Every startup is spending its way toward one of two outcomes: profitability or a dead bank account. Burn rate tells you how fast you are moving. Runway tells you how much road is left. Together they are the two numbers that decide when you need to raise, cut, or change course, and they are simpler to calculate than most founders assume.
Key takeaways
- Burn rate comes in two forms: gross burn (total monthly spending) and net burn (spending minus revenue). Net burn is the metric that matters for planning.
- Runway equals your cash balance divided by your net monthly burn. The formula assumes burn stays constant, which it rarely does, so treat the answer as a floor, not a promise.
- Stripe’s guidance is to start fundraising before runway falls below six months; later-stage companies often start sooner.
- These numbers are working tools for budgeting, strategizing, forecasting, and fundraising, not just figures investors ask for.
What burn rate actually measures
Burn rate is the speed at which a company spends cash, usually expressed per month. As Mercury puts it, it is what you would call negative cash flow on a cash flow forecast. The metric matters most for early-stage startups that raise capital to fund operations before they are profitable, which is why investors use it to gauge how long a company can keep operating without new money.
There are two versions, and confusing them produces bad plans. Gross burn rate is the total amount a company spends each month: payroll, rent, software, marketing, everything. Net burn rate subtracts revenue and other income from that total. Stripe’s guide defines the pair the same way, and Brex calls net burn the preferred metric for most startups, because it reflects the real rate at which the bank balance is shrinking.
Brex also offers a clean way to compute net burn from bank balances over any period: subtract your ending cash balance from your starting cash balance, then divide by the number of months. In its worked example, a fictional startup that goes from $2.3 million to $1.9 million over six months has a net burn of about $66,667 per month.
Runway: cash divided by burn
Once you have net burn, runway is one division away. Stripe’s formula: cash balance divided by net monthly burn rate equals startup runway. Using Brex’s example, $1.9 million in the bank at roughly $66,667 of monthly net burn works out to about 28 months of runway.
That math carries a warning. Mercury points out that the simple formula assumes your burn stays constant, and rising expenses mean the formula can overestimate how long you actually have. A growing team, a new office, or a pricing change all move the denominator. Mercury recommends a rolling cash flow forecast for a more realistic view, which is a reasonable upgrade once the business is complex enough that burn changes month to month.
How long is long enough?
There is no universal correct runway, and anyone who gives you one without asking about your stage is guessing. What the guides do agree on is the fundraising clock. Stripe advises startups to begin raising before runway drops below six months, noting that later-stage companies often start sooner, both to negotiate from strength and to avoid panic-driven decisions like giving away too much equity to stay afloat.
What counts as a healthy burn also depends on context. Mercury frames the right burn as a function of your operating cadence: what you are building, what stage you are at, and how deliberately you review spending. The practical takeaway across sources is that burn is neither good nor bad on its own. High burn with a plan and a fundraise timeline is different from high burn with neither.
Burn also changes shape by stage. Stripe’s guide tracks how cash burn typically evolves from seed through Series B and beyond, as headcount and operating costs scale. A seed-stage company burning modestly to find product-market fit runs a different race than a Series B company funding aggressive growth.
Why these two numbers get so much attention
Stripe’s burn-rate guide notes that burn rate indicates the sustainability of the business itself: where other metrics speak to how a product is performing, burn tells you how long the company can keep operating before it becomes profitable or needs more financing. Mercury adds that investors scrutinize burn rate because early-stage startups often are not yet making money and are burning through cash reserves to fund operations, develop products, and acquire customers.
For founders, the same numbers serve a different purpose: they turn anxiety into arithmetic. If the math says you have eight months, you know when spending reviews happen, when a raise must start, and what happens to that timeline if revenue grows or a hire is delayed. The founders who get surprised are usually the ones who stopped dividing.
Burn and runway connect to the rest of your founder finances. Your own salary is one of the largest lines in early-stage burn, and the 2026 benchmarks for what founders actually pay themselves can help you sanity-check that line. For the broader planning picture, our guide to financial strategies for startup founders covers budgeting and cash flow alongside these metrics. If the runway math is already tight, it is worth understanding what a down round is before you are negotiating one, and what it takes to become default alive instead of default dead.
This article explains how the metrics work. It is not financial advice, and the right burn and runway for your company depend on your stage, market, and plan. A good accountant who knows startups is worth the line item.
