Startup Runway and Burn Rate: How Many Months of Cash You Actually Have

There is one number that quietly decides almost every founder decision — the hire you make, the raise you delay, the office you sign for. It is not revenue and it is not valuation. It is how many months of cash you have left. That number is your runway, and most founders carry a fuzzy version of it in their head when they should know it to the week.

This post walks through what runway is, how burn rate feeds into it, the difference between gross and net burn that trips up a surprising number of operators, and the rule of thumb for when to start raising. The math is plain arithmetic, but getting the inputs honest is where the real work is.

What runway actually measures

Runway is the number of months your company can keep operating before the bank balance hits zero, assuming nothing changes. The core formula is short:

runway (months) = cash on hand / net monthly burn
net burn = monthly expenses − monthly revenue

That is it. Two divisions and a subtraction. If you have $500,000 in the bank and you are burning $50,000 net every month, you have 10 months of runway. When that month count gets low, every other decision tightens around it.

The trap is that the formula looks too simple to get wrong, so people don't check their inputs. They use last month's spend even though three offers signed last week. They forget the annual insurance premium that hits next quarter. They count revenue that hasn't actually landed. The formula is honest; the numbers fed into it usually aren't.

Gross burn versus net burn

This is the distinction that separates founders who can answer an investor cleanly from those who fumble.

Gross burn is everything you spend in a month — payroll, rent, cloud bills, software subscriptions, contractors — regardless of any money coming in. Net burn is gross burn minus revenue: how much your bank balance actually drops each month.

Picture two companies, each spending $100,000 a month. The first has zero revenue, so its net burn is the full $100,000. The second pulls in $60,000 of revenue, so its net burn is only $40,000. On the same cash pile, the second company has two and a half times the runway. Same gross burn, wildly different survival.

When an investor asks about your burn rate, they almost always mean net burn, because net burn is what determines runway. Quoting gross burn as if it were your runway divisor is a classic way to scare yourself into a bad decision — or to undersell how much room you really have. Always separate the two and know both.

A worked example

Let's run the numbers all the way through.

Say you raised a seed round 14 months ago. The account shows $500,000. Your monthly spend has crept up to $50,000, and you have $10,000 of monthly recurring revenue.

Net burn = $50,000 − $10,000 = $40,000 per month
Runway = $500,000 / $40,000 = 12.5 months

So you have until roughly the middle of next year before the balance reaches zero. Now hold that thought, because the cash-out date is only half the picture. The date that actually governs your calendar is the day you have to start raising — and that comes much sooner than 12.5 months out.

If instead the same $500,000 sat against a flat $50,000 net burn with no revenue at all, you'd be at exactly 10 months of runway. Revenue, even modest revenue, buys time, which is exactly why net burn and not gross burn belongs in the denominator.

When I was helping a friend pressure-test his deck, we did this on a napkin and his face changed when he saw the cash-out date as an actual calendar day instead of a vague "next year." Abstract months are easy to ignore. A date in March is not. That single reframing pulled his fundraise forward by a quarter, and he closed from a position of growth instead of need.

The fundraise timing rule of thumb

Here is the rule worth tattooing somewhere visible: start raising when you have at least 6 months of runway left.

The reasoning is mechanical. A seed or Series A round typically takes 3 to 6 months from the first partner meeting to money actually in the bank. Term sheets slip. Diligence drags. Holidays eat weeks. If you start raising with 6 months of cash, you have buffer for the round to close before zero. If you start with 2 months, you are negotiating with a balance sheet that everyone in the room can see is about to break — and investors can smell desperation in the diligence questions.

So the date that matters is not your cash-out date. It is:

start-raising-by date = cash-out date − raise lead time (≈ 6 months)

In the worked example above, with 12.5 months of runway and a 6-month lead time, your start-raising-by date lands about six and a half months from now. Not a panic — but close enough that the next two quarters of traction decide whether you raise from strength or from need.

Run your own numbers through the startup runway calculator. It takes cash, spend, and revenue, computes net burn and runway, and turns the abstract "how many months" into a hard cash-out date and a start-raising-by deadline — and it flags it plainly when that deadline is already behind you.

Default alive, and why growth changes everything

The simple division assumes revenue stays flat. For a growing company, that understates your runway, sometimes badly. If revenue compounds month over month, each future month burns a little less, and the cash curve bends.

Paul Graham framed the key question as: if revenue keeps growing at the current rate and expenses stay flat, does the company reach profitability before the money runs out? If yes, you are default alive — you can survive without raising again, and any fundraise becomes a choice rather than a deadline. If no, you are default dead, and you should know that now, not in month nine.

Take $100,000 of cash, $50,000 of spend, and $40,000 of revenue growing 10% a month. Naive division says 10 months of runway. But run the month-by-month simulation and growing revenue overtakes spend around month four, with the balance bottoming out near $80,000 before climbing back. You never hit zero. That is default alive, and it completely changes whether the next raise is a rescue or an option.

One more honest caveat: a flat-spend, constant-growth model is a planning tool, not an audited forecast. It ignores churn, seasonality, and new hires. For board-grade numbers you layer in a hiring plan and scenario ranges. For deciding whether to panic this quarter, cash divided by net burn with a growth curve is exactly the right back-of-envelope.

Putting it to work

Three habits keep your runway number honest:

Divide by net burn, never gross. Spend minus revenue is the only denominator that tells the truth about your balance.
Fold in committed costs. A $24,000 annual premium is $2,000 of monthly burn. A signed offer starting next quarter is burn you already own. Leave them out and your runway is fiction.
Manage to the start-raising-by date, not the cash-out date. The six-month buffer is what lets you raise from strength.

If you are also pricing the hire that pushes your burn up, model where you actually turn cash-flow positive with the break-even calculator before you commit the headcount. Runway tells you how long you have; break-even tells you whether you'll need the runway at all.

Know your number to the week. It is the cheapest insurance a founder has.

Made by Toolora · Updated 2026-06-13