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Every month after the books close, Sai resets your runway to actual cash and net burn, checks how last month's forecast held up, and republishes a simulator where you can test hires, revenue slips, and churn — and watch the cash-out date move.
The runway number founders quote is almost always older than they think. It was worked out after the last raise, adjusted once when the big contract closed, and has been carried around in their head ever since — while two hires started, a customer churned, and invoices started getting paid a month late. Runway is cash divided by net burn, and both of those change every month. A runway figure that is not recomputed monthly is not a slightly outdated number; it is the specific mistake that leaves a company three months shorter than it believes.
After each month's books close, Sai records actual cash and actual net burn, resets the model's starting point to today's real balance, and republishes an interactive simulator. The headline is one number — months of runway and the month cash runs out — with a chart underneath and sliders for the decisions you are actually weighing. Move a slider and the cash-out date moves with it.
Most runway surprises do not come from a bad forecast of one thing. They come from forgetting what a decision drags along with it.
Every month opens with a comparison: what last month's model said would happen, against what did. Over a few months this becomes the most honest thing in the whole model, because it shows whether the plan runs consistently optimistic — revenue always landing a month later, hires always costing more — and by how much. No single-use runway calculator can tell you that, because it only ever sees one month.
Actuals already recorded are never recalculated, so the history reflects what the numbers were, not a later revision of them. The baseline burn is a rolling three-month average, which smooths one unusual month without hiding a trend. A missing input disables its lever instead of being filled with an assumption. Each month keeps its own link, so the version you showed the board in March is still exactly what it was.
Today's runway appears on the runway slide of the weekly executive briefing deck; this simulator answers what happens if something changes. The size of a new raise — and what it costs you in ownership — is modeled in the cap table and dilution calculator. For the classic framing of whether a startup survives on its current trajectory, see Paul Graham's Default Alive or Default Dead?.