What the Rule of 40 actually measures
The Rule of 40 is a single-number test of whether a software company is buying its growth at a sensible price. You add the year-over-year revenue growth rate to the profit margin, both expressed in percent, and compare the total to 40. A company growing 45% while losing 5% of revenue scores 40. So does a company growing 10% at a 30% margin. Both pass; they simply sit at different points on the same trade-off.
The reason a crude sum works at all is that growth and profit are substitutes in a subscription business. Every dollar you spend on sales, marketing and engineering today is a dollar that does not reach the margin line, but it buys recurring revenue that persists for years. Spending less lifts the margin and slows the growth; spending more does the reverse. The Rule of 40 says the exchange rate between those two should be roughly one-for-one, and that the combined total should not fall below forty points.
That makes it a diagnostic, not a valuation. It does not tell you what a company is worth, and it says nothing about retention, gross margin or unit economics — a company can score 60 with terrible retention if it is simply outrunning its churn for a year. Read it alongside net revenue retention and LTV to CAC, never on its own.
The formula, and the three decisions hidden inside it
The arithmetic is trivial. The judgement is in what you feed it, and there are exactly three choices to make.
First, which revenue. The growth term is year-over-year revenue growth: current-period revenue divided by revenue for the same length of period ending twelve months earlier, minus one. Use GAAP revenue if you have audited statements. If you are pre-audit and running off ARR, use ARR consistently on both sides — mixing recognised revenue in one term with ARR in the other inflates the growth rate whenever your bookings are front-loaded.
Second, which profit. EBITDA is the most common choice because it strips out capital structure and non-cash amortisation of acquired intangibles, which makes companies with different debt loads and acquisition histories comparable. Free cash flow is the strictest and hardest to flatter, because it charges you for capitalised software, prepaid commissions and working capital swings. Operating income sits in between. The formula is indifferent; your investors are not.
Third, whose denominator. The margin goes on current-period revenue, the same figure that sits in the numerator of the growth term. Dividing profit by prior-year revenue would flatter a fast-growing company by the full growth rate.
Because both terms are percentages of the same base, they are additive, and rearranging is easy: the margin you need to reach a target score is the target minus your growth rate. That single subtraction is the most useful thing the rule gives an operator, because it converts a vague demand for “efficiency” into a specific dollar figure on your own revenue base.
Worked example: $36M to $48M with $3.6M of EBITDA
Take a company that finished last year at $36.0M of revenue and this year at $48.0M, with $3.6M of EBITDA on the current year.
- Growth term. $48.0M ÷ $36.0M = 1.33333. Subtract 1 to get 0.33333, then × 100 = 33.33%.
- Margin term. $3.6M ÷ $48.0M = 0.075, × 100 = 7.50%.
- Add them. 33.33 + 7.50 = 40.83 points.
- Compare to the target. 40.83 − 40 = +0.83 points. The company clears the bar, but barely.
Now run the two rearrangements, because they are what you actually act on.
- Margin needed at today's growth. 40 − 33.33 = 6.67%. In dollars on $48.0M of revenue: 0.0667 × $48.0M = $3.20M of EBITDA. The company has $3.6M, so it is $400,000 clear.
- Growth needed at today's margin. 40 − 7.50 = 32.50%. Applied to last year's $36.0M, that means current-year revenue of 1.325 × $36.0M = $47.70M. Actual revenue is $48.0M, so the company beat the growth requirement by $300,000 of revenue.
Notice how small the cushion is. A single $500,000 unbudgeted expense drops the margin to 6.46% and the score to 39.79 — a fail. That sensitivity is why the score is worth recomputing every quarter rather than once a year, and why the choice of profitability basis matters so much: $500,000 of capitalised engineering salary changes free cash flow but not EBITDA.
How to read your score
Read the score, then immediately read the split that produced it, because two companies at 40 can be in completely different situations.
A score at or above the target with growth carrying most of it is the normal shape for a company early in its life. It is also the fragile shape: growth decays, and a company at 55% growth and −15% margin has to convert fifteen points of margin improvement out of thin air as growth slows, or the score collapses. The score tells you nothing about how hard that conversion will be. Look at what the spend is buying.
A score at or above the target with margin carrying most of it is the mature shape. The risk there is different: if growth keeps decaying and margin is already high, there is less room left to offset. A company at 8% growth and 32% margin passes today, but each further point of growth decay must be met with another point of margin, and margin expansion is bounded by gross margin.
A score below the target tells you how many points you owe, and the calculator splits the bill two ways — the margin you would need at today's growth, and the growth you would need at today's margin. Real plans mix the two. If you owe eight points, four points of margin plus four points of growth is usually far more achievable than either alone.
A negative score means revenue is shrinking, losses exceed the shrinkage, or both. Nothing about the rule breaks; the number is just a long way from 40, and the diagnosis has to come from elsewhere — start with churn and runway.
