LTV/CAC Ratio Explained: The 3:1 Rule and When It Lies
Published 2026-08-02
TL;DR
LTV = (ARPU × gross_margin) / monthly_churn. LTV / CAC ≥ 3 and CAC payback ≤ 18 months is the healthy zone. Both conditions matter.
Where the formula comes from
Under a constant-churn assumption, expected customer lifetime is 1 / monthly_churn. Contribution per month is ARPU × gross_margin. Multiply and you get lifetime contribution: LTV.
Worked example
A B2B SaaS:
- ARPU: $500/month
- Gross margin: 82%
- Monthly churn: 1.8%
- CAC: $2,400
Contribution per month = $500 × 0.82 = $410. LTV = $410 / 0.018 ≈ $22,800. Ratio = 22,800 / 2,400 = 9.5x. Payback = 2,400 / 410 ≈ 5.9 months.
Excellent. See it in the calculator.
When 3:1 lies
Long payback with a great ratio. A ratio of 5:1 with 30-month payback means you are financing your customers for two-and-a-half years. If runway runs out, the ratio doesn’t matter.
Infinite LTV from tiny churn. A 0.3% churn produces enormous LTV, but three churned customers this month might be measurement noise. Floor churn for planning purposes.
Gross-margin games. LTV is contribution-margin based. If gross margin is 40% (unusual for SaaS but common for services-heavy businesses), the ratio compresses fast.
Cohort mixing. Blended ARPU across a SMB tier and an enterprise tier inflates the average and hides that the SMB CAC is not being recovered.
When lower than 3:1 is fine
- Very early stage, when CAC includes sunk experimentation costs.
- Products with strong network effects where LTV grows non-linearly.
- Businesses shifting from paid acquisition to organic (blended CAC drops).
Common mistakes
- Using revenue-multiple LTV instead of gross-margin LTV.
- Excluding sales team fully-loaded costs from CAC.
- Ignoring the cost of onboarding as part of CAC.
- Using LTV as a target instead of a leading indicator.
- Comparing your LTV/CAC to a public company’s without normalizing for stage.
Related
- LTV / CAC Calculator
- Quick Ratio Calculator — MRR gained vs lost
- MRR / ARR Growth — extrapolate the ratio into growth
FAQ
Should I use monthly or annual churn?+
Monthly for SMB; annual (converted to monthly) for enterprise. The formula uses monthly; annual churn / 12 is a decent approximation.
What if my LTV is infinite?+
It means monthly churn rounds to zero, which is either brilliant (net revenue retention above 100%) or a measurement artifact. Use a floor churn of 0.5-1% for planning.
Does 3:1 hold for enterprise SaaS?+
Enterprise runs higher: 5:1 or better because CAC is huge and churn is low. If enterprise LTV/CAC is 3:1 with 24-month payback, something is wrong.
How do I include expansion revenue?+
Raise ARPU to reflect net revenue retention. If NRR is 120%, use ARPU × 1.2 or bake it into a negative churn number.
Is CAC payback a better metric?+
For cash-constrained companies, yes. LTV/CAC assumes you have the runway to wait for lifetime; payback is the number that keeps the lights on.