Runway calculations using cash divided by monthly burn assume a static burn rate and often conceal risk from cost creep, delayed revenue and premature hires. Small increases in monthly spend can materially shorten actual runway, sometimes compressing timelines by months without board visibility. Spending typically drifts upward as founders add hires, marketing or infrastructure without updating forecasts, making corrective levers harder to pull later. Instead of a single projection, build three scenarios to reflect real-world volatility. The base case should mirror the current plan with expected revenue growth, controlled spend and hiring on schedule.
More importantly, runway is usually presented as a single number - static, linear and unchallenged. In reality, startup burn is a dynamic organism. It evolves with each new hire, vendor negotiation or go-to-market experiment. Yet pitch decks rarely reflect that complexity. This is not about being pessimistic. It is about planning for the turbulence that every early-stage company inevitably hits.
The standard formula is straightforward: cash divided by monthly burn equals runway. But what happens when that burn isn't static? In practice, spending tends to drift upward. Founders approve a new hire, increase marketing spend or scale infrastructure without immediately adjusting the model. In one case I observed, a startup believed it had 16 months of runway. With just a few unexpected expenses, that dropped to 11 - without a single board-level discussion.
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