Burn Rate & Runway: How to Calculate Both

Burn rate and runway answer the only financial question that is existential for a pre-profit startup: how much time do you have left? Every other metric — CAC, gross margin, pipeline coverage — matters because of what it does to this one. Yet in our work with hundreds of startups, burn is also one of the most commonly miscalculated metrics, usually because founders compute it off the income statement while their books are on an accrual basis. This guide covers the formulas, worked examples, and the accrual adjustments that separate an accurate runway number from a comfortable fiction.

What Is Burn Rate?

Burn rate is the pace at which your company consumes cash, measured monthly. If your bank balance drops from $800,000 to $750,000 over a month with no fundraise in between, you burned $50,000. It's negative operating cash flow with a more memorable name, and it's the denominator in your runway calculation — which is why getting it right matters more than getting it flattering.

The term matters most during the period when outside capital — a priced round, SAFEs, venture debt — funds your operations. Once you sustain positive cash flow, burn stops being the headline metric, though the underlying discipline of tracking cash monthly shouldn't retire with it.

Gross Burn vs. Net Burn: What's the Difference?

There are two versions of the metric, and they answer different questions.

Gross burn is total cash out the door each month — payroll, rent, software, COGS, everything — ignoring any cash coming in. It answers: what does it cost to run this company?

Net burn is cash out minus cash in. It answers: how fast is the bank balance actually declining?

• Net burn = monthly cash outflows − monthly cash inflows
• Gross burn = monthly cash outflows

A worked example. Suppose in a given month your startup collects $40,000 from customers, spends $10,000 on cost of goods sold, and spends $80,000 on operating expenses (mostly payroll):

• Gross burn = $10,000 + $80,000 = $90,000/month
• Net burn = $90,000 − $40,000 = $50,000/month

Investors will generally anchor on net burn, since it reflects the business as it actually operates. But gross burn is the stress-test number: if your revenue is concentrated — one customer representing 40% of collections, say — your net burn is one churn event away from looking like your gross burn. When revenue concentration is high, plan against gross burn and treat net burn as the optimistic case. Keep both numbers in view; they diverge exactly when it matters.

What Is Runway, and How Do You Calculate It?

Runway is your cash balance divided by your monthly burn — the number of months until the balance hits zero if nothing changes.

Runway = cash balance ÷ monthly net burn

Continuing the example: with $600,000 in the bank and $50,000 of monthly net burn, you have 12 months of runway. Run the same math on gross burn and you get 6.7 months — that's your runway if revenue goes to zero. The gap between those two numbers is a useful measure of how much your survival depends on revenue continuing to perform.

Two adjustments make the calculation honest:

Use net cash, not the raw bank balance. Subtract drawn credit lines, credit card balances, and other near-term obligations from cash on hand. $625,000 in the bank with $25,000 on cards is $600,000 of net cash. The bank balance overstates your position by exactly the amount you already owe.

Pre-revenue companies divide by gross burn. If you're not yet collecting revenue, there is no net burn distinct from gross — your runway is net cash divided by average monthly expenses. When revenue starts, recalculate on a net basis; the runway extension you see is the point of the exercise.

One more caveat: a single month's burn is a noisy input. Legal fees, annual insurance premiums, a laptop refresh — one-off items distort any single month, in either direction. Use a trailing three-month average for the denominator, and your runway estimate stops swinging with the invoice calendar.

How Do You Calculate Burn Rate on Accrual-Basis Books?

Here's where most founders' calculations quietly break. If your books are on a cash basis, your income statement approximates cash movement and the formulas above work as written. If your books are on an accrual basis — and if you're a SaaS company past your seed round, they should be — your income statement records revenue when earned and expenses when incurred, not when cash moves. Net loss and net burn are no longer the same number, and the difference can be large.

The reconciliation runs through the balance sheet: start with your net loss, add back non-cash expenses, then adjust for changes in working capital accounts.

