Every Rule of 40 conversation starts with a tradeoff. Grow faster or run leaner. Pick your ratio, defend it to the board, come back next quarter.
The tradeoff is real. It just isn't universal, and the exception is worth more than the rule.
Two terms and one assumption
Growth rate plus profit margin, forty or better. The arithmetic is honest and I have no argument with it.
Underneath it sits an assumption nobody says out loud. Growth costs margin. Buy more growth and you pay for it in EBITDA, take the margin back and you watch growth slow, so the score reads as a statement about how you balanced the two.
That's true. For one engine.
It's on the list. It isn't in the plan.
Ask anyone how to improve a Rule of 40 score and you get two lists. Accelerate growth. Optimize margin.
Expansion turns up on the first list, usually third, somewhere between pricing changes and channel partnerships. Grow your existing accounts, the bullet says, it's cheaper than acquiring new ones.
Nobody disagrees with that bullet, and it still isn't doing any work.
I ask this at events. A room of SaaS CEOs, raise your hand if you've ever tried to sell more to existing customers, and every hand goes up. Keep it up if it works consistently and you're down to one or two. So expansion is on every list and in almost no forecast. Hopes and dreams. Thoughts and prayers.
That isn't stupidity and it isn't anyone's fault. They tried it, nobody owned it, nothing triggered it, no forecast contained it, and it didn't work. That outcome became the evidence. On the occasions it does work it arrives looking like luck, which is worse, because luck doesn't get a line item.
Here's the part that decides everything downstream. Something that isn't repeatable can't be forecast, and something that can't be forecast can't be a channel. It stays a pleasant surprise. The growth lever only accepts what you can plan.
So expansion sits on the list as a wish and gets budgeted like every other way to buy growth, which is exactly the one thing it isn't.
The number that separates them
CAC counts customers. Revenue Acquisition Cost follows the dollars, and split by source the spread is the whole argument.
A new-logo dollar carries the full load. Marketing, sales development, commission. Typically 35 to 50 cents per dollar acquired.
An expansion dollar carries almost none of that. The customer is already qualified, the trust already exists, and nobody has to be convinced the company is real. Low teens.
Renewals, single digits.
Same dollar of ARR, a third of the cost or less, decided entirely by where it came from.
Now run it through the Rule of 40. A new-logo dollar raises growth and lowers margin, which is the tradeoff working exactly as advertised. An expansion dollar raises growth and raises margin, because you spent thirteen cents to get it instead of forty.
It's the only input on either list that moves both terms the same direction.
Delivery, which is the part people skip
Acquisition cost isn't the whole gap. There's a second one on the delivery side, and strategic unbundling is what opens it.
Hold an item out of the initial sale and attach it to the milestone that earns it. By the time you deliver, the customer has arrived at the problem it solves. They asked for it. They're ready.
Deliver that same item at signing and you're doing the work into a vacuum. Nobody's ready, the thing sits unused, and the hours went into something that produced no outcome anyone can point at.
Same item, same price, and a completely different cost to deliver. Timing did that.
So the expansion dollar is cheaper to acquire and cheaper to serve. Both terms again.
What this actually changes
Nothing about the metric. Forty is still forty and the arithmetic still holds.
What changes is what you do at 31 when the board wants a plan. The two-lever framing offers a choice between spending more and cutting more, and both of those are moves inside the engine that produced the 31.
There's a third move and it isn't a harder version of either one. It's a second channel, with its own owner, its own timing and its own forecast. Build that and the tradeoff stops being the shape of the problem.
The Rule of 40 was never wrong. It assumed you had one way to grow.
Lincoln Murphy formally named and popularized Customer Success starting in 2010 and has spent 15 years connecting it to expansion revenue and commercial outcomes. Read The Premise.