What Is the Rule of 40? Six Ways to Score Exactly 40

The arithmetic, and the indifference built into it

The calculation is a single addition. Take the year-over-year revenue growth rate as a percentage, take the profit margin as a percentage, add them, and compare the total to 40. Growth of 30 percent with a 10 percent margin gives 40 and passes. Growth of 25 percent with a 5 percent margin gives 30 and fails, even though a company growing a quarter a year while making money is a perfectly good business. There is no weighting, no adjustment for company size, and no term for how the two numbers were reached. That simplicity is why the rule spread — you can compute it from the first two lines of any board deck — and it is also the source of every objection to it.

The indifference is the part worth dwelling on. Because the two inputs are simply added, the rule assigns identical scores to opposite strategies. On $10M of revenue, a company growing 60 percent at a minus 20 percent margin adds $6.0M of new revenue and burns $2.0M of cash; a company growing 10 percent at a 30 percent margin adds $1.0M and generates $3.0M. Both score 40. One is a venture bet that will run out of money without another round, the other is a self-funding business that has stopped compounding, and the rule ranks them equal. Anyone using the score as a filter is implicitly claiming those two are equally attractive — which depends entirely on whether you are buying growth or buying cash flow, a question the rule does not ask.

Which margin you use changes the verdict

There is no canonical margin in the Rule of 40. In practice people use EBITDA margin, free-cash-flow margin, operating margin, or occasionally net margin, and they rarely say which. In the same company these differ enormously. Take a company with $10M of revenue, 25 percent growth, a 20 percent EBITDA margin and a 5 percent free-cash-flow margin — an entirely ordinary spread once you account for capitalised development costs, working-capital swings and stock-based compensation. On the EBITDA definition it scores 45 and passes comfortably. On the free-cash-flow definition it scores 30 and fails clearly. The same year, the same company, a 15-point swing, and a $1.5M gap between the two profit figures.

The practical rule is therefore to state the definition every time you state the score, and never to compare two companies' scores unless you know both were computed the same way. The most conservative choice is free-cash-flow margin, because it is the one that cannot be improved by an accounting decision: capitalising development shifts cost off the profit line but not out of the bank account. EBITDA margin is the most commonly used and the most flattering, which is precisely why it is the most commonly used. If you compute your own score both ways and the two answers straddle 40, that gap is more informative than either number — it tells you exactly how much of your reported profitability is an accounting presentation rather than cash.

What the rule is actually good for

Used as a screening question, the rule earns its keep. It forces the two conversations that matter into a single sentence, and it catches the two failure modes that a growth chart alone hides: growth bought at a burn rate the company cannot sustain, and profitability achieved by stopping investment. A board that reviews the score every quarter and asks which of the two inputs moved is doing something useful. A board that treats 40 as a target to be optimised is inviting the obvious gaming — cut sales and marketing in the last quarter of the year and the margin rises immediately while the growth cost shows up only in the following year's number.

Two limits are worth stating explicitly. The rule is close to meaningless below roughly $1M of revenue, because percentage growth off a tiny base is trivially large — doubling from $200K to $400K is 100 percent growth and says almost nothing about the business. And the rule was built for subscription software, where revenue is recurring and gross margins sit above 70 percent; applied to a business with 30 percent gross margins, the margin term is structurally capped and 40 becomes unreachable regardless of quality. If you want one number to sit next to it, use net revenue retention: it measures whether the revenue you already have grows on its own, which is the thing the Rule of 40 cannot see at all.

Frequently asked questions

Which margin should I use in the Rule of 40?

There is no official answer, which is the problem. EBITDA margin is the most common and the most flattering; free-cash-flow margin is the most conservative and the hardest to manipulate with accounting choices. Pick one, state it whenever you quote the score, and compute it the same way every quarter. If you compute both and one passes while the other fails, the gap between them is the number worth investigating.

Does the Rule of 40 apply to businesses that are not subscription software?

Poorly. The threshold of 40 was calibrated on subscription software, where revenue recurs by default and gross margins routinely exceed 70 percent. A retailer or an agency with a 30 percent gross margin cannot reach a 30 percent profit margin at all, so the margin term is structurally capped and only extreme growth can clear the bar. The structure of the rule — trade growth against profit, and hold the sum to a benchmark — still transfers; the number 40 does not. Set your own threshold from the margins your industry actually achieves.

Is a score well above 40 always better?

Not automatically. A score of 70 built from 65 percent growth and a 5 percent margin is a very different asset from a 70 built from 20 percent growth and a 50 percent margin, and the rule cannot distinguish them any more than it can distinguish the two ways of reaching 40. A very high score driven entirely by margin often means the company is under-investing and will hand the market to a competitor that spends. Read the two components, never the sum alone.