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"What's your burn rate?" and the four seconds of silence that follow.

You can ship a feature in a day. Can you tell me, right now, your monthly burn and how many months of runway it buys? Most founders can't, and it's not because they're bad at math.

By Pelle Brændgaard

The call is going well. Then it comes: “So, what’s your monthly burn, and how much runway does that give you?” And you realize you are about to answer a question about your own company with a guess.

I have been on both sides of that pause. It is a specific kind of dread, because the number is not hard to define. It is hard to know, because it lives in six places and none of them are your real books.

And it does not only bite early. I am six years into the company I run now, with a VP of finance, and there are still versions of this I cannot answer: the real CAC for a particular kind of customer, or the true gross profit per customer. Those are exactly the numbers I would use to decide which customers to chase and how to price for them. Six years in, with a finance team, and the stack still will not just hand them to me.

Founders describe the feeling better than I can:

“Six months ago I had no idea how much runway I had left. Not roughly. Not ‘somewhere between 8 and 12 months.’ I genuinely didn’t know.” (founder, Indie Hackers)

And when she finally did the math, the gap was not small:

“When I finally ran the numbers, I had 4.5 months left. Not the 8+ I’d been assuming. I’d been making hiring decisions, signing annual contracts, and spinning up paid acquisition, all based on vibes.” (same founder, Indie Hackers)

That is the real cost. Not the awkward pause on the call. The hiring and spending decisions made on a runway number that was double the truth.

What is burn rate, really?

Burn is simple to say and easy to get wrong. Two versions matter:

• Gross burn: everything you spend in a month.
• Net burn: what you spend minus what you bring in. This is the one that decides how long you live.

Runway is just your cash in the bank divided by your net burn. If you have $180,000 and you are net-burning $15,000 a month, you have twelve months. That is the whole formula.

So why can’t you answer instantly? Because every input has a trapdoor.

Why the number lies

Say you have $180,000 in the bank and it feels like plenty. Here is what the simple version misses:

• Your AWS bill reads near-zero, because you are on credits. When they run out, your real spend jumps and your runway was always shorter than the dashboard said.
• You collected a year of an annual plan up front in January. Your bank balance looks great, but eleven months of that is money you have not earned yet and may owe back.
• Two of your biggest invoices are sixty days overdue, so the revenue you are counting has not actually arrived.

Get any of those wrong and your runway is fiction. A founder said this exactly:

“A founder signs $100K in MRR and assumes they’ve got 15 months of runway, but a chunk is still in pilot, some is overdue, and the real number is nothing like that. Treating ARR like cash is one of the most common early-stage mistakes.” (YC founder on private YC forum)

Why is this so hard to just look up?

Because for most early founders the number does not exist anywhere. It gets assembled by hand, every single time:

“Every month same routine — export CSV from Stripe, paste into Excel, spend 2 hours figuring out MRR, churn, failed payments.” (u/Timely-Curve1425, r/SaaS)

“I’ve looked everywhere and tried quite a few tools, but nothing can accurately give us runway, burn rate, MRR/ARR, gross margin, CAC payback: basically all the metrics an investor expects us to know when raising.” (YC founder on private YC forum)

When the number is stitched together from a Stripe export, a bank statement, and a memory of which invoices are paid, you do not trust it. And when you do not trust it, you hesitate. The investor is not really testing your mental math. They are testing whether you know if you are default alive. The four seconds of silence answers that question for them.

What if nobody’s asking?

Maybe you are not raising. Plenty of the best founders I know are default alive on their own money — no board, no investor call, nobody outside the company who will ever ask them for a burn number. It sounds like that lets you off the hook. It does the opposite. It means the only person who will ever make you answer is you, and it is very easy to never ask.

The bootstrapped version of the question is quieter, but sharper. Not “how many months until I run out of someone else’s money,” but “am I actually making money, and do I know it per customer?” Those are the numbers — real unit economics, contribution margin, the true cost to serve one account — that tell you whether to raise prices, which customers to chase, and whether you can afford to hire. And if you ever want to sell what you have built, a buyer’s very first move is diligence on exactly these figures. Books that survive that read hold your price; a spreadsheet that does not add up hands the buyer a reason to discount you.

Same silence, same fix. The only thing that changes is who is on the other side of the table — an investor, an acquirer, or just you on a Sunday night.

The fix is not a better spreadsheet

The reason burn is hard is not the formula, it is that your numbers live in six disconnected places and none of them are real, accrual-basis books. Fix that underneath and burn stops being a monthly archaeology project and becomes something you can just read.

That is the deeper problem, and it is the next one worth understanding: why the spreadsheet you are keeping is not actually books.

Read next: Your books are a spreadsheet and a folder of receipts