The Rule of 40 frames the central tension in SaaS between growth and profitability: \(\text{growth rate (\%)} + \text{profit margin (\%)} \geq 40\). A company growing 50% with a 0% margin passes the rule, as does one growing 20% with a 20% margin. The benchmark captures the idea that a SaaS can be unprofitable but fast, or profitable but slow, and still be a healthy business—so long as the combined score clears 40. It is most useful as a comparative heuristic across stages and segments rather than a strict target, since the optimal balance depends on market size, competition, and capital availability.
The Magic Number focuses the same idea on sales and marketing efficiency. It is computed as \((\text{Net new ARR in a quarter} \times 4) / \text{prior-quarter S\&M spend}\), annualizing the quarterly ARR output and comparing it against the prior quarter's marketing investment. A Magic Number above 1.0 suggests S&M is efficient, above 0.75 is reasonable, and below 0.5 suggests inefficient spend. Because the formula uses prior-quarter spend in the denominator, it implicitly assumes a lag between marketing investment and resulting ARR—a reasonable approximation of how pipeline converts. The typical S&M ratio for high-growth SaaS is 50–80% of revenue, declining as the company matures and customer acquisition compounds; this declining ratio is exactly what the Magic Number is designed to detect.
S&M efficiency and LTV:CAC answer adjacent but distinct questions. S&M efficiency is a period measure of marketing spend versus new ARR produced, useful for tuning go-to-market motion in the near term. LTV:CAC is a lifetime economic ratio that compares the present value of a customer's revenue stream against the cost to acquire them. Both are needed: S&M efficiency tells operators whether to step on the gas, while LTV:CAC tells investors and boards whether the underlying economics justify the spend. Together with the Rule of 40, they form the trinity most commonly used to evaluate SaaS health.