Burn Rate and Runway: How to Calculate How Long Your Startup Has Left
Gross burn vs net burn, how to calculate runway correctly, and the warning signs that tell you it's time to start fundraising — explained with real formulas.
"How long do we have left?" is the single most consequential question a startup can ask itself — and the most commonly answered with a back-of-envelope number that overstates the real picture. Runway looks like simple division: cash in the bank divided by monthly spend. The part that determines whether that number is useful or dangerously optimistic is which "spend" you actually divide by.
This guide walks through gross burn vs. net burn, how to calculate runway in a way that won't mislead you, and the timing signals that tell you when it's time to start raising — well before the number gets uncomfortably small.
Gross burn vs. net burn — the distinction that changes everything
Gross burn
Gross Burn
Gross Burn = Total Monthly Operating Expenses
Gross burn is the full cost of running the business each month — payroll, infrastructure, software, rent, contractors, everything that leaves the account regardless of what comes in. It tells you the size of your operation, but on its own it overstates your actual cash decline for any business with revenue.
Net burn
Net Burn
Net Burn = Total Monthly Operating Expenses − Monthly Revenue
Net burn is the number that actually determines how fast your bank balance falls — it's gross burn minus whatever revenue offsets it. This is the figure that should drive your runway calculation, because it reflects the real net outflow, not the gross size of your spending.
A pattern worth recognizing
Gross burn rising while net burn falls is one of the healthiest patterns a growing company can show — it means revenue is growing faster than costs, even as the operation itself gets bigger. Conversely, gross burn staying flat while net burn rises is a warning sign: revenue is shrinking or costs are quietly creeping in ways that aren't yet visible in the topline expense figure.
Calculating runway — and the trap in doing it with one month's data
The runway formula
Runway (months) = Current Cash Balance ÷ Net Burn
The formula is one line. The trap is in what you use as "net burn." A single month is a noisy sample — it can be distorted by an annual software renewal, a one-off bonus, a sudden customer prepayment, or a seasonal dip in sales. Plugging that one number into the formula can make your runway look dramatically better or worse than it actually is.
Use a trailing three-month average net burn instead. It smooths out one-time noise and gives you a figure that reflects the underlying trend rather than a single month's anomalies — which is what you actually need to make a decision as consequential as "when do we start raising."
It's also worth running the number under more than one assumption: your current trajectory, a scenario where a key revenue source slips, and a scenario where a planned cost-saving measure lands on schedule. Runway isn't a single fixed number — it's a range that shifts with decisions you haven't made yet, and seeing that range is far more useful than anchoring to one point estimate.
Try it yourself
Burn Rate & Runway Simulator
Model gross burn, net burn, and funding runway with monthly projections — and see exactly how a hiring plan or revenue scenario shifts your timeline.
When to start fundraising — and why the timing matters more than the number
A widely cited rule of thumb is to begin fundraising with roughly 9 to 12 months of runway remaining. The reasoning isn't about the runway number itself — it's about negotiating leverage and how long raises actually take:
- Raises take longer than the plan says. Between building the narrative, running the process, navigating diligence, and closing, a raise that "should" take eight weeks often takes considerably longer. Starting early absorbs that slippage without forcing a panic.
- Urgency is visible to investors. A founder negotiating with six weeks of runway left is negotiating from a position of obvious desperation — and experienced investors price that into the terms they offer, often substantially.
- Optionality compounds. Starting a raise early — and being able to walk away from a bad term sheet because you're not out of time — is one of the most valuable (and least visible) advantages a founder can have in any negotiation.
The practical implication: don't wait for runway to feel uncomfortably short before starting the conversation. By the time it feels urgent to you, it's already visible to everyone you're negotiating with.
The levers that actually extend runway
- Grow revenue faster than costs. This is the only lever that improves your position structurally rather than temporarily — it's the difference between extending runway and merely delaying the same outcome by a few months.
- Cut discretionary costs before cutting strategic ones. Tooling, vendor contracts, and non-essential spend are the first places to look — cutting into the team or the roadmap that drives revenue growth often shortens runway in the medium term even as it extends it in the short term.
- Re-sequence hiring against revenue milestones. Tying headcount growth to specific revenue or product milestones — rather than a fixed calendar — keeps gross burn growth aligned with the business's actual ability to support it.
- Re-run the model whenever a major assumption changes. A new hire, a lost customer, a delayed launch, or a pricing change all shift net burn — and therefore runway — immediately. Treat the calculation as a living model you revisit monthly, not a one-time exercise you did for the board deck.
Frequently asked questions
What's the difference between gross burn and net burn?
Gross burn is the total cash going out of the business each month — payroll, infrastructure, rent, tools, everything. Net burn subtracts incoming revenue from that figure, showing the actual monthly decline in your bank balance. A company can have rising gross burn and falling net burn at the same time if revenue is growing faster than costs — which is exactly the pattern investors want to see.
Should I calculate runway using a single month's burn?
Not if you can avoid it. A single month can be skewed by one-time costs (an annual software renewal, a one-off hire bonus, a conference sponsorship) in either direction. A trailing three-month average net burn produces a far more stable, decision-worthy runway figure than any single month in isolation.
When should a startup start fundraising relative to its runway?
A common rule of thumb is to begin fundraising with somewhere between 9 and 12 months of runway remaining. Raises routinely take longer than founders expect, and negotiating from a position of six weeks of runway left puts you at a severe disadvantage — investors can sense urgency, and it shows up in the terms they offer.
Does revenue growth automatically extend runway?
Only if it grows faster than costs. Revenue growth that is fully offset by proportional increases in headcount, infrastructure, or customer acquisition spend leaves net burn — and therefore runway — unchanged. The number that matters for runway is net burn, which only improves when the gap between revenue and costs narrows, not simply when either one grows.
Runway is only as reliable as the burn figure behind it — and net burn, averaged over a meaningful window, is the number that actually predicts when you'll run out of cash. Build your monthly projections in the Burn Rate & Runway Simulator above to see how a hiring decision, a revenue scenario, or a cost cut would shift your timeline before you commit to it.