How Long Will Your Startup’s Cash Last? The Burn Rate Math Every Founder Avoids

There’s a number every founder technically knows and many refuse to look at directly: the month their company runs out of money. It’s not hidden — it’s two divisions away from your bank balance — but it’s uncomfortable, so it gets rounded up, mentally adjusted for revenue that hasn’t closed, and checked less often as it shrinks. This is the case for doing the math coldly, and what to do with the answer.

Burn rate: know both versions

Gross burn is everything you spend in a month — payroll, rent, tools, contractors, cloud bills. Net burn is that minus the revenue that actually arrived. The distinction matters because they answer different questions: gross burn is your risk (it’s what you owe the world if revenue stops), net burn is your clock (it’s what actually drains the account). Founders who quote only net burn in good months are borrowing optimism from customers who haven’t churned yet. Use a three-month average for both — single months lie, in both directions.

Runway: the honest division

Cash in bank ÷ average net burn = months of runway. Two adjustments make the number honest. First, subtract cash that isn’t really yours to burn — security deposits, taxes collected but not remitted, that grant with clawback conditions. Second, if burn is growing (you just hired three people), project forward with the new burn, not the trailing average that still includes cheaper months. The difference between naive and honest runway is routinely three or four months — which happens to be exactly the amount that decides whether a fundraise is calm or desperate.

The fundraising clock

Here’s why 12 months of runway is treated as the practical minimum: a seed or Series A takes three to six months from first meeting to money in the bank, and investors can smell the difference between a founder choosing to raise and one who has to. Start conversations at 12 months and you negotiate; start at five and you accept terms. The brutal corollary: your real decision deadline isn’t when cash hits zero — it’s roughly nine months before that, when you must choose between raising, cutting to profitability, or winding down with dignity.

Default alive, or default dead?

The most clarifying question in startup finance: at current growth and current burn, does the company reach profitability before the cash runs out? If yes, you’re “default alive” — fundraising becomes optional leverage. If no, you’re “default dead,” and every plan you make should acknowledge that survival depends on someone else’s money arriving on schedule. Neither answer is shameful at an early stage; not knowing which one applies to you is.

Do the math this week

All of this is ten minutes of arithmetic, and there’s a free burn rate and runway calculator that runs the whole picture interactively — plug in cash, spending and revenue, then stress-test the scenarios: what happens to the runway if that big client churns, or if the next hire starts next month? Put the resulting date in your calendar, nine months early, labeled “decide.” Companies rarely die from the math being bad; they die from nobody doing it until the options were gone.