The formula
The Rule of 40 adds two numbers: revenue growth rate (year-over-year, in percent) and a profitability margin (in percent). A company satisfies the rule when the sum is 40 or above: Revenue Growth % + FCF Margin % ≥ 40.
A company growing at 50% with a −15% FCF margin scores 35 — below the threshold. One growing at 20% with a 25% FCF margin scores 45 — above. Neither growth alone nor margin alone determines health; the metric rewards balance between the two.
Try any pair of inputs with the Rule of 40 calculator.
Why 40 is the threshold
The number 40 is empirical, not derived from first principles. Early-stage software businesses that succeeded at scale tended to cluster around a growth-plus-margin sum of 40 or higher. Investors and operators adopted it as a quick health benchmark: a score above 40 signals that the business is growing fast enough, profitable enough, or a productive combination of both.
A score of exactly 40 is not a hard cutoff — it is a benchmark. Scores consistently above 40 suggest a business can sustain itself without perpetual external capital. Scores consistently below 40 raise questions about whether growth investment is generating sufficient returns. Elite software companies typically score 60 or above.
Why it's SaaS-specific — and the scale caveat
The Rule of 40 was designed for subscription software businesses. These companies front-load costs (sales, onboarding, infrastructure) and earn revenue over the lifetime of a subscription, so early-stage margin is structurally negative even for businesses that will ultimately be highly profitable. The rule acknowledges this by trading off growth against margin: burning cash is acceptable if growth is high enough.
The metric is less meaningful below roughly $15–20M in annual recurring revenue (ARR). At that scale, hyper-growth rates (200%, 300%) are often a function of a tiny base, not operational efficiency, making the rule trivially easy to pass. At early stages, other signals — net revenue retention, payback period, gross margin — are more informative.
Outside software — hardware, retail, industrial, financial services — the metric has no established benchmark and produces misleading cross-sector comparisons.
FCF vs EBITDA variant
Two profitability measures are used in practice:
- Free cash flow (FCF) margin — operating cash flow minus capital expenditures, divided by revenue. FCF is favored by most investors because it is difficult to engineer and reflects the actual cash the business generates. It is the default for this tool and for most modern software analysis.
- EBITDA margin — earnings before interest, taxes, depreciation and amortization, divided by revenue. Older literature and some analysts use EBITDA because it was the metric available before SaaS-specific cash flow reporting matured. EBITDA ignores capital expenditures and can be flattered by high depreciation.
The two measures produce meaningfully different scores for capital-intensive software businesses. For asset-light pure SaaS, the gap is usually small. When comparing Rule of 40 scores across sources, always confirm which profitability measure each source uses.
Limitations
- Sector-specific. The benchmark is calibrated for software. Applying it to other sectors produces misleading comparisons.
- Scale-dependent. Below $15–20M ARR, hyper-growth from a small base makes the rule easy to pass regardless of underlying efficiency.
- One-dimensional. Two companies can score identically via very different paths: 70% growth with −30% margin vs. 5% growth with 35% margin. The raw score does not distinguish these profiles; investors must judge which is healthier given stage and market conditions.
- Gaming via accounting. Companies can inflate the metric by delaying sales commissions, capitalizing R&D, or timing one-time cost cuts. FCF margin is harder to engineer than EBITDA, which is one reason investors have shifted toward it.
- Point-in-time. A single quarter score can be distorted by seasonal cash timing or large one-time items. Analysts typically look at trailing-twelve-month (TTM) figures or multi-year trends rather than a single data point.
Related concepts and tools
- Rule of 40 calculator — enter revenue growth and FCF (or EBITDA) margin to score a company instantly.
- Profit margin explained — the gross, operating, and net margins that feed the profitability half of the rule.
- Profit margin calculator — compute gross margin from revenue and cost of goods sold.
- CAGR explained — compound annual growth rate, the measure behind the revenue growth input.