Free Tool
SaaS CAC Payback Calculator
A customer who costs 300 to acquire and pays 25 a month in gross margin takes a year to pay back — and less than that if they churn first. This calculator shows the payback in months.
Planning estimate, not financial advice. All figures on this page are estimates or scenarios for planning purposes only, built from the numbers you enter. They are not financial, tax or legal advice, and no earnings are promised. Read the full financial disclaimer.
What this tool does
Enter the customer acquisition cost, the monthly revenue per customer, the gross margin percentage and the monthly churn rate. The calculator shows the monthly gross margin per customer, the payback period in months, and how that compares with the average customer lifetime the churn rate implies.
Payback is the decision the CAC and LTV tools do not make on their own. A CAC that looks fine next to a three-year LTV is a problem when the customer leaves after eight months — the payback period against the lifetime is the comparison that catches it.
Try it
Waiting. Enter the CAC, monthly revenue, margin and churn, then press Calculate.
How it works
Monthly gross margin per customer is monthly revenue multiplied by the gross margin percentage. The payback period is the CAC divided by that monthly margin. Average lifetime is one hundred divided by the monthly churn percentage, and the lifetime value is the monthly margin multiplied by the lifetime.
The comparison is payback against lifetime. A payback inside the lifetime means the average customer pays back their acquisition cost and then contributes; a payback beyond the lifetime means the average customer leaves before paying back.
Assumptions and limitations
Payback is a monthly-margin calculation. The margin has to be real.
- Gross margin is revenue minus the direct cost of serving the customer — hosting, support, payment fees — not the full business cost. Using revenue instead of margin understates the payback badly.
- CAC is the full cost of acquiring one customer: advertising, sales time and tools, divided by the customers won. An ad-spend-only CAC flatters the payback.
- Lifetime assumes a steady churn rate. Early customers churn differently from mature ones — re-run the numbers as the base ages.
- The calculator does not model expansion revenue. A customer who upgrades pays back faster; the payback shown is the conservative case.
Worked example
Example (illustrative inputs, not a real product). A 300 CAC, 29 monthly revenue, 80% gross margin, 5% monthly churn:
| Item | Figure |
|---|---|
| Monthly gross margin (29 × 80%) | 23.20 |
| CAC payback (300 ÷ 23.20) | 12.9 months |
| Average lifetime (100 ÷ 5) | 20.0 months |
| Lifetime value (23.20 × 20) | 464.00 |
Frequently asked questions
What is a good CAC payback period?
It depends on the lifetime. The rule that matters is payback versus lifetime: the payback must sit comfortably inside the average customer lifetime, or the average customer leaves before paying back. A 12-month payback is fine at a 20-month lifetime and fatal at a 10-month one.
Why use gross margin instead of revenue?
Because revenue is not what pays the CAC back — margin is. Hosting, support and payment fees come out of the revenue first. Using revenue instead of margin can halve the payback on paper and hide a real problem.
What goes into CAC?
Everything spent to win the customer: advertising, sales time, tools and commissions, divided by the customers won. Ad spend alone is not the CAC — it is the cheapest part of it.
How does churn change the payback?
It does not change the payback months — it changes whether the customer is still there to pay them. That is why the tool shows lifetime beside payback: the two numbers together are the decision.