €16, €14, €12, €11 and €10 per customer in months 1 to 6. Cumulative margin builds up like this:
| Month since acquisition | Margin in month | Cumulative | Remaining to CAC |
| 0 | €40 | €40 | €80 |
| 2 | €16 | €74 | €46 |
| 4 | €12 | €100 | €20 |
| 6 | €10 | €121 | repaid |
The curve crosses CAC in the sixth month: at the start of the month 120 minus 111 = €9 is missing and the month brings €10, more precisely 5.9 months. For the whole cohort this means the €60,000 invested in January is back in margin by the end of July.
Why payback matters more to cash flow than LTV
LTV says how much comes back over 24 months; payback says when the money is back and can fund acquisition again. At a 6-month payback the same cash turns over twice a year, at 3 months four times, at 12 months once. The company in the example, with a €60,000 monthly budget, must finance roughly six months of acquisition at all times; if payback grew to 12 months, the cash required would double even though LTV stayed the same. Two channels with the same LTV can therefore differ in how much growth the company can afford without outside money.
Three rules of interpretation. Payback is calculated per segment and channel; the account average hides slow cohorts. It is compared with the LTV horizon: a 6-month payback against a 24-month LTV means 18 months of pure margin; a 20-month payback against the same horizon means almost nothing. And the sensitivity to CAC is direct: if CAC rose 25% to €150, payback at a constant €20 of monthly margin would rise from 6 to 7.5 months.
Why payback dictates the pace of growth
A worked example: CAC of 120, a customer bringing in 20 of margin per month, payback of 6 months. Acquiring 100 customers a month at that pace means 72,000 invested at any given moment in six cohorts that have not yet repaid their CAC. Doubling the growth pace doubles that amount. That is exactly why two companies with the same LTV/CAC can have completely different options: the one with the shorter payback recycles cash faster and scales from its own resources.
From our own practice
Managing budgets of more than EUR 52 million in total, we use payback as the second criterion when scaling: a channel with a worse LTV/CAC ratio but fast payback can be more valuable for growth than a channel with an attractive ratio and a one-year return. With seasonal goods there is an extra rule: payback has to land inside the season. Acquiring in September with a six-month payback means the money returns after the peak it was supposed to work in.
Common mistakes
- Calculating payback from revenue. Only margin can repay CAC. Revenue payback is fiction.
- Ignoring it when LTV/CAC looks good. A ratio without time is half the information. Growth is killed by cash flow more often than by economics.
- Averaging across segments. Payback differs by channel and product, and the average hides the slow segments that tie up cash.
Related terms
See also CAC, LTV, LTV/CAC ratio, contribution margin and AOV.
Frequently asked questions
How do I calculate payback precisely?
By cohort: for customers acquired in a given month, draw the cumulative margin curve and find the month it crosses CAC. A spreadsheet is enough.
What shortens payback?
Higher first-purchase margin, a faster second purchase (CRM, reminders) and cheaper acquisition. The fastest lever is usually the second purchase.
What payback is too long?
One the company cannot finance at its target growth pace. There is no absolute threshold, only the relation to available cash.
How we can help
We calculate cohort-based acquisition payback as part of our Performance marketing agency service.