"Runway" is the most-used and least-rigorous number in venture-stage finance. Founders quote a single number ("we have eighteen months") that is almost always wrong, usually in the company's favor. The correction is not painful. It is mechanical.
Runway is three numbers, not one.
Real runway operates at three resolutions simultaneously. A founder who is only tracking one of them is steering on incomplete information.
1. Cash runway.
Cash on hand divided by average net monthly burn. This is what most founders quote. It is the floor of the problem, not the ceiling. The problem with cash runway as a single metric: it averages a burn rate that is not actually flat. Hiring waves, contract renewals, working capital swings, and one-time payments all distort the average.
2. Forward burn runway.
Cash on hand divided by projected monthly burn over the next six months, based on the actual hiring plan and committed expenses. This is the number that matters. It is almost always shorter than cash runway because the next six months include hires, contract renewals, and capex that the trailing average does not.
3. Decision runway.
Time until the next capital decision needs to be triggered. Working backwards from when you need cash in the bank, subtract the diligence and close timeline (typically three to four months for a priced round), and that is when fundraising has to start. Decision runway is usually four to six months shorter than forward burn runway. This is the number that should drive the calendar.
The three structural mistakes.
1. Trailing average burn.
If your last three months were unusually quiet (no hires, no big contracts, no offsite), your average understates real burn. Investors normalize this. Founders rarely do.
2. Ignoring credit card cycles.
Credit card spend lags one statement cycle. A founder who looks at bank account changes alone systematically misses the next month's card payment, which can be 5 to 15% of monthly burn. Cash runway has to be net of card balances, not net of card payments.
3. Counting committed but undeployed cash as "deployed."
SAFEs that have not converted, grants that have not landed, AR that has not been collected: all of it is in some founders' runway calculations. None of it is real cash until it is in the bank. Runway is a function of bank balance, not balance sheet.
How a CFO models it.
The right structure: a 24-month monthly cash flow forecast with hiring, infrastructure, and one-time items modeled by month. Three scenarios layered on top (base, downside, upside). Runway is the column where cash crosses zero in each scenario.
The base case is the operating plan. The downside case strips out optimistic revenue and adds three months of hiring. The upside case assumes the raise closes on time.
If base case runway is fourteen months, downside case is nine months, and upside case is twenty-two months, the real planning runway is somewhere around nine to twelve months. That is the window founders should be making decisions inside.
The lowest cash point matters more than the average.
Runway calculations often miss the trough. A company that has $1M of cash, burns $80K/month, but has a $200K annual contract renewal in month six does not have twelve months of runway. It has a cash position that dips dangerously close to zero in month seven, regardless of the average.
The lowest cash point inside the forecast period is the constraint. Investors look at this. Founders rarely do.
The conversation with the board.
When a CFO walks into a board meeting, they do not say "we have fourteen months of runway." They say:
- Cash on hand today.
- Forward burn for the next six months, modeled by month.
- Lowest cash point in the next twelve months and the month it hits.
- Date by which fundraising must be initiated to avoid that point.
That is the runway conversation. Everything else is a guess wearing a number.