Startup Runway Calculation: Formula, Mistakes

Runway is the most-quoted startup metric and one of the most mis-calculated.

Startup Runway: The Formula, Common Mistakes, and Board Definitions

Runway seems simple — cash divided by burn — but the definition contains three common mistakes that mislead founders and boards. Wrong definitions of burn, wrong assumptions about revenue, and failure to account for growing burn all inflate reported runway. A founder who thinks they have 18 months when they actually have 11 is one who runs out of cash 7 months earlier than planned.

The right formula

Runway = Current cash balance ÷ Projected monthly net burn. Net burn = Cash operating expenses − Cash revenue. Use projected burn (forward-looking, based on hiring plan and known cost increases), not trailing burn (backward-looking, understates future costs). Use cash accounting, not GAAP — a signed multi-year contract that hasn't been paid yet doesn't add cash.

Common mistakes

(1) Using trailing 3-month burn instead of projected next-12-month average — hides upcoming cost increases from hiring plans. (2) Treating deferred revenue as available cash — it's cash you've been paid but committed to deliver services against; don't burn it. (3) Ignoring seasonality in revenue — Q4 heavy SaaS may burn more in Q1-Q3 than the annual average. (4) Forgetting tax obligations, payroll taxes, and other lumpy expenses. (5) Assuming your fundraise will close by month X — plan runway to hit fumes if it doesn't.

Gross vs. net burn

Gross burn = Total cash operating expenses (no revenue offset). Net burn = Gross burn − Cash revenue. Report both. Gross burn shows the operational cost base — how much you spend each month regardless of revenue. Net burn shows actual cash depletion. Investors want to see both because gross burn tells them your fixed cost base while net burn tells them cash trajectory.

Runway thresholds and actions

24+ months: strong position, focus on execution. 18-24 months: healthy, start considering next raise timing. 12-18 months: begin raise planning, target closing at 12 months. 9-12 months: raise or extend runway now — waiting past 9 months means raising under duress. Below 9 months: emergency mode. Every additional month of delay compounds risk.

Frequently asked questions

Should we count committed but unfunded capital as runway?
No. Runway is cash in the bank. Committed capital (signed SAFEs waiting for wire, term sheet not closed) can fall through. Report runway based on cash, then separately disclose 'plus $X in committed but unfunded capital' if relevant.
How do we present runway when it's contingent on hitting revenue targets?
Show two scenarios: 'plan case' (hitting revenue projections) and 'downside case' (revenue at 70-80% of plan). Never present only the plan-case runway to boards — the downside case is what they need for risk management. Best-in-class founders present both by default.
How often should we recalculate runway?
Monthly, in your investor update. Runway shifts with every hire, contract, and revenue swing. Boards should see the trend line, not just a snapshot. If runway is dropping faster than expected, the trend surfaces the problem 2-3 months before it becomes urgent.

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