The Rule of 40: how investors use it to judge a scaleup's health
A simple formula that blends growth and profitability has become a shorthand test for whether a software scaleup is built to last, but it only works if you understand what it hides.
What the Rule of 40 actually is
The Rule of 40 is a quick sanity check used mostly for software and SaaS businesses. You add a company’s revenue growth rate to its profit margin (usually EBITDA margin or free cash flow margin), and if the total comes to 40 or more, the business is considered to be in reasonably healthy shape. Score well below that and investors start asking harder questions about whether the business is growing too slowly, burning too much cash, or both.
The logic behind it is that a young company doesn’t have to be profitable and doesn’t have to be growing extremely fast, but it should be doing well on at least one of those two fronts, and ideally a bit of both. A company growing revenue by 60% a year while losing money at a rate that drags its margin to minus 25% would score 35, just under the line. A company growing more modestly at 20% but running at a 25% profit margin would score 45, comfortably over it. Both are plausible, healthy-looking scaleups, which is exactly the point: the rule doesn’t insist on a single path to a good number.
Why investors like a single number
Founders and boards can get lost in metrics: annual recurring revenue, net revenue retention, gross margin, customer acquisition cost payback, burn multiple, and so on. The Rule of 40 is popular precisely because it compresses two of the most important dimensions, growth and efficiency, into one figure that’s easy to compare across a portfolio or a sector. A venture or growth investor screening dozens of SaaS businesses can use it as a first filter before digging into the detail behind the number.
It’s also useful because it discourages growth at any cost. A company that is spending itself into the ground to post an impressive top-line growth number will show up badly on the Rule of 40 once its poor margin is factored in. Equally, a company that has throttled growth to protect margin will also fail the test if it isn’t growing enough to compensate. In principle, it rewards discipline rather than a single flattering metric viewed in isolation.
How it tends to be calculated in practice
There’s no single official formula, which is one of the metric’s weaknesses. Common variations include:
- Revenue growth rate (year on year, often annual recurring revenue growth for SaaS) plus EBITDA margin
- Revenue growth plus free cash flow margin, which some investors prefer because it strips out accounting adjustments and reflects actual cash generation
- Revenue growth plus operating margin
Because the inputs can differ, the same company can post noticeably different Rule of 40 scores depending on which profitability measure is used. Founders raising a round should be ready to say clearly which version they’re quoting, and should expect an investor to recalculate it their own way using the underlying numbers rather than take the headline figure at face value.
Where the Rule of 40 falls short
The metric was built with mature, capital-efficient SaaS businesses in mind, and it stretches awkwardly outside that context. A very early-stage company with tiny revenue can post triple-digit growth rates almost by accident, since doubling a small number is easy, which can make the Rule of 40 look brilliant while telling you almost nothing useful about the underlying business. Pre-revenue or very early-revenue startups are generally too immature for the metric to mean much at all.
It also says nothing about the quality of growth. A company growing through heavily discounted contracts, aggressive one-off sales pushes, or unsustainable customer acquisition spending can hit a good score for a quarter or two without that growth being durable. Net revenue retention, gross margin and cash runway all matter alongside it, not instead of it.
Sector context matters too. Capital-intensive businesses, hardware-heavy companies, or those in regulated markets with long sales cycles may never comfortably clear 40 even when they’re performing well for their stage and sector, so applying the rule mechanically across very different business models is a mistake investors are increasingly wary of making.
How UK founders should use it
Treat the Rule of 40 as a conversation starter, not a scorecard. If you’re preparing for a growth-stage raise, it’s worth calculating your own number under a couple of different definitions before an investor does it for you, and being ready to explain the trade-off you’re making between growth and margin, and why it’s the right one for your stage and market. A founder who understands why their number looks the way it does, and can point to a credible path to improving it, will get more credit than one who quotes a favourable figure without being able to defend the methodology behind it.
It’s also worth remembering that the Rule of 40 is one lens among several that growth investors use. It sits alongside metrics like net revenue retention, the burn multiple, and CAC payback period, and boards should expect it to be discussed as part of a wider set of numbers rather than as a single pass or fail test.
Where to check current practice
Because there’s no statutory definition and methodologies vary between investors, founders should look at how leading growth-equity firms and industry bodies frame the metric before quoting it in a pitch, and cross-check any specific benchmark figures against current, dated sources rather than assuming an old rule of thumb still applies.