The rule of thumb is LTV:CAC of at least 3:1, earn back roughly three times what it costs to acquire a customer. Below 1:1 you lose money on every sale; way above 3:1 usually means you're underspending on growth. Just as important is CAC payback: recover your acquisition cost within 12 months or cash flow will strangle you.
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David Skok's For Entrepreneurs work is the foundational, near-universally cited source on SaaS unit economics and modeling. It's where the LTV:CAC and CAC-payback conventions were popularized.
The reference primer on the metrics and market-sizing logic investors use, including bottom-up market sizing that keeps founders honest about how big a market really is. Canonical a16z source.
A focused a16z piece on the single ratio that most captures a business's efficiency and drives its valuation. Great for founders who need to understand the 3:1 benchmark and its limits.