Rule of 40: Balancing Growth and Profitability

What it is

The Rule of 40 is a shorthand diagnostic for businesses that must grow to win yet must also earn real profits. It states that a company's annual revenue growth rate plus its profit margin should equal at least 40%. A software firm growing 30% with a 10% margin passes; one growing 50% while losing 20% fails. The rule is a truce between two camps that too often fight: the growth-at-any-cost believers and the value purists who distrust growth entirely.

The elegance is in the trade-off. Above 40%, extra growth can justify thinner margins; below 40%, a business is either not growing enough or not profitable enough to compound owner value. It is a screen, not a verdict — but a useful first filter for capital allocation discipline.

Why Buffett cares

Buffett rarely invokes the Rule of 40 by name, but the principle is thoroughly his. He has warned that growth is only valuable when it occurs behind a durable economic moat and at returns above the cost of capital. Growth in a mediocre business destroys value, because each reinvested dollar earns less than a dollar back. The Rule of 40 is simply the modern, quantified expression of that old insight: growth and margin are two sides of the same return.

His 1985 letter made the iron law explicit — size and growth eventually dampen exceptional economics. The Rule of 40 respects that by insisting growth be paired with profitability before it is celebrated.

How to spot it

Apply the test to any company whose story is "growth first, profits later." Compute trailing revenue growth and a margin (operating, free-cash-flow, or owner) and add them. Consistently above 40% signals a business with genuine operating leverage: each new dollar of revenue costs less to serve than the last. Sitting near or below 40% for years signals a company buying growth with investor capital.

Beware the manipulation. Margin can be inflated by cutting R&D, and growth can be rented through acquisitions. The honest version uses sustainable margins and organic growth, measured over a full cycle rather than a single quarter.

A second check is trajectory. A company falling from 55 to 42 is improving its balance; one rising from 20 to 38 is healing. The Rule of 40 is a snapshot, not a verdict, and the direction of travel often matters more than the current reading. Pair it with returns on capital: a business clearing 40% while earning high returns on invested capital is rare and precious, whereas one clearing it only through aggressive, low-quality growth deserves suspicion rather than celebration.

Examples

Visa and Mastercard are rare giants that clear the bar with room to spare — high margins from network economics plus steady double-digit growth, a combination that compounds spectacularly. Apple blends modest unit growth with extraordinary margins to stay well above 40%. The danger zone is populated by companies that grow 40% while losing money; history suggests most never reach profitability before capital runs out. The Rule of 40 is the investor's reminder that a business must eventually pay its own way.