The Runway Number Everyone Miscalculates The Same Way

Ask a founder how much runway they have and you'll get a fast, confident answer. Cash in the bank divided by this month's burn. Eighteen months. Plenty of time.

That number quietly assumes burn stays flat for the entire period. Burn almost never stays flat. That one wrong assumption routinely overstates real runway by months.

I've watched founders plan an entire raise around a runway number that was fiction. They find out halfway through, when the pressure is already showing in every investor meeting.

How do you calculate startup runway?

The simple formula is cash on hand divided by monthly net burn. Net burn is what you spend in a month minus what comes in. Gross burn is total spending before any revenue.

That formula is fine for a snapshot. It's wrong as a forecast, because it treats this month's burn as if it will repeat unchanged until the money runs out.

The right way is to model burn forward, month by month, using what you already know is coming: planned hires, planned spending increases, annual bills and seasonal swings. Then count the months until the cash line hits zero.

Why flat-burn runway is almost always wrong

A growing company almost always means growing burn. New hires. More tools and infrastructure. Marketing spend that ramps as growth targets climb.

A runway number built on this month's burn ignores every one of those increases. It looks more comfortable than the business's actual path supports.

This matters most at the exact moment you're deciding whether there's still time before you need to start raising. Overstate runway by two or three months and you can start the raise too late, right as desperation starts showing up in your conversations with investors.

The runway math, worked both ways

Say a hypothetical company has $1,080,000 in the bank and a current net burn of $60,000 a month. The flat math says $1,080,000 divided by $60,000 is 18 months.

Now add what's already planned. A first hire starts in month 4, adding $15,000 a month. In month 7, a second hire adds another $15,000 and marketing ramps by $10,000. Burn is now $100,000 a month.

Real runway is about 12¾ months, not 18. Nothing went wrong. Every increase was planned. The founder just didn't put the plan into the math.

What do founders forget in their burn rate?

• The fully loaded cost of a hire. Payroll taxes, benefits, equipment and software push the real cost above salary.
• Annual and lumpy bills. Insurance, annual software contracts, legal and accounting fees, tax payments.
• Revenue that isn't booked. Counting hoped-for sales as if they're signed makes net burn look smaller than it is.
• Money that isn't in the bank. A verbal commitment from an investor isn't cash until it's wired.
• Slow collections. If customers pay in 60 days, revenue shows up later than the invoice date suggests.

When should you start raising?

Work backward from the real number, not the easy one. Raises take longer than founders expect: building the list, taking meetings, diligence, documents and closing all take time. My rule is to start while you still have enough runway that you could walk away from a bad deal.

Run it on the example above. Suppose you assume the raise takes six months and you want three months of cushion when it closes. The flat model says start by month 9, because 18 minus 6 minus 3 is 9. The real model says start by month 3, because 12.75 minus 6 minus 3 is 3.75. Same company, more than five months less time than the founder thinks.

That's also why the size of the round matters. Your runway model tells you how much you need to reach the next real milestone, which is the heart of how much you should actually raise. The Raise Academy builds this number with founders before they ever ask for a dollar.

The objection: "Isn't a month-by-month model overkill this early?"

With so much uncertainty, doesn't a detailed model just create false precision around numbers that are guesses anyway?

No. Even a rough monthly model built on known plans beats a flat-burn number that assumes zero change. The goal isn't precision. It's avoiding the one common error of assuming burn won't grow when it almost always does. Update the model every month with actual numbers, and your guesses get better fast.

Two founders, same cash balance

Consider two hypothetical founders who start with the same cash.

The first uses flat burn, sees 18 months and starts raising casually. Two planned hires come on and marketing ramps as intended. Partway through the raise, a recalculation shows closer to 12 months. The rest of the raise happens under pressure.

The second models the same hires and spending from day one and sees about 12 months right away. That founder starts raising months earlier, with urgency but without desperation, and keeps the leverage to say no.

Same cash. Same plan. The only difference was which number they believed.

What to ask your CPA or finance lead

• Is this runway based on net burn, and does it include every planned hire at fully loaded cost?
• Which annual or one-time payments land in the next 12 months?
• What happens to runway if revenue comes in 25% below plan?

Once you have the real number, share the trend in your investor updates. Investors trust founders who see the cliff coming.

What to do this week

Open a spreadsheet and rebuild your runway month by month for the next 18 months. Add every planned hire, spending increase and annual bill in the month it lands, and find the month your cash hits zero.

The real number is almost always shorter than the easy one. Better to know that now than discover it under pressure.

Key takeaways

• Cash divided by current burn assumes spending never grows, and it almost always does.
• Model burn month by month with every planned hire and annual bill included.
• Start raising from the real runway number, while you can still walk away.

Frequently asked questions

What is the formula for startup runway?

The simple formula is cash on hand divided by monthly net burn. It works as a snapshot, but for planning you should model burn month by month, adding planned hires and spending increases, and count the months until cash reaches zero.

What is the difference between gross burn and net burn?

Gross burn is everything you spend in a month. Net burn is that spending minus the cash that comes in from revenue. Runway is usually based on net burn, so only count revenue you've actually booked and collected.

How many months of runway should I have before raising?

Start raising while you still have enough runway to walk away from a bad deal, because raises take longer than most founders expect. Build the real month-by-month number first, then work backward to your start date.