StartupsBurn Rate and Runway: How to Calculate Both (and What to Do With Them)

There is a particular kind of math founders do at 11:40 p.m. It involves the bank app, a calculator, and a lot of rounding in the optimistic direction. "We have about a year," you tell yourself, and then you go to sleep feeling mostly fine.

The trouble is that "about a year" was built on one good month, a payroll you forgot was semi-monthly, and an annual software bill that hasn't hit yet.

This article walks through a burn rate and runway calculation you can do from real numbers, the mistakes that make runway look longer than it is, and what to actually do with the answer once you have it. No spreadsheet wizardry required. A calculator and some honesty will do.

What is burn rate, exactly?

Burn rate is the speed at which your company uses cash. There are two versions, and mixing them up is how founders end up surprised.

• Gross burn is every dollar of cash going out in a month: payroll, rent, software, contractors, ad spend, loan payments.
• Net burn is gross burn minus the cash that came in from customers in that same month.

If you have no revenue yet, gross and net burn are the same number. Once customers start paying, net burn is the one that drives runway. Gross burn still matters, though, because it tells you how expensive the machine is to run. Investors often ask for both.

Notice the word "cash" keeps showing up. That's on purpose.

How do you calculate runway?

Here's the runway calculation in one line:

Runway (months) = cash on hand / average monthly net burn

That's it. The formula is easy. The inputs are where things go sideways (like a recipe that says "season to taste" and you've never tasted it).

Use these rules for the inputs:

1. Take cash from reconciled bank balances, not the number in the banking app on a random Tuesday.
2. Average net burn over the last three months, so one weird month doesn't set the whole forecast.
3. Count only cash you can actually spend. A credit line you haven't drawn is a backup plan, and it doesn't belong in the numerator.
4. Adjust for anything you already know is coming: a new hire starting next month, an annual insurance renewal, a price increase from a vendor.

A worked example with real-looking numbers

Here is a made-up case. You run a seed-stage software company with eight people. Your reconciled cash balance at the end of September is $900,000. Here's the last quarter:

Average net burn is $75,000. So runway is $900,000 / $75,000 = 12 months.

Now add what you know. Two engineers start in November, and together they add about $30,000 a month in fully loaded cost (salary plus payroll taxes and benefits). Net burn jumps to roughly $105,000. Runway drops to about 8.5 months, before revenue growth is counted.

Same company. Same bank balance. Very different conversation at the next board meeting.

Why your P&L burn and your bank burn don't match

Plenty of founders pull burn from the profit and loss statement. It's close, but it isn't cash, and the gap can be large.

A P&L built on accrual accounting (recording revenue when earned and expenses when incurred, rather than when cash moves) will smooth some things out and miss others:

• A $24,000 annual software contract paid upfront hits cash in one month, but the P&L spreads it over twelve.
• A customer who prepays a year of service gives you cash today, and most of it shows up as deferred revenue (money received for work you haven't delivered yet) on the balance sheet rather than income.
• Loan principal payments and equipment purchases use cash but barely touch the P&L.

The P&L tells you whether the business model works. The bank tells you how long you can keep finding out. For runway, start from cash, then use the P&L to explain why burn moved. If your books aren't clean enough to trust either, that's a setup problem, and our piece on the first-year startup accounting setup founders usually skip covers where to begin.

What runway number should make you act?

There's no magic threshold, but here's a practical way to think about it. Raising money usually takes longer than founders plan for, and you want to negotiate from a position where walking away is possible. So work backward:

1. Estimate how long a raise would realistically take you, from first meeting to cash in the bank.
2. Add a cushion of a few months for delays (there are always delays).
3. That total is your "start acting" line. When runway drops near it, you're either raising, cutting, or growing revenue fast enough to change the math.

Many founders treat 12 months as a mental line for starting a raise and 6 months as the point where cuts are no longer optional. Treat those as conversation starters, not rules. Your sector, your traction, and your investors change the answer.

What to do with the number once you have it

A runway figure that lives in one founder's head isn't doing much work. Put it to use:

• Recalculate monthly, right after the books close. If close takes six weeks, your runway figure is always stale.
• Model scenarios. What happens if the next hire waits a quarter? If a big customer pays 30 days late? A short-term view like the 13-week cash flow forecast catches the timing problems a monthly average hides.
• Share it. Burn and runway belong in every update to investors. Here's what to include in a monthly investor update so the number arrives with context instead of panic.
• Separate one-time from recurring. A $40,000 legal bill for a financing round inflates one month's burn. Note it, then exclude it from the run-rate.

If you want someone to keep these numbers accurate every month, our startup accounting service handles the bookkeeping, reconciliations and monthly close, and fractional CFO support adds the forecasting and scenario work on top.