SaaS Unit Economics Explained: LTV, CAC, Payback & The Rule of 40
A complete guide to calculating Customer Lifetime Value, Customer Acquisition Cost, Payback Period, and Rule of 40 benchmarks for subscription businesses.
September 2, 2026
12 min read
The Financial Foundations of Subscription Software
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For SaaS startups and growth-stage technology companies, revenue growth without sustainable unit economics leads to rapid capital depletion. Investors evaluate unit economics to determine whether capital injected into sales and marketing produces profitable compounding returns.
1. The Four Essential SaaS Unit Metrics
- Customer Lifetime Value (LTV): Total gross profit generated across a customer relationship:
LTV = (ARPU × Gross Margin %) ÷ Monthly Churn Rate. - Customer Acquisition Cost (CAC): Fully loaded sales and marketing spend divided by new customers acquired:
CAC = Total S&M Spend ÷ New Customers. - LTV : CAC Ratio: The universal efficiency standard. Benchmark: 3x to 5x is healthy. Below 1x loses money on every acquisition.
- CAC Payback Period: Months required to recover acquisition costs:
Payback = CAC ÷ (ARPU × Gross Margin %). Ideal: < 12 months for SMB SaaS.
2. The SaaS Rule of 40
The Rule of 40 states that an elite software company's Year-over-Year Growth Rate (%) plus its Profit Margin (%) should meet or exceed 40% (e.g., 60% growth with -15% margin = 45% Rule of 40 score).