One structural caveat: the rule is scale-blind. A company going from $1M to $1.4M scores the same 40% growth as one going from $100M to $140M, and the first is a far less impressive achievement. Below roughly the point at which a company has a repeatable sales motion, the growth term is dominated by noise and the rule stops discriminating.
Growth and margin pairs that score exactly 40
| Revenue growth | Margin required | Profit on $48.0M of revenue |
|---|---|---|
| 0% | 40.0% | $19.20M |
| 10% | 30.0% | $14.40M |
| 20% | 20.0% | $9.60M |
| 30% | 10.0% | $4.80M |
| 33.33% | 6.67% | $3.20M |
| 40% | 0.0% | $0 |
| 50% | −10.0% | −$4.80M |
| 60% | −20.0% | −$9.60M |
| 80% | −40.0% | −$19.20M |
| 100% | −60.0% | −$28.80M |
The margin column is 40 minus the growth rate; the dollar column is that percentage of $48.0M. Substitute your own revenue to price the frontier for your business.
Adjusted EBITDA is where scores are manufactured
The growth term is hard to fake, because revenue is audited. The margin term is not. Every add-back — restructuring, an acquisition earn-out, a litigation settlement, capitalised software, and above all stock-based compensation — moves the score directly, point for point on revenue. A company that adds back $2.0M on $160M of revenue moves its score by 2.0 ÷ 160 = 1.25 points. The same arithmetic runs the other way: enter a one-time gain as a negative adjustment, or a settlement received flatters the margin exactly as much as a settlement paid depresses it.
If you are a public filer, SEC Regulation G governs this: any non-GAAP measure you present must be reconciled to the most directly comparable GAAP figure, and the GAAP figure must be given equal or greater prominence. Private companies face no such rule, which is exactly why a diligence team will rebuild your EBITDA from the general ledger rather than accept your bridge. Score yourself on the definition you would be comfortable defending line by line.
Mistakes that produce a flattering score
- Annualising a strong quarter. Multiplying one good quarter by four and comparing it to last year's full revenue mixes a peak with an average. Compare like periods: this quarter against the same quarter a year ago, or trailing twelve months against the prior twelve.
- Putting the margin over prior-year revenue. Both terms must share the current-period revenue base, or a fast-growing company gets credit twice.
- Mixing ARR growth with GAAP margin. ARR leads recognised revenue whenever contracts are signed mid-period, so the growth term runs ahead of the base the margin is measured on.
- Counting services and one-time revenue as growth. A large implementation project can carry the growth term for a year and then vanish. Split recurring from non-recurring before you start.
- Switching basis between periods. Reporting an EBITDA-based score this year and a free-cash-flow-based score last year makes the trend meaningless.
- Reading the score without the split. The headline number hides whether growth or margin is carrying it, and that is the whole diagnosis.
- Applying it to a company with no repeatable sales motion. At very small revenue the growth term is dominated by a handful of deals.
Where the rule came from, and what to use instead
The rule entered common use through a February 2015 post by the venture investor Brad Feld, The Rule of 40% For a Healthy SaaS Company, which described it as a heuristic he had heard from a board discussion rather than as a finding from data. That provenance matters. Forty is a convention, not a measured optimum, and there is no standards body behind it — which is why this calculator lets you move the target.
Three tools do jobs the Rule of 40 cannot. Burn multiple — net cash burned divided by net new ARR added — measures the efficiency of the spending directly rather than inferring it from a margin, and it does not let a strong prior-year comparison mask a bad current quarter. CAC payback answers how long a customer takes to repay its acquisition cost; work it out with the CAC payback period calculator. And net revenue retention tells you whether growth is coming from the existing base or from ever-larger new-logo spend.
The Rule of 40's real value is as a constraint on planning. Once you know that the margin you need equals your target minus your planned growth rate, next year's operating plan has a hard ceiling on spend that follows arithmetically from the growth you have committed to. Build the growth side of that plan with the subscription revenue forecast calculator, check the margin side against your EBITDA margin, and confirm the cash side with the burn rate and runway calculator.
Key terms
- Rule of 40
- The convention that year-over-year revenue growth percent plus profit margin percent should total at least 40 for a healthy software business.
- EBITDA
- Earnings before interest, taxes, depreciation and amortisation. A proxy for operating cash generation that ignores capital structure and non-cash charges.
- Adjusted EBITDA
- EBITDA with further add-backs for items management considers non-recurring. Not defined by GAAP, so its contents vary by company.
- Free cash flow margin
- Cash from operations less capital expenditure, divided by revenue. The strictest of the three profitability bases because it charges you for capitalised costs.
- Burn multiple
- Net cash burned in a period divided by net new ARR added in the same period. A direct measure of how much cash each dollar of new recurring revenue costs.
- Regulation G
- The SEC rule requiring public companies that present a non-GAAP measure to reconcile it to the nearest GAAP measure and give the GAAP figure equal prominence.