6 min
Every founder knows the formula. Money out, minus money in, divided by months left. It takes ten seconds to calculate. So why do founders still run out of cash without seeing it coming?
Paul Graham has a simple test. Once you're past eight or nine months, there's only one question that matters. Will you make it to profitability with the cash you have left? But as he says, "half the founders I talk to don't know whether they're default alive or default dead."
19 August 2026
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Here's the problem. That formula only looks at the past. It doesn't know your costs are about to change. And getting new money is getting harder too.
In 2025, startups on Carta closed just 4,859 new funding rounds, the lowest in six years, down 41% from 2021. Total money raised still hit $119.5 billion, but it went to fewer companies.
So counting on your next round to show up on time "is not a forecast, it is a hope with a decimal point."
Ask three founders for their burn rate, and you'll get three different answers. So before we get to any formula, let's make sure you know exactly which number you're looking at.
Gross burn rate is easy to picture. It's every dollar that leaves your bank account in a month. Payroll, rent, software, contractors, taxes, all of it. It doesn't matter how much money came in. This number only cares about what went out. Think of it as the price tag for running your company for one month. Nothing more, nothing less.
Net burn rate is gross burn minus cash collected. Not revenue booked. Cash collected. That difference is everything.
Here's why. A signed deal isn't money yet. A sent invoice isn't money yet. Only cash sitting in your bank counts as real. Say your customers take 60 days to pay you. During those 60 days, you still have bills to pay. That gap is what separates a company that makes payroll from one that doesn't.
Cash runway is your cash balance divided by net burn, measured in months. It tells you how much time you have left at your current pace.
But here's the catch. Cash runway is only as honest as the net burn number underneath it. Get that number wrong, and your runway is wrong too. That's why investors never ask for just one number. They ask for net burn and cash runway together, because either one alone can be made to look reassuring.
This is the part you'll want to screenshot. Here's how to calculate burn rate and startup runway, using one real example from start to finish.
Runway (months) = Cash on hand ÷ Net monthly burn
Net burn = Gross burn − Cash collected
Take a company with $1,200,000 in the bank. It pays out $260,000 in costs each month and collects $80,000 in cash. Net burn comes out to $180,000. Divide $1,200,000 by $180,000, and the burn rate formula gives you 6.7 months of runway. That's the headline number, and it's the one most founders stop at.
But this company already has two decisions on the calendar. Two engineers start next month, adding $15,000 a month in combined cost. A $24,000 annual insurance premium is due in month 3. Once those are layered in, here's what the next six months actually look like.
Month | Opening Cash | Net Burn | Closing Cash |
1 | $1,200,000 | $180,000 | $1,020,000 |
2 | $1,020,000 | $195,000 | $825,000 |
3 | $825,000 | $219,000 | $606,000 |
4 | $606,000 | $195,000 | $411,000 |
5 | $411,000 | $195,000 | $216,000 |
6 | $216,000 | $195,000 | $21,000 |
Here's the bitter pill. The headline said 6.7 months. But look at the table. Only $21,000 is left after month 6. That means the money runs out in month 7, not 6.7 months from now.
And nothing went wrong to cause this. No surprise costs. Just two new hires and one insurance bill, both already planned. Together, they quietly used up about three weeks of runway.
So was the formula wrong? No. The formula itself isn't wrong. It is a snapshot being used as a projection. Trailing-average burn describes a company that has stopped changing, and that's not a description of any startup worth funding.
The bitter pill from before wasn't bad luck. It happens for three simple reasons, and once you see them, you'll spot them in almost every startup's numbers.
New hires, new office space, yearly contracts, none of these creep up slowly. They show up all at once, like steps on a staircase, not a smooth ramp. Remember the two engineers who added $15,000 a month? Or the $24,000 insurance bill? Neither one crept in slowly. Both hit in one single month. But a burn rate formula built on averages pretends everything rises smoothly. No real bank account has ever looked like that.
Say a customer signs a deal with you. That's great news. But it's not cash yet. If they pay you in 30 or 60 days, you have to wait. Meanwhile, your bills don't wait. This gap is what changes your net burn rate without you noticing. Here's the part that surprises people. Growing your company faster can actually make your burn worse. Not better. That's because new costs hit right away, but new cash takes time to show up.
Think about legal fees on a financing, a compliance audit, a deposit, or a severance payment. Each one shows up once, feels unusual, and gets waved away as an exception when founders build their burn rate formula. Fair enough, taken one at a time. But look closer, and there's always another exception waiting right behind it. That's the trap. All of them together are a permanent line in the cash flow of a company that never stops having exceptions.
Here's something important. The number on paper and the number you can really count on are two different things. Let's look at why, and what you can do about it.
When a company needs more money, it can't just ask for it and get it the same day. Getting new money takes months of work.
And the deal needs to be done and signed before the cash runs out, not right when it runs out. So imagine a company has 9 months of cash left. But raising new money takes 4 months. That company doesn't really have 9 months to work with. It only has 5 months before it needs to start the process.
Paul Graham, a well known startup expert, explains this simply. He says raising money should never be your only plan, just "plan A." He believes every founder should have answers to two questions written down. What will you do if you can't raise more money? And exactly when will you switch to that other plan?
Without answers to these questions, a company can end up in trouble. It runs low on cash, isn't growing fast enough, and doesn't have time left to fix it. This didn't happen because the problem was too hard to solve. It happens because nobody noticed the warning signs early enough.
Startup runway is the tool that helps you notice early. That's why it should be something you check often, not something you only look at right before a meeting with investors.
Knowing your cash runway number is not enough. You have to actually use it. Here are four simple habits that make the number useful.
Practice 1 - Look twelve months ahead, not just at the average. Write down every new hire, every bill, and every renewal, in the exact month it happens. What matters most is the lowest point your cash reaches, not the average amount you spend each month.
Practice 2 - Watch two numbers, not just one. If the amount of cash you're losing each month goes down because customers are paying faster, that's good news. If it goes down because you stopped hiring people, that's a different story. Looking at only one number hides which one is really true.
Practice 3 - Pick a specific date or amount ahead of time. Decide now, not later, at what point you'll need to change your plan. Write it down. Most people find out too late, not because the problem was impossible to see, but because nobody was watching for it.
Practice 4 - Plan for more than one outcome. Make three plans. One where everything goes exactly as expected. One where customers pay a little slower than expected. One where raising money takes twice as long as expected. Three numbers give you something real to prepare for. One number just leads to arguments about whether it's correct.
Most startup numbers can be argued about. Burn rate and runway can't. The cash is either there, or it isn't. Simple as that.
That's why it's worth doing this the right way. Learn how to calculate burn rate properly. Then update your cash walk every single month. Do that, and you'll spot trouble two quarters before it hits, back when your choices are still cheap and easy.
Here's the real point. The value isn't in the math itself. It's in “looking ahead”. Every hire, every renewal, every payment date goes on a calendar, so you can see exactly what's coming your way.
It takes one afternoon to set up. After that, just a few minutes each month. That's a small price for always knowing your startup runway, down to the day.