The Rule of 40 is the rare finance heuristic that fits in one line: a healthy software company's revenue growth rate and profit margin should add up to at least 40 percent. Grow 100 percent a year and you may burn heavily. Grow 10 percent and you had better be solidly profitable. The line between those points is the rule.
One line is also enough room to fool yourself. Which growth rate, which margin, and at what stage the sum means anything are where the rule earns or loses its usefulness, so this is the whole of it: origin, calculation, the tradeoff it prices, and the ways companies get it wrong.
Where the rule came from
Brad Feld published the rule in 2015 on his blog, crediting it to conversations with late-stage investors who were using it as a screen for SaaS businesses. It spread because it compresses the central tension of software economics — growth costs money, profit proves the model — into a single number a board can glance at.
Feld's own framing carried a boundary that gets dropped in retellings: he described it as applying from roughly $1M of ARR upward. Below that scale, growth rates swing too wildly for the sum to mean much. The glossary entry holds the citable one-line definition; this post is the longer argument.
How to calculate it
growth rate = (revenue this year − revenue last year) / revenue last year margin = profit / revenue this year score = growth rate % + margin % → pass at 40 or more
A worked example. A company did €2.0M of revenue last year and €2.9M this year, and will end the year with an operating loss of €290,000:
- Growth rate: (2.9M − 2.0M) / 2.0M = 45%
- Operating margin: −290,000 / 2.9M = −10%
- Score: 45 − 10 = 35. Below the line.
That company is not failing — it is growing well. The score says its growth is costing slightly more than the rule considers healthy, and the interesting conversation is whether next year's plan closes the gap from the growth side or the margin side. That conversation is what the rule is for. The Rule of 40 calculator runs this from your own revenue and profit figures, deriving the growth rate from two years of revenue rather than asking you to remember it, because remembered growth rates are reliably generous.
The tradeoff it prices
Growth and profitability are not independent virtues; they are usually bought with the same money. Every euro into sales, marketing and product is a euro that could have been margin, so judging a company on either axis alone rewards the wrong behavior: pure growth-judging rewards buying revenue at any price, pure profit-judging rewards harvesting a business instead of building it.
Adding the two puts a fast-burning grower and a slow profitable compounder on one scale, and 40 is where convention drew the healthy line. A company at 80/−40 and a company at 10/30 both score 40, and the rule deliberately refuses to prefer one — what it flags is the company at 20/0, growing modestly while earning nothing, which each single-axis view might excuse.
That refusal is also the rule's limit. The two components are not truly interchangeable — a margin collapse and a growth collapse are different problems with different fixes — so the score is a screen, not a diagnosis. When it drops, the next question is always which half moved.
When it matters, and when it does not
The rule starts to bite when growth naturally slows and the question becomes whether efficiency arrived to replace it. That makes it a later-stage yardstick: useful from about $1M of ARR, genuinely important at growth stage, and standard vocabulary in any fundraise or acquisition conversation from Series B onward.
Before that, it mostly generates noise. A pre-revenue company cannot compute it. A seed company tripling from €50,000 scores in the hundreds and learns nothing. At those stages the numbers that decide things are burn multiple — what a euro of growth costs in burn — and runway, which is the clock all of it runs against. The SaaS metrics guide places the rule among the other eleven numbers worth tracking, and it is deliberately last on that list.
The common mistakes
Mixing margin definitions. Operating margin, EBITDA margin and free cash flow margin all circulate, and companies drift toward whichever flatters the current quarter. GAAP operating margin and an adjusted EBITDA with generous add-backs can differ by ten points or more — enough to turn a fail into a pass. Pick one measure, state it, and keep it.
Mismatched numerators. ARR growth against a revenue-based margin mixes two different revenue definitions in one sum. If ARR grew 50 percent but recognized revenue grew 35, the honest score uses 35 with a revenue margin, or a consistently ARR-based pair — not the best half of each.
Counting one-time revenue as growth. Services projects, implementation fees and a lumpy enterprise deal inflate this year's growth and then subtract from next year's. The rule is meant to describe the recurring engine; feeding it non-recurring revenue describes a coincidence.
Annualizing a spike. One strong quarter times four is not a growth rate. Trailing-twelve-month figures move slower and lie less.
Benchmarking against the wrong peer set. The 40 line came from venture-scale software. A bootstrapped company that grows 20 percent at a 25 percent margin passes honestly — but a services company or a hardware business scoring itself against a SaaS heuristic learns little either way, because its margin structure was never the one the rule assumed.
What to do with a number below 40
Nothing dramatic, usually. The score is one reading, not a verdict: a deliberate investment year can sit below the line on purpose, funded and planned. The score becomes a problem when it is below 40 and drifting, with no mechanism in the plan that turns it — no margin leverage arriving, no growth reacceleration anyone can name.
The practical use is trend plus decomposition. Compute it the same way each quarter, watch which component moves, and treat a falling score as a prompt to look at the burn multiple and the SaaS magic number underneath it — the rule says whether the engine is healthy, those two say which part is expensive. Keeping the inputs current is the unglamorous prerequisite for all of it, and it is the part a live model does for you: revenue and costs recorded as they happen, so growth and margin are simply there when someone asks.