Cash runway is the most important financial metric for any startup. It tells you how long the business can operate before needing additional capital — which determines when you need to raise, how much negotiating leverage you have in a fundraising process, and how quickly you need to reach profitability or the next funding milestone. Getting the runway calculation right — and keeping it current — is one of the most important things a finance function does for an early-stage company.

A good runway model is not just a current burn rate divided into a current cash balance. It's a dynamic model that updates as assumptions change, shows the impact of different spending decisions on runway, and gives the founding team clear visibility into the decision points ahead.

The basic calculation and its limitations

The most common runway calculation is simple: cash in bank divided by monthly burn rate. If you have $600k in the bank and you're spending $75k per month, you have 8 months of runway. This calculation is useful as a rough guide but has significant limitations that matter for actual decision-making.

The most important limitation is that burn rate is not constant. It changes as you hire, as revenue grows (partially offsetting burn), as large one-time payments hit (annual software renewals, tax payments, capital expenditure), and as you execute seasonal or project-based spending plans. A static burn-rate calculation ignores all of this variability and produces a runway estimate that's often significantly wrong by month three or four.

Building a dynamic runway model

A dynamic runway model builds the cash position month by month from first principles. It starts with the current cash balance, then adds expected cash inflows (revenue collections based on your sales plan and payment terms) and subtracts expected cash outflows (salaries and benefits, rent, software, marketing, and any other planned expenditure) for each future month. The result is a monthly cash balance that shows you exactly when you expect to hit your minimum cash threshold — typically 1–2 months of operating costs — under your current plan.

The value of building it this way rather than using a simple burn-rate calculation is that you can see the timing impact of specific decisions. If you hire two engineers in month 3, the model shows you the exact impact on your cash balance in month 3 and all subsequent months. If a large customer is expected to pay in month 5, the model shows you how that changes your runway. This specificity changes how you make decisions — from guessing about impact to modelling it.

Scenario analysis

Every runway model should have at least three scenarios: a base case that reflects your current plan, an upside case that shows the impact of stronger-than-expected growth, and a downside case that shows what happens if growth is slower than planned or if a specific risk materialises. The downside case is the most important for decision-making — it tells you whether you have enough buffer to absorb a setback without running out of money, and it determines how urgently you need to either reduce burn or accelerate fundraising.

For most seed-stage companies, the downside scenario should assume revenue growth at roughly 50–60% of the base case assumption, with costs approximately as planned. This is a realistic stress test — not catastrophic, but meaningfully more conservative than the plan. If the downside scenario still shows at least 9–12 months of runway, you have adequate buffer. If it shows less, you have a decision to make about either reducing planned spending or accelerating your fundraising timeline.