Rule of 40

The Rule of 40 is a single-number test for whether a SaaS company is trading growth against profitability at a defensible rate. A company growing 60% with a −20% margin scores 40. So does a company growing 10% at a 30% margin. Both are considered acceptable; a company growing 20% at a −30% margin scores −10 and is not.

The appeal is that it refuses to let either half of the equation be ignored. Hypergrowth funded by unlimited losses fails the test, and so does a profitable business that has stopped growing.

Pick a margin definition and keep it

The formula does not specify which margin, and the choice changes the score by 20 points or more:

• EBITDA margin — the most common in public-company reporting.
• Free cash flow margin — the most conservative and arguably the most honest, since it captures working capital and capitalised costs.
• Operating margin — sits between the two.

Companies under pressure have a habit of quietly switching to whichever definition scores best. State the definition next to the number.

Where it applies and where it does not

The Rule of 40 was formulated for SaaS businesses at scale — roughly $50M ARR and above. Applied to an early-stage company it produces nonsense: a company growing 300% from a small base scores enormously well while burning through its runway, and a seed-stage company with a −200% margin fails a test it was never meant to take.

It also says nothing about quality of growth. Read it alongside NRR, which distinguishes growth from the existing base from growth bought with sales spend.