The Rule of 40 says that a SaaS company’s growth rate plus its profit margin should add up to at least 40. Grow fast and you are allowed to lose money. Grow slowly and you had better be profitable. It is the most widely quoted heuristic in software, and the most widely misapplied.
The calculation
Rule of 40 score = year-over-year revenue growth rate (%) + profit margin (%).
A company growing 55% with a −20% margin scores 35. A company growing 12% with a 30% margin scores 42. The second clears the bar and the first does not, which is exactly the point: the rule treats growth and profitability as interchangeable currencies.
Which margin, though?
This is where most disagreements start, because the choice can swing the score by twenty points.
- EBITDA margin — the most common choice, and the most flattering, because it excludes depreciation and amortisation. For a company capitalising significant software development cost, this hides real spending.
- Free cash flow margin — the most honest, because it reflects cash actually generated. It is also the most volatile, since annual billings and deferred revenue swing it.
- Operating margin — a reasonable middle ground, and the easiest to tie back to the income statement.
- Net income margin — rarely used, because tax and interest tell you little about operating quality.
Pick one, state it, and use it consistently. A company that quotes EBITDA margin in a good year and free cash flow margin in a better one is telling you something about itself.
Growth rate: ARR or revenue?
The second common fudge. ARR growth and GAAP revenue growth diverge, sometimes sharply, because ARR is a point-in-time snapshot and revenue is recognised over the period.
A company that signed most of its new ARR in December will show strong ARR growth and modest revenue growth for the same year. Using ARR growth with a GAAP margin mixes two bases and produces a number that means nothing. If you use ARR growth, pair it with a cash-based margin; if you use revenue growth, pair it with an accounting margin.
What the rule gets right
Its real value is as a constraint on the conversation, not as a target. It forces a board to state the trade explicitly: we are choosing to spend this much to grow this fast, and here is the score that results.
It also catches the two failure modes that kill software companies. Growing fast while burning disproportionately — the score collapses. Protecting margin while growth stalls — the score collapses from the other direction. Either way the number moves before the narrative does.
Three situations where it misleads
Below roughly $10M ARR
Early-stage companies routinely score 100 or more simply because growth from a small base is percentage-large. A company going from $1M to $3M ARR grew 200%; it could lose money at an impressive rate and still clear 40 easily. The rule tells you nothing until growth rates have come down into double digits, which usually means somewhere past $10–20M ARR.
During a deliberate investment year
A company that decides to build a second product, enter a new geography or move up-market will depress margin for 18 months before the growth arrives. The score dips, and reading that dip as decline is a mistake. The useful question is whether the investment has a thesis and a measurable milestone, not whether the score held.
When the business is not really recurring
The rule assumes revenue persists. A company with heavy services revenue, one-off licence sales or high churn does not have the compounding base the rule implicitly credits. A 45 score built on revenue that has to be resold every year is worth far less than a 38 built on 120% net revenue retention.
How the score moves, in practice
A company at 32 wanting to reach 40 has two routes, and they have very different time constants.
Cutting cost moves the score within a quarter. Reducing sales and marketing by five points of revenue adds roughly five points to margin immediately — and subtracts from growth four to six quarters later, when the pipeline that spend would have built fails to arrive. Boards that cut their way to 40 often find themselves at 34 eighteen months later with a smaller company.
Improving efficiency moves it durably but slowly. Lifting win rate, shortening the CAC payback period, or building expansion revenue all raise growth without raising spend. These are the levers that hold.
The diagnostic question is simple: is the score low because we are spending too much, or because the spending is not working? The magic number answers that, and the answer determines which route is available.
Using it well internally
Three habits make the rule useful rather than ceremonial.
- Plot it quarterly over three years. The trajectory tells you about the business; a single score tells you about one quarter’s accounting.
- Show the two components separately. A 40 made of 60% growth and −20% margin is a different company from a 40 made of 10% growth and 30% margin. They have different risks, different buyers and different appropriate strategies.
- Set it as a constraint, not a goal. “We will not go below 30 while we build the new product” is a useful commitment. “Hit 40 this year” invites the cost cut that hollows out next year.
Four companies, all scoring 40
The clearest way to see why composition matters is to put four profiles side by side.
- 80% growth, −40% margin. A venture-stage land grab. Defensible if the market is winner-take-most and the capital is committed; catastrophic if funding tightens, because the business cannot reach breakeven quickly.
- 45% growth, −5% margin. Arguably the healthiest of the four. Growing fast, nearly self-funding, with the option to go either way next year.
- 20% growth, 20% margin. A solid, boring, durable software business. Unexciting to growth investors, attractive to private equity.
- 5% growth, 35% margin. Mature or stalling. The margin is real, but 5% growth in software usually means the product is being displaced somewhere you cannot yet see.
Same score, four completely different conversations about what to do next. Anyone who reports the Rule of 40 without the split is hiding the only interesting part.
Common ways the score gets inflated
Because the rule drives valuation conversations, it attracts creative accounting. Four patterns to watch for in your own reporting as much as anyone else’s.
Capitalising development cost. Moving engineering salaries from the income statement to the balance sheet lifts EBITDA margin without changing the cash spent. Legitimate under accounting standards, and worth disclosing.
Quoting ARR growth with EBITDA margin. Mixing a point-in-time growth figure with a period accounting margin, as discussed above. The mismatch usually favours the company.
Excluding stock compensation. Common, and defensible for cash analysis, but it can add ten points of margin in a company that pays heavily in equity.
Using the best quarter annualised. A single strong quarter extrapolated to a year is not a growth rate.
None of these are dishonest in themselves. All of them should be labelled, because an unlabelled adjustment is what turns a useful heuristic into a sales pitch.
Frequently asked questions
Where did the Rule of 40 come from?
It emerged from growth-equity and venture investors in the early 2010s as a shorthand for comparing software companies with very different growth-profitability mixes. It is a convention, not a law, and no formal derivation supports the specific figure of 40.
Does the Rule of 40 apply to private companies?
It applies as a planning constraint, but private companies at high growth rates routinely and deliberately sit below it. The rule becomes a hard expectation mainly when a company approaches an exit or an IPO.
Is a score above 40 always good?
Not necessarily. A score of 70 built on very high margin and low growth can signal underinvestment in a market that is still open. The composition matters as much as the total.
Should the Rule of 40 be in a board pack?
Yes, with both components shown and the margin definition labelled. It is a useful single line to anchor the growth-versus-burn discussion, as long as it does not become the only line.
