Rule of 40 in SaaS: what it is and how to apply it
Growth + margin ≥ 40 %. The number VCs and CFOs use first to decide if a SaaS is healthy or burning cash without return. How to measure it right.
The Rule of 40 is the fastest single check on SaaS health. Add your growth rate and your profit margin: if it clears 40 %, you’re in good shape. If it doesn’t, you’re burning cash without a proportional return.
The rule was popularised by Brad Feld in 2015 and it’s now the first number a VC looks at in a Series A or B. Here’s how to calculate it, which inputs to use, and why most founders apply it wrong.
The formula
Rule of 40 = Growth rate (%) + Margin (%) ≥ 40The math is trivial. The trap is which specific metrics you put on each side.
- Growth: typically YoY ARR growth. Some teams use total revenue growth, but in SaaS with recurring billing, ARR is more stable.
- Margin: no consensus here. The three most common choices are EBITDA margin, free cash flow margin, and operating margin.
For a typical Series A B2B SaaS, the most-used combo is EBITDA + ARR growth. At later stages, FCF margin + ARR growth.
Why it matters
Investors lean on it because it captures an uncomfortable truth in SaaS: growth without profitability and profitability without growth are both bad in the long run.
- Company A: grows 60 %, margin –20 % → sums to 40. Pass.
- Company B: grows 10 %, margin 30 % → sums to 40. Pass.
- Company C: grows 50 %, margin –30 % → sums to 20. Fail.
All three burn cash differently, but the first two are building value. The third is destroying capital with no proportional return.
How to calculate it with real data
Imagine a SaaS company with these numbers:
| Metric | Current year | Prior year |
|---|---|---|
| ARR | $4,000,000 | $2,500,000 |
| EBITDA | –$600,000 | –$800,000 |
ARR growth: (4,000,000 − 2,500,000) / 2,500,000 = 60 %
EBITDA margin: −600,000 / 4,000,000 = −15 %
Rule of 40: 60 + (−15) = 45
✓ Pass. Investors accept losses because the company is growing fast enough to justify them.
If next year growth drops to 30 % and margin stays at −15 %, the sum is 15. Fail. You either cut spend, reignite growth, or both.
Common mistakes when measuring it
1. Using monthly growth instead of annual
MoM growth amplifies one-off spikes. For the Rule of 40 always use YoY (12 months), which smooths seasonality.
2. Confusing gross margin with the Rule of 40 margin
Gross margin measures product efficiency (revenue − COGS). The Rule of 40 uses operating margin, EBITDA, or FCF margin — metrics that also include sales, marketing, and G&A. Don’t conflate them.
3. Ignoring free cash flow when there’s debt
If you carry significant debt, EBITDA inflates real profitability. For Series B+ companies or anyone with debt on the balance sheet, FCF margin is the more honest input.
4. Reading it as a snapshot, not a trajectory
The Rule of 40 is useful month over month, but the trajectory matters more. A company moving from 25 to 38 to 45 over three years is on a healthy path. A company moving from 50 to 42 to 38 is losing efficiency, even if it still “passes”.
When NOT to apply the Rule of 40
The rule is built for growing B2B SaaS, typically between $1M and $100M ARR. Outside that band, accuracy drops:
- Pre-$1M ARR: growth percentages can be absurdly high and margins deeply negative. The rule tells you very little.
- Mature companies above $500M ARR: growth stabilises below 30 %, and the focus shifts to finer unit economics: NRR, magic number, CAC payback.
- Non-SaaS businesses: ecommerce, marketplaces, services. Margin and revenue dynamics behave differently.
How it connects to other metrics
The Rule of 40 is an aggregate. When you fail it, the metrics that tell you why are:
- Growth rate → if it’s dropping, look at CAC payback, retention, top-of-funnel.
- Gross margin → if it’s below 70 %, your product isn’t efficient.
- EBITDA and EBIT → if margin is deeply negative, find the spend line responsible.
- MRR and churn → predict next year’s growth.
Passing doesn’t mean everything is fine — it means the aggregate is healthy. Keep watching the components.
How NextScenario measures it
NextScenario calculates the Rule of 40 in real time from your Stripe + bank + ad-platform data. No CSV imports, no waiting on month-end. Cash, ARR, and margin updated daily turn the Rule of 40 into a live KPI, not a rear-view mirror six weeks late.
If you want to see your own Rule of 40 with your real numbers, book a 20-minute demo.