Rule of 40

How do you calculate it without fooling yourself?#

1. Choose the growth measure#

Use year-over-year growth for the same period. Never combine quarterly sequential growth with an annual margin.

2. Choose the profitability measure#

Free-cash-flow margin is closest to liquidity but swings with working capital and capex. Operating margin is more standardised but includes non-cash items. EBITDA and adjusted EBITDA support operating comparison, but the exclusions can be very large — Blackbaud's FY2025 adjusted EBITDA adds back $92.9 million of stock-based compensation on $1,128 million of revenue, which is 8.2 points of margin on its own.

SEC staff guidance is the right discipline here even for private companies: non-GAAP adjustments can be misleading, labels must be clear, reconciliation is required, and free cash flow has no uniform definition.

3. Show both components, always#

growth rate + margin = Rule of 40 score

A 45 built from 60% growth and a −15% margin carries entirely different financing risk from a 45 built from 20% growth and a 25% margin. The first needs a funded balance sheet; the second needs a growth thesis.

4. Anchor on a trailing period#

Use a trailing-twelve-month actual score for accountability. Show the forward plan separately, with named assumptions for hiring, pricing, churn, collections, and capex.

5. Corroborate with the causal metrics#

The score is an outcome. The diagnosis lives in Gross Margin, NRR and GRR, CAC Payback Period, Burn Multiple, customer concentration, and runway.

6. Read it as a decision matrix, not a pass/fail stamp#

Worked example#

A SaaS startup reports, for the year:

Step 1 — growth.

ARR growth = ($6,500,000 - $5,000,000) / $5,000,000
           = $1,500,000 / $5,000,000
           = 30.0%

Step 2 — free cash flow.

FCF = cash from operations - capex
    = -$500,000 - $150,000
    = -$650,000

Step 3 — margin, and the denominator choice. Divided by GAAP revenue:

FCF margin = -$650,000 / $5,800,000 = -11.2%
Score      = 30.0% + (-11.2%) = 18.8

Divided by average ARR, as the SaaS Metrics Standards Board's ARR calculation specifies:

Average ARR = ($5,000,000 + $6,500,000) / 2 = $5,750,000
FCF margin  = -$650,000 / $5,750,000 = -11.3%
Score       = 30.0% + (-11.3%) = 18.7

Here the denominator choice is worth only a tenth of a point, because revenue and average ARR happen to be close. That will not be true for a company mid-ramp, where ARR runs well ahead of recognised revenue and the ARR-denominated margin looks materially better. Check the gap before assuming it is immaterial.

Step 4 — the margin definition, which is not immaterial. Adjusted EBITDA of $290,000 is exactly 5.0% of $5,800,000 of revenue:

Adjusted-EBITDA version = 30.0% + 5.0% = 35.0

The company can report 18.8 or 35.0 for the same year. The 16.2-point difference is the $940,000 gap between adjusted EBITDA and free cash flow — non-cash charges, working capital, and capex. A useful board slide shows both and reconciles the gap; a bad one picks the flattering number.

Step 5 — the decision the score does not make. Management considers adding $600,000 of annual sales expense, forecast to lift ARR growth to 40% while pushing FCF margin to −18%:

Plan score = 40.0% + (-18.0%) = 22.0

The score improves by 3.2 points against the 18.8 baseline. That is not the decision. At −18% of $5.8 million the plan burns roughly $1,044,000 of free cash flow in the year, against $650,000 today — an extra $394,000 of cash for 10 points of growth. Whether that is a good trade depends on CAC payback, on retention, and on how many months of runway the extra burn removes. The Rule of 40 makes the exchange visible; it does not price it.

What are the common mistakes?#

• Mixing definitions across periods or peers. ARR growth plus FCF margin is not comparable with organic revenue growth plus adjusted EBITDA margin. As the table above shows, the gap can exceed 25 points.
• Treating 40 as a law. It came from one late-stage investor's rule of thumb for scaled software companies. Stage, gross margin, capital intensity, and the cost of capital all change the appropriate trade-off.
• Publishing the sum without the components. The same total describes opposite financing profiles.
• Letting adjusted EBITDA do the work. If the adjustments are large — recurring stock compensation, restructuring, acquisition costs — the score is measuring the adjustment policy.
• Using it as evidence of quality. A strong score can coexist with weak retention, dangerous concentration, or a low gross margin. It is a lens on one trade-off, not a verdict.

When does the Rule of 40 break?#

Before scale. For a pre-revenue or sub-$5M company the growth percentage is arithmetic noise off a tiny base, and the margin is dominated by fixed cost. The rule was framed for companies an order of magnitude larger.

Outside recurring-revenue software. Usage businesses with volatile consumption, hardware-heavy firms, marketplaces reporting gross transaction value, and services- or acquisition-driven businesses put a numerator and a denominator on the page that do not describe the same economic engine.

When a point of growth and a point of margin are not worth the same. The formula weights them equally by construction. Growth bought through long payback periods, discounts, channel incentives, or low-gross-margin revenue is worth less than the arithmetic implies.

When working capital does the talking. Annual prepayments can lift free-cash-flow margin in one period and reverse it in the next. Report a rolling history and the reconciliation, not a point estimate.

When the question is actually about something else. The metric is silent on market size, concentration, competitive advantage, governance, and balance-sheet risk. It does not translate into a valuation multiple, however often it is used that way.