Rule of 40: How Investors Judge Software Companies

The Rule of 40 is a quick benchmark for software and subscription businesses: a company’s revenue growth rate plus its profit margin should add up to at least 40%. A company growing 30% a year with a 10% margin passes; so does one growing 10% with a 30% margin. The rule captures the central trade-off in software, spending for growth versus earning profits today, in a single number. It is widely used by investors, analysts, and boards, though it is a heuristic rather than a law of economics.

The formula

The calculation is simple:

Rule of 40 score = revenue growth rate (%) + profit margin (%)

A score of 40 or higher is considered healthy. Some examples:

Company profile	Revenue growth	Profit margin	Score	Passes?
Early hypergrowth	60%	-20%	40	Yes
Balanced grower	25%	15%	40	Yes
Mature cash generator	8%	35%	43	Yes
Slowing and unprofitable	15%	-10%	5	No

The point is that a negative margin can be acceptable if growth is fast enough to justify it, and slow growth can be acceptable if the business throws off substantial profit. What the rule flags is the company doing neither.

Which numbers to use

The rule leaves room for interpretation, and the choice of inputs changes the result considerably.

Growth rate. Most people use year-over-year revenue growth. For subscription businesses, growth in annual recurring revenue (ARR) is a common alternative because it reflects the forward trajectory of contracted revenue. Whichever you pick, use the same measure consistently when comparing companies.

Profit margin. This is where the variation lives. Common choices include:

• Free cash flow margin — free cash flow divided by revenue. Popular because cash is harder to dress up than accounting earnings. See what free cash flow is for how it is calculated.
• EBITDA margin — often adjusted EBITDA, which strips out several costs. Covered in what EBITDA is.
• Operating margin — GAAP operating income divided by revenue, the strictest of the common choices. The differences among margin types are laid out in gross vs operating vs net margin.

The same company can pass using adjusted EBITDA and fail using GAAP operating margin. When someone cites a Rule of 40 score, the first question should be which margin they used.

Why the rule exists

Software businesses have unusual economics. Once a product is built, serving an additional customer costs relatively little, so gross margins tend to be high. That leaves management with a genuine choice about how much of each revenue dollar to reinvest.

A company can spend aggressively on sales, marketing, and product development to grow faster, accepting losses in the near term, or it can pull back spending and harvest profit. Both can be rational. The Rule of 40 provides a common yardstick that lets investors compare a fast-growing, cash-burning company with a slow-growing, highly profitable one.

It also reflects how operating economics change over a company’s life. Early on, growth dominates the score. As a company matures and growth naturally decelerates, margins are expected to rise to compensate, partly through operating leverage as fixed costs are spread over a larger revenue base.

What a strong score signals

Companies that sustain scores above 40 tend to share a few traits:

• Efficient growth. They acquire customers without spending a disproportionate amount on sales and marketing.
• Strong retention. Existing customers stick around and often expand their spending, a dynamic captured by net revenue retention. Growth from existing customers is far cheaper than growth from new ones.
• Pricing power. They can raise prices or move customers to higher tiers without heavy churn.

Investors have historically rewarded companies that clear the bar with higher valuation multiples, which is why management teams often cite their score in earnings presentations.

The limits of the rule

The Rule of 40 is useful precisely because it is simple, but that simplicity hides several weaknesses.

It treats growth and profit as interchangeable. One point of growth and one point of margin count the same in the formula, but they rarely have the same value. For a young company with a long runway, an extra point of durable growth may be worth far more than a point of margin today.

The margin choice can be gamed. Using adjusted figures that exclude stock-based compensation can flatter the score substantially. A company paying employees heavily in equity can look efficient while diluting shareholders.

It ignores scale. A small company growing 50% off a tiny base and a large company growing 20% at scale may have similar scores but very different risk profiles.

It is a snapshot. A single year’s score can be distorted by a large one-time contract, a deferred revenue shift, or a temporary spending cut. Looking at the trend over several years is more informative. Changes in deferred revenue in particular can move cash flow margins independently of underlying performance.

It only fits certain businesses. The rule was developed for software and subscription companies. Applying it to hardware makers, retailers, or capital-intensive businesses with structurally different margins produces misleading results.

Using the Rule of 40 well

Treat it as a starting filter rather than a verdict:

1. Decide your inputs and stick to them. Free cash flow margin with stock-based compensation subtracted is a conservative and consistent choice.
2. Compare within peer groups. Companies at similar scale and in similar markets give more meaningful comparisons.
3. Look at the components, not just the sum. A score of 40 built on rising growth tells a different story from one built on margins propped up by spending cuts.
4. Track the trend. A company drifting from 50 to 30 over several years is more concerning than one holding steady at 35.

The takeaway

The Rule of 40 adds a software company’s revenue growth rate to its profit margin and asks whether the total reaches 40%. It is a compact way to weigh growth against profitability and to compare companies at different stages. Its value depends on the inputs: know which margin is being used, prefer measures that charge for stock-based compensation, compare against true peers, and watch how the score and its components change over time.