Rule of 40 definition
The Rule of 40 is an empirical benchmark for evaluating the health of a SaaS business. It states that a company's annual revenue growth rate (as a percentage) plus its EBITDA margin (as a percentage) should equal or exceed 40%.
A business growing at 60% ARR with a −25% EBITDA margin scores 35 — below the threshold. A business growing at 20% ARR with 25% EBITDA margin scores 45 — healthy. A business growing at 50% ARR with 10% EBITDA margin scores 60 — exceptional. The metric acknowledges that in SaaS, growth and profitability are traded off against each other, and that either a very high growth rate or strong profitability (or some combination) is acceptable — but you cannot sustainably have neither.
Why investors use the Rule of 40
The Rule of 40 became widely adopted after a 2015 blog post by venture investor Brad Feld. It spread rapidly because it addresses a genuine problem in SaaS valuation: how do you compare a high-growth, loss-making business to a slower-growth, profitable one?
Traditional metrics struggle with this comparison. Revenue multiples favour high-growth businesses but ignore profitability. P/E multiples are meaningless for loss-making companies. EBITDA margins alone penalise businesses that are rationally investing in growth. The Rule of 40 collapses this complexity into a single, comparable score that can be tracked over time and benchmarked against peers.
In practice, public SaaS companies that score consistently above 40 trade at a meaningful premium to those below. Private equity and growth investors use Rule of 40 as a filter and a target when evaluating and managing SaaS businesses.
Growth vs profitability: the Rule of 40 trade-off
The core insight of the Rule of 40 is that growth and profitability are interchangeable currencies in the SaaS business model — up to a point. Investing in sales and marketing accelerates ARR growth but burns cash and depresses EBITDA. Pulling back on S&M improves margins but slows compounding.
For a SaaS CFO, the Rule of 40 is useful as a guardrail in budget planning: it provides a clear expression of investor expectations that can be translated into annual growth and margin targets. If the board or investors expect a Rule of 40 score of 45 and the business is projecting 30% growth, the CFO knows the EBITDA margin target must be at least 15%.
The key FP&A exercise is building a model that shows the Rule of 40 score across different S&M investment levels and growth outcomes — allowing the leadership team to make an explicit, informed decision about where they want to operate on the growth-profitability frontier.
Limitations of the Rule of 40
Like all single-number benchmarks, the Rule of 40 has limitations. It does not account for capital efficiency — a business can score 40 while burning enormous amounts of cash on unsustainably low CAC payback. It does not reflect cash flow (EBITDA is an approximation, not free cash flow). And it treats growth and margin as perfectly interchangeable, which is only true if the growth is high-quality (i.e., backed by strong NRR and healthy unit economics).
Sophisticated investors therefore use the Rule of 40 alongside NRR, CAC Payback Period, and gross margin to build a complete picture of SaaS business quality. A 60 Rule of 40 score built on a collapsing NRR base is far less valuable than a 40 score built on 120% NRR and improving unit economics.
