The Rule of 40's blind spots: when it misleads investors on UK scaleups
The Rule of 40 is a useful shorthand for balancing growth and profit, but it can flatter or unfairly punish a scaleup depending on its stage, sector and how the number is built.
A quick recap of the maths
The Rule of 40 says a healthy software business should have its revenue growth rate plus its profit margin add up to 40% or more. A company growing at 50% a year with a -10% margin passes. So does one growing at 15% with a 25% margin. It is popular because it forces a single number to hold two things investors care about at once: how fast a business is expanding and whether it is burning cash responsibly to do it.
That simplicity is exactly why it gets misapplied. Two businesses can score the same 40 and be in completely different states of health.
The stage problem
A pre-seed or seed company often has no meaningful revenue base, so its growth rate can look enormous purely because it is growing off a tiny number. Doubling revenue from £50,000 to £100,000 is a 100% growth rate, but it says almost nothing useful about durability or product-market fit. Applying the Rule of 40 to a company this early can produce a flattering score that has no relationship to whether the business will still exist in three years.
At the other end, a scaleup that has reached a large revenue base naturally slows down. Growth compounding on £2m of annual recurring revenue looks very different from the same percentage growth on £20m. Investors who apply the Rule of 40 as a flat pass/fail bar without adjusting for scale risk penalising mature, capital-efficient businesses that are, in reality, performing well for their size.
Which margin, and whose numbers
The rule does not specify which profit margin to use, and this is where founders and investors can talk past each other. Gross margin, EBITDA margin, operating margin and free cash flow margin can all produce very different scores for the same company. A business with high gross margin but heavy sales and marketing spend might look strong on one measure and weak on another.
Free cash flow margin is generally seen as the most honest version because it reflects actual cash generated or consumed, not accounting profit that can be shaped by how costs are capitalised or deferred. When a scaleup or its investor quotes a Rule of 40 score, the useful follow-up question is always which margin definition sits underneath it, and whether that definition is applied consistently period to period.
Sector and capital intensity distort it further
The Rule of 40 was built around SaaS economics: high gross margins, low marginal cost of serving an extra customer, and revenue that recurs. It travels less well outside that world. A hardware, marketplace, fintech-with-balance-sheet, or deep tech company often has structurally lower gross margins or heavier upfront capital needs, so a lower combined score does not automatically mean weaker underlying health. Comparing a robotics scaleup’s Rule of 40 score directly against a pure-play SaaS company’s score is comparing different economic models, not just different levels of execution.
Companies with lumpy, contract-based revenue, such as those selling large annual enterprise deals or government contracts, can also show wild swings in growth rate quarter to quarter that have nothing to do with underlying momentum, making a single snapshot Rule of 40 score noisy and easy to misread.
How it gets gamed
Because the rule is additive, a business under pressure to hit 40 can chase growth at the expense of margin, or squeeze margin at the expense of growth, in whichever direction is easier to move short term. A founder near a fundraise might pull forward revenue recognition, delay marketing spend, or cut headcount temporarily to hit the number for a board deck or pitch, without those choices reflecting a genuinely healthier business afterwards. Because the formula rewards the sum rather than the balance, a company can also swing from very high growth and deeply negative margin one year to modest growth and thin profit the next, hitting 40 both times while its actual trajectory is unstable.
What a more careful read looks like
Rather than treating the Rule of 40 as a pass or fail test, more experienced investors use it as a starting prompt: what is driving each half of the number, is the margin figure cash-based or accounting-based, how does the score trend over several periods rather than one, and how does it compare to genuinely similar businesses in the same sector and at the same revenue scale. Net revenue retention, gross margin trend, and cash runway alongside the Rule of 40 usually tell a fuller story than the single combined figure on its own.
For founders, the practical takeaway is to know your own number, know which margin definition you are using, and be ready to explain the story behind it rather than let a single score stand in for the underlying business.
Where to check current benchmarks
Benchmark data and definitions evolve, so founders and investors should check current guidance and market data through their own advisers and industry bodies rather than relying on a fixed figure.