The Rule of 40 and software metrics: ARR, churn, net retention

Subscription software (SaaS) companies often grow quickly and report modest profits, which makes the usual yardsticks hard to apply. Investors and managers therefore use a handful of specialized metrics: the Rule of 40, annual recurring revenue, churn and net revenue retention. They are useful shortcuts for judging the health of a subscription business, but most are not standardized, so definitions matter.

The Rule of 40

The Rule of 40 is a rule of thumb that says a healthy software company's growth rate and profit margin should add up to at least 40%.

Rule of 40 score = Revenue growth rate (%) + Profit margin (%)

It captures a trade-off: a company can grow fast and spend heavily, or grow slowly and be very profitable, and a score of 40 or more suggests the balance is sound. Which margin to use is not fixed. Common choices are free cash flow margin, operating margin and adjusted EBITDA margin. Growth is usually year-over-year revenue growth. Always check which version is being quoted, because adjusted figures can look much better than free cash flow.

Example (illustrative numbers)

Company A clears the bar. Company B is profitable but not growing enough to compensate. Company C grows quickly but burns too much cash to meet the guideline. Neither a pass nor a fail settles the question, since the number says nothing about the price paid, the competitive position or the reasons behind the spending.

ARR and MRR

Monthly recurring revenue (MRR) is the normalized monthly value of active subscriptions. Annual recurring revenue (ARR) is the annualized equivalent, often MRR multiplied by 12. ARR counts only recurring subscription fees, excluding one-time items such as setup fees or professional services. It is a management metric, not an accounting figure. Revenue under accounting rules (see revenue recognition) is what appears on the income statement, and ARR can differ from it because of timing, discounts and contract terms. Companies define ARR differently, and not all report it.

Churn

Churn measures how much business is lost.

• Customer (logo) churn is the share of customers that cancel in a period.
• Gross revenue churn is the share of recurring revenue lost from cancellations and downgrades, excluding any expansion.

Gross revenue retention = (Starting ARR - churned ARR - contraction) / Starting ARR

Gross retention cannot exceed 100%. Because the lost customers have to be replaced just to stand still, high churn forces a company to spend heavily on new sales. Annual churn of 10% means the base shrinks by about a tenth each year before any new customers arrive.

Net revenue retention

Net revenue retention (NRR, sometimes called net dollar retention) looks at a group of customers that existed a year ago and asks how much their recurring revenue is now, including upgrades and extra purchases.

NRR = (Starting ARR + expansion - contraction - churned ARR) / Starting ARR

New customers are excluded. An NRR above 100% means the existing customer base grows by itself.

Example (illustrative numbers)

A company begins the year with $100 million of ARR from existing customers. Over the year, those customers add $20 million through upgrades and extra seats, reduce spending by $5 million and cancel $10 million.

• NRR = (100 + 20 - 5 - 10) / 100 = 105%
• Gross revenue retention = (100 - 5 - 10) / 100 = 85%

The company grows 5% from its installed base before winning a single new customer. At the same time, 15% of the base is leaking away, so the headline number hides churn that is being offset by expansion. Looking at both metrics together gives a fuller picture.

Other metrics you will see

• Customer acquisition cost (CAC) and payback. How much sales and marketing spending it takes to win a customer and how long the gross profit takes to repay it.
• Gross margin. Software margins are often high, and a drop can signal rising hosting or support costs.
• Remaining performance obligations and deferred revenue. Contracted revenue not yet recognized, found in the filings.
• Free cash flow and stock-based compensation.

Common mistakes and limits

• Comparing different definitions. ARR, churn and NRR are not governed by accounting standards, so two companies may calculate them differently, and a company may change its method.
• Using only the flattering margin. A Rule of 40 score built on adjusted EBITDA that excludes stock compensation can overstate economics. Dilution is a real cost to shareholders.
• Ignoring quality of growth. Growth bought with discounts, or from acquisitions, is different from organic growth.
• Treating 40 as a law. It is a heuristic. Large mature companies, small high-growth ones and different sub-industries are not held to the same bar.
• Forgetting valuation. A strong score does not make a stock cheap. See price-to-sales ratio for how revenue multiples are read.
• Extrapolating. Growth rates slow as companies scale, so a past score is not a forecast.

Key takeaways

• The Rule of 40 adds revenue growth and a profit margin, and a score of 40 or more suggests a healthy balance, though the margin used varies.
• ARR, churn and net revenue retention are management metrics without a standard definition, so read the footnotes.
• NRR above 100% means existing customers grow on their own, but compare it with gross retention to see how much is leaking.
• Always check the cost side, including free cash flow and stock-based compensation, and keep valuation in view.