SaaS Unit Economics: LTV, CAC, Payback Period & Scenario Modeling
Lifetime Value (LTV)
Simple Formula: LTV = ARPU / Churn Rate.
Example: $50/month ARPU with 5% monthly churn = $50 / 0.05 = $1,000 LTV.
Cohort-Based LTV
The simple formula is a starting point, but cohort-based LTV is far more accurate:
- Group customers by signup month.
- Track cumulative revenue per cohort over 6, 12, and 24 months.
- Extrapolate the curve to estimate full lifetime value.
The cohort method captures expansion revenue — upgrades, add-ons, and seat increases — that the simple formula misses entirely. If your product has any upsell motion, cohort LTV will be significantly higher than simple LTV.
Customer Acquisition Cost (CAC) by Channel
Don't use blended CAC — it hides which channels actually work. Track CAC per channel:
- Organic (SEO, content marketing, word of mouth): $20–80
- Paid (Google Ads, Facebook, LinkedIn): $100–500
- Referral (referral programs, affiliate partnerships): $30–120
When calculating CAC, include all costs: ad spend, marketing team salaries, tools and software, agency fees. Leaving out salaries is the most common mistake — it makes your CAC look artificially low.
The Magic Ratio: LTV:CAC
This single ratio tells you whether your business model works:
- Below 1:1 — You're losing money on every customer. Stop and fix before scaling.
- 1:1 to 3:1 — Marginal. Be cautious with spending and focus on improving either LTV or CAC.
- 3:1 and above — The sweet spot. You can scale confidently.
- Above 5:1 — You're likely under-investing in growth. Spend more to capture the market before competitors do.
Payback Period and Cash Flow
Formula: Payback Period = CAC / (ARPU × Gross Margin)
Example: $300 CAC / ($50 × 0.80) = 7.5 months to recover your acquisition cost.
Benchmarks
- SMB SaaS: Target under 12 months payback.
- Enterprise SaaS: Target under 18 months payback.
The longer your payback period, the more working capital you need to fund growth. A 12-month payback means you need to front a full year of acquisition costs before seeing returns — that adds up fast when you're scaling.
Scenario Modeling
Never plan around a single set of assumptions. Model three scenarios:
| Scenario | Churn | CAC | ARPU | LTV:CAC |
|---|---|---|---|---|
| Pessimistic | 8% | $400 | $40 | 1.25:1 |
| Base | 5% | $250 | $50 | 4:1 |
| Optimistic | 3% | $150 | $65 | 14.4:1 |
Use pessimistic for survival planning, base for budgeting, and optimistic for understanding your upside. If even the base case doesn't work, revisit your fundamentals before spending more.
When Unit Economics Signal a Pivot
Sometimes the numbers are telling you something. Watch for these red flags:
- LTV:CAC stays below 2:1 after 6 months of optimization.
- Payback period exceeds 18 months while bootstrapped.
- Churn keeps climbing despite product improvements.
- No channel produces a CAC below 30% of LTV.
These signals might mean you're in the wrong market, have wrong pricing, or have wrong positioning — not necessarily a bad product. Before pivoting the product, test changing your target customer, pricing model, or go-to-market strategy first.