Rule of 40: which variant you pick is the whole argument

You are writing the Q1 board package for a Series B SaaS company. Revenue growth was 42%. Which margin do you use for the Rule of 40?

The answer the company deserves depends on two things:

1. What the board is trying to evaluate.
2. What’s currently distorting the efficiency number.

If you pick the wrong variant, the narrative lies — not because the math is wrong, but because you’ve handed the board the wrong mental model.

The four variants worth knowing

Rule of 40 (FCF margin). Revenue growth + FCF margin ≥ 40%. This is the original variant Brad Feld described. It is the hardest test, because it captures capex, working capital, and stock-based compensation all at once. It punishes a company mid-migration far more than it should.

Rule of 40 (operating margin). Revenue growth + operating margin ≥ 40%. GAAP operating margin (pre-SBC in practice, for SaaS). This is the variant I reach for most often at Series B. It separates the ongoing business from one-time investment cycles.

Rule of 40 (EBITDA margin). Revenue growth + EBITDA margin. Closer to operating margin, more forgiving of depreciation and amortization. Some boards find it intuitive; others find it too generous.

Rule of 40 (gross margin). Revenue growth + gross margin ≥ some higher threshold (60%+). Useful only for early-stage companies where operating margin is structurally negative and you’re trying to test unit economics, not efficiency.

A concrete test

Here’s the same Series B company computed four ways, based on real anonymised Q1 numbers:

Variant	Revenue growth	Margin	Rule of 40
FCF margin	42%	-18% (migration capex landing)	24
Operating margin	42%	-1%	41
EBITDA margin	42%	3%	45
Gross margin	42%	71%	113

Four different scores. One company. Which one tells the truth?

It depends what you are measuring. If the board is evaluating long-run efficiency while a multi-quarter cloud migration lands in Q2–Q3, FCF margin understates the business. The company is investing, and by the board’s own prior approval. Leading with a FCF-margin Rule of 40 in that quarter tells a story the board has already ruled out.

The operating-margin variant — 41 — is the story the board actually asked for. It says: the business is passing the efficiency test, before a planned investment cycle that was budgeted and approved.

What goes in the variance narrative

Once you’ve picked the variant, the narrative should say three things:

1. The score, and the variant. “Rule of 40: 41% (operating-margin variant).”
2. Why this variant, in one clause. “Operating margin excludes the multi-cloud migration capex approved at January’s board, landing Q2–Q3.”
3. The revisit trigger. “We’ll restate on FCF margin once migration completes, currently tracking to Q3.”

That last line matters. It’s the commitment that makes the choice defensible. Without it, the variant choice looks like an excuse. With it, it looks like a plan.

The board-deck lie I try to never tell

The worst version of this call is switching variants silently between quarters. One quarter it’s FCF margin, the next it’s operating margin, and the third it’s Rule of 40 “adjusted.” The board stops trusting the number. They should.

Once you pick a variant, the decision should live in the decision ledger with the reason and the revisit trigger. Next quarter, the package should reference the prior call and restate only if the trigger fired. No silent switches.

What a memory layer does for this

The first time my CFO edits a Rule-of-40 narrative to use the operating-margin variant for a Series B client mid-migration, that correction should live forever — attributable, with a rationale and a revisit trigger. Next month’s draft lands on the same variant without anyone re-deriving the argument. When the migration completes, the revisit trigger fires and the package restates on FCF margin, with the prior decision cited.

That’s what “institutional finance memory” is, in one board-room pattern. The variant is a judgment call. The memory is what keeps the judgment coherent across twelve packages and two fractional CFOs.