How to Calculate Your Startup's Runway (Before It's an Emergency)
Runway is cash on hand divided by net burn, recalculated monthly — here's the formula, worked with real numbers, and why the six-month mark matters more than zero.
In this article 6 sections
Quick answer: Runway is your current cash balance divided by your monthly net burn — the amount you're actually losing each month after subtracting any revenue coming in. If you have $240,000 in the bank and you're burning $30,000 a month net, you have 8 months. The math takes thirty seconds. The part founders get wrong is what counts as "burn," and how late they let themselves run the calculation for the first time.
I've sat across the table from three founders in the last year who found out their runway number was wrong by finding out the hard way — payroll was due and the account didn't have what they thought it did. In two of those cases, the founder had a number in their head. It just wasn't the right one. They'd been tracking gross burn, or they'd forgotten a $40,000 annual software renewal that hits every March, or they'd counted a signed contract as cash before the invoice had actually cleared.
None of them were bad at math. They were busy, and runway is the kind of number that punishes you quietly until the day it doesn't.
The Napkin Math Most Founders Get Wrong
Ask a founder how much runway they have and most will give you an answer within five seconds. Ask them to walk you through how they got that number, and the confidence usually drops. The most common shortcut — current cash divided by last month's total expenses — isn't wrong exactly, but it's incomplete in a way that matters.
Say your company spent $85,000 last month and brought in $20,000 in revenue. Your gross burn is $85,000. Your net burn — what's actually draining the account — is $65,000. Divide $500,000 in the bank by the wrong number and you get 5.9 months. Divide by the right one and you get 7.7 months. That's almost two extra months of decision-making room, and it's the difference between panicking in April and having a real plan by June.
The other trap is treating every month like an average month. Payroll is steady. Annual insurance renewals, quarterly tax payments, and that AWS bill that spikes every time you onboard a new enterprise customer are not. If you're only looking at a trailing average, you'll be blindsided by the month those all land at once.
Gross Burn vs. Net Burn: The Number That Actually Matters
This is where the gap between cash flow and profit actually bites founders — a company can look "on track" on a P&L and still be six weeks from a payroll problem, because burn is about cash timing, not accounting periods.
| Metric | What it measures | Why it's misleading alone |
|---|---|---|
| Gross burn | Total cash going out per month | Ignores revenue you're already collecting, understates your real runway |
| Net burn | Cash out minus cash in per month | The number that actually predicts when you hit zero — use this one |
| Burn multiple | Net burn ÷ net new revenue | Useful for judging efficiency, not runway itself — a separate conversation |
Net burn is the only one of the three that answers the question "how many months until the account hits zero." Track gross burn too if you want to understand your cost structure, but don't calculate runway with it.
The Runway Formula, With Real Numbers
Here's the actual formula, worked through with numbers that resemble a typical seed-stage company:
Runway (months) = Cash on hand ÷ Average monthly net burn (trailing 3 months)
- Cash on hand: $620,000 (pull this from your actual bank balance, not your accounting software's "cash" line, which can lag by days)
- Monthly revenue (avg. last 3 months): $28,000
- Monthly expenses (avg. last 3 months): $102,000
- Net burn: $102,000 − $28,000 = $74,000/month
- Runway: $620,000 ÷ $74,000 ≈ 8.4 months
Use a trailing three-month average, not a single month — a single strong or weak month will swing your number more than your actual trajectory has changed. And be honest about revenue: only count cash that's actually landed, not invoiced amounts sitting in accounts receivable. If you want the fuller version of this exercise, the break-even number is worth calculating alongside runway — one tells you when you run out of money, the other tells you what has to be true to stop needing more of it.
This is also where founders under-account for lumpy expenses. According to the Federal Reserve's Small Business Credit Survey, uneven cash flow — not a lack of profitability — is the reason cited most often by small businesses for financial distress, which is exactly the pattern that catches founders who only look at monthly averages.
The Six-Month Rule (and Why Waiting Until Zero Is a Mistake)
Fundraising takes longer than founders plan for, even in a good market. Three to six months from first pitch to wired funds is normal; longer isn't rare. That means if you wait until you have three months of runway left to start raising, you're already behind, and every investor conversation you have will smell like desperation, which is its own problem — terms get worse when the other side knows you don't have options.
The six-month mark is when the real decision-making should start, and it's a three-way fork, not a single move:
- Raise — if the metrics support it and you have time to run a real process, not a fire sale.
- Cut — extend runway by reducing net burn, which is faster and fully in your control, unlike a raise.
- Do both — cut first to buy yourself a longer runway number and a stronger story, then raise from a position that isn't purely defensive.
A useful comparison: businesses that survive their first five years overwhelmingly aren't the ones with the biggest initial war chest — they're the ones that adjusted spending before the number forced their hand. The Bureau of Labor Statistics' Business Employment Dynamics data puts roughly one in five new establishments out of business within the first year, and cash exhaustion — not lack of demand — is a recurring thread in why. Nobody plans to run out of money. They just don't recalculate often enough to see it coming.
If cutting is on the table, it's worth revisiting whether the original bootstrap-versus-raise decision still fits where the company actually is now — sometimes the honest move is slowing growth to match the cash you have rather than spending against cash you're hoping to get.
What to Do Once You Have the Number
Knowing your runway is only useful if you act on it before the number gets urgent. A few habits that separate founders who see problems coming from founders who get surprised:
- Recalculate monthly, on the same day every month, using actual bank balances — not once a quarter, and not from memory.
- Keep a running list of known lumpy expenses (renewals, taxes, insurance) for the next 12 months so they don't ambush the average.
- Set a trigger date, not just a trigger number — "start raising by March 1st" is easier to act on under pressure than "start raising when we hit six months," because the second one is easy to talk yourself out of by month five.
- Share the number with your leadership team, not just your own notebook — a runway number nobody else knows about doesn't change anyone's spending decisions but yours.
The founders I've watched handle this well treat runway less like a doomsday clock and more like a speedometer — something you glance at constantly, not something you check when you're already worried you're going too fast.
Conclusion
Runway math isn't complicated. Cash on hand, divided by real net burn, recalculated monthly with honest numbers, gets you a figure you can actually plan around. What trips founders up isn't the formula — it's letting weeks go by without running it, or running it with the wrong inputs. Do the calculation this week if you haven't in the last thirty days, and put a trigger date on the calendar for when six months of runway is left. That single habit prevents most of the emergencies people call "unexpected."
