Most founders think they know their runway. They take the cash balance, divide by last month's burn, and call it a day. That's backwards — and it's why founders get surprised by a cash crunch they should have seen coming six months earlier.
Runway is a variable, not a number
Runway isn't a single number you check at the board meeting. It's a live variable that changes every time you hire someone, sign a contract, or lose a customer. The formula itself is simple: cash divided by net monthly burn. But the denominator is where almost everyone gets it wrong.
The denominator problem
Most founders calculate net burn as last month's outflows minus last month's inflows — and assume next month will look the same. It won't. If you've got a hire starting in two weeks, your burn just went up. If a customer you're counting on hasn't signed yet, that revenue doesn't count toward extending your runway. Projected revenue is not cash in the bank — and treating it that way is the single most common mistake I've seen across the startups I've worked with.
Burn spikes
The second mistake is treating burn like a flat line. It isn't. Early-stage burn has spikes:
- a tax payment
- a contract renewal
- onboarding a contractor
- annual software licenses that bill all at once
If your runway model assumes the same burn every month, your zero-cash date is wrong. Not by a little — by months.
The timing trap
Then there's the timing trap. Your cash runs out on what's called the cliff date. But you can't wait until the cliff to do something about it. The practical deadline — the date you need cash in the bank, a term sheet signed, or costs cut — is months earlier.
| Situation | Minimum runway target |
|---|---|
| Actively fundraising | 12+ months |
| Steady operating buffer | 6+ months |
| Survival mode | 4 months or fewer |
A healthy rule of thumb: 12+ months of runway when you're fundraising, 6+ months as a minimum operating buffer. If you're at 4 months, you're not in planning mode — you're in survival mode. And negotiating from survival mode costs you twice as much in dilution or desperation.
The founders who get this right don't have a magic spreadsheet. They have a living model that shows them best-case, base-case, and worst-case scenarios — and they update it monthly, not quarterly.
I've built these models for more startups than I can count — as VP Finance at DialSource (now Conquer), as a fractional CFO at Kranz Consulting, and now through Setness Consulting.
What honest inputs look like
You don't need an MBA for this. What you need is honest inputs: what's actually in the bank, what's contractually going out, and what revenue has actually closed — not what's in the pipeline. Everything else is a forecast, and forecasts don't pay rent.
If you want a place to start, I built a free Cash Runway Calculator that does the three-scenario math — best, base, and worst — and shows you exactly when the cash runs out in each one. It takes two minutes. The hard part is being honest with the numbers.
This post is for general educational purposes only and does not constitute tax, legal, accounting, or investment advice. Consult a qualified professional for your specific situation. Content reflects the author's understanding as of the publication date; rules, rates, and interpretations may change.