The runway calculation that lied by six months
He thought he had eighteen months. He had eleven. The difference was the assumptions he never wrote down.
A founder told me in January he had eighteen months of runway. By July he had four. He had not lost a customer, missed a launch, or had a bad month. The eighteen months were never real — they were a number calculated correctly against assumptions he had never written down.
The model assumed new ARR would land on schedule (it landed at sixty percent), that the next two hires would happen on plan (they happened three weeks late at higher comp), that the SaaS stack would stay flat (it grew twelve percent), and that no one-time wires would surprise the account (two did). Each minor on its own. Stacked, they ate seven months of cash without ever showing up in a board update.
Runway is the number founders trust most and audit least. It feels precise because it has a denominator. It is, in practice, the average of a set of optimistic assumptions, presented as a single integer because the integer is easier to say out loud. The integer is also the part that ends up wrong.
The honest version is not a number. It is a range stated under three scenarios. One — the plan you actually built. Two — the plan minus your top customer. Three — the plan with your next big milestone slipping sixty days. The right number to lead with — to the board, to yourself, to the person you are about to hire — is the worst of those three. Anything else is a wish denominated in months.
Cash burn also lies in its own direction. Headcount is the lagging indicator everyone watches. The leading indicators are quieter: vendor renewals creeping up, the tooling stack expanding by one SaaS subscription a month, one-time costs nobody flags as recurring because they technically are not. By the time the headcount line moves, the burn rate has been wrong for two quarters.
The founder I started with eventually raised a bridge at terms he hated, because he had to. The bridge would not have been necessary at the runway he believed in. It was necessary at the runway he actually had. The six-month gap was not bad luck. It was the predictable result of trusting an integer.
The cheapest CFO upgrade most companies can make is to stop reporting runway as a number and start reporting it as a range. The story you tell the board gets less confident. The story you tell yourself gets honest. Honest is what survives the quarter.