LTV / CAC Ratio: Formula, Benchmarks, and Common Traps

LTV / CAC: The Ratio That Justifies Growth Investment

LTV / CAC is the second-most-cited SaaS metric after growth rate. Above 3, investors will fund more S&M spend. Below 1, the business is destroying value. But the calculation is genuinely tricky — and easy to game unintentionally. Here's how to get it right.

The formula

LTV = ARPA × gross margin × average customer lifetime (in months). CAC = fully-loaded S&M cost per new customer. Ratio: LTV / CAC. Example: $12K annual ARPA × 80% gross margin × 4-year lifetime = $38.4K LTV. $12K CAC → LTV / CAC = 3.2x.

Benchmarks

Under 1: business destroys value with each new customer — pause growth spend. 1-3: sub-scale, improve efficiency before scaling. 3-5: healthy, invest more. Above 5: highly efficient but may signal under-investment (worth diagnosing). Growth-stage investors expect 3+ minimum for Series B, 4+ for Series C.

The lifetime estimation problem

Customer lifetime = 1 / (annual churn rate). Example: 10% annual churn → 10-year lifetime. Problem: at early stage, you don't have 10 years of data to validate. Solution: use conservative lifetime estimates (cap at 4-5 years even if churn implies longer) and cite the assumption explicitly. Investors will discount aggressive lifetime assumptions.

Common mistakes

Using revenue instead of gross margin in LTV (inflates by 20-30%). Including expansion revenue in ARPA without adjusting for expansion CAC. Estimating lifetime based on a few outlier long-tenured customers. Excluding fully-loaded S&M from CAC (only counting variable spend, not sales team overhead). Each mistake compounds — a "5x" LTV/CAC calculated incorrectly might be 2x actual.

Frequently asked questions

Should we include expansion revenue in LTV?

Yes for pure LTV calculation (expansion is real value from the customer), but be explicit — some investors prefer new-logo LTV separated from expansion LTV. Report both.

How does LTV / CAC relate to CAC payback?

CAC payback is the leading indicator; LTV / CAC is the lagging outcome. Payback under 18 months typically produces LTV / CAC above 3x. Both together give a complete efficiency picture.

What if churn is too new to estimate lifetime?

Use conservative caps (4-5 year lifetime max) and cohort-based churn from your longest-tenured cohort. Disclose the calculation methodology. Investors respect honest under-estimation more than aggressive assumptions.

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