The Rule of 40 says that a healthy SaaS business should have year-over-year revenue growth rate plus free cash flow margin add up to at least 40 percent. It is the single most-cited public-market benchmark for SaaS, and the framing is genuinely useful: at any growth-vs-profitability trade-off, the sum should hit 40. A 50 percent grower at minus 10 percent margin is fine; a 10 percent grower at 30 percent margin is also fine.
The formula
Rule of 40 = YoY revenue growth % + FCF margin %You can substitute different denominators for “profitability” and the spirit holds:
- FCF margin: Strictest. Public-market preferred.
- EBITDA margin: Common in private SaaS reporting.
- Operating margin: Sometimes used. Less honest because it hides capex.
Pick one and stick with it. Mixing definitions across quarters defeats the metric.
How to calculate it cleanly
- YoY growth. Trailing-12-month revenue this period over trailing-12 prior. Avoid quarter-on-quarter, which is noisy at small scale.
- FCF margin. Operating cash flow minus capex, divided by revenue. Be honest about stock-based compensation — most SaaS companies report adjusted FCF that ignores SBC, but the IPO market increasingly does not.
- Use the same period for both terms. TTM growth against TTM FCF margin.
A company at 35 percent growth and 5 percent FCF margin sits at 40 — exactly the bar. A company at 60 percent growth and minus 30 percent margin sits at 30 — under the bar despite eye-popping growth. The rule punishes both extremes.
Benchmarks
For public SaaS:
| Performance | Rule of 40 score |
|---|---|
| Top quartile | 50 or higher |
| Healthy | 40 to 50 |
| Acceptable | 30 to 40 |
| Concerning | Under 30 |
Private SaaS investors apply the same bar from Series C onward. Earlier-stage companies should not be evaluated on Rule of 40 — they are deliberately sacrificing margin for growth.
Common pitfalls
- Using gross margin or operating margin. They flatter the score versus FCF. Use FCF for honest comparisons.
- Including non-recurring revenue. Services, perpetual licenses, one-time fees inflate growth. Use ARR or recurring revenue only.
- Ignoring the trade-off direction. A team at 40 hitting it via 80 percent growth and minus 40 percent margin is in a different place than a team at 40 hitting it via 20 percent growth and 20 percent margin. Both pass, but they need different next-year plays.
- Using non-GAAP numbers without disclosure. SBC-adjusted FCF can be 20 to 40 percent higher than GAAP. Be explicit about which.
Related
- LTV:CAC — the unit-economics input
- Magic Number — the sales-efficiency input