Burn = net loss + non-cash expenses (depreciation, stock-based comp) ± changes in working capital

A SaaS example makes the mechanism concrete. Suppose you close a $120,000 annual contract, invoiced and collected upfront in January. Under ASC 606 you recognize $10,000 of revenue per month, with the balance sitting in deferred revenue:

• January: income statement shows $10,000 of revenue, but $120,000 of cash arrived. Your P&L dramatically understates how good the month was for cash — burn is $110,000 lower than the net loss suggests.
• February through December: the P&L shows $10,000 of monthly revenue against which no cash arrives. Your P&L now overstates cash performance every month, by $10,000, for eleven months.

A founder who computes burn off the income statement in this scenario will overestimate runway for most of the year — and the error compounds with every annual prepay you close. The same mechanics apply in the other direction to accounts receivable (revenue booked, cash not yet collected), prepaid expenses (cash gone, expense not yet recognized), accounts payable and accrued expenses (expense recognized, cash not yet gone), and inventory if you carry it.

The simplest cross-check requires no accounting at all: true burn is the month-over-month change in your bank balance, excluding financing inflows. If that number and your P&L-derived burn disagree materially, the difference lives in working capital — find it before it finds you. This reconciliation is a standard part of a monthly close; if nobody is doing it on your books, that's a signal worth acting on.

How Much Runway Should a Startup Have?

The honest answer is that it depends on stage, growth rate, and the fundraising climate — but "it depends" still resolves to usable thresholds:

• 12–18 months after closing a round is the working standard. It's enough time to hit the milestones that justify the next round, plus the 6+ months a raise actually takes.
• Below 12 months, runway extension should be on the management agenda: accelerate revenue, cut costs, or start the raise earlier than feels comfortable.
• Below 6 months, you are out of planning territory and into action territory — material cost reduction, a bridge, or both. Options narrow quickly from here, and they narrow further the longer you wait, because every acquirer, lender, and investor you approach can do this same math.

There's also an investor-perception dimension worth understanding. CB Insights' post-mortem research has consistently ranked running out of cash among the top reasons startups fail — investors know this, and a short runway reads as risk before anyone evaluates the product. Raising while simultaneously trying to execute the milestones the raise depends on is the position the 12-month threshold exists to prevent.

Can Burn Be Too High? Can It Be Too Low?

A high burn rate is not inherently a problem — it's a bet, and the question is whether the bet is underwritten. Cash converted into durable growth (sales capacity that pays back, product that compounds retention) is capital doing its job. Cash converted into expenses that don't move revenue or retention is just a shorter runway. The test isn't the burn number itself; it's whether each major expense line has a defensible return attached.

Burn can also be too low, which surprises founders the first time an investor raises it. If you raised a large round and are burning a fraction of plan, the capital isn't compounding — and investors funded a growth plan, not a savings account. Under-deployment usually signals either an inability to find productive uses for capital or a plan that was never realistic, and both are questions you'll answer at the next board meeting. As a general rule the burn number that's "right" is the one your model says buys the milestones — which is an argument for having a financial model that actually ties spend to outcomes.

Where Does Burn Actually Hide?

When founders go looking for their burn, payroll is never the surprise — it's typically 70%+ of a startup's cost base and everyone knows it. The drift accumulates in the lines nobody owns:

• Software subscriptions — SaaS sprawl is real; seats and tools outlive the people and projects that justified them. An annual audit of the vendor list against actual usage routinely recovers 10–20% of the software line.
• Prepaid annual contracts — cash leaves in month one while the expense amortizes, so cash-basis burn spikes invisibly against your P&L (the accrual mechanics above, working against you this time).
• Professional services, travel, insurance — lumpy, irregular, and rarely budget-owned, which is exactly why a trailing average and a monthly budget-versus-actuals review catch them and a single month's P&L doesn't.

Reducing burn deliberately — which levers to pull and in what order, without cutting the muscle that drives growth — is its own topic, covered in How to Reduce Burn Rate: 5 Levers to Extend Runway.

How Do You Extend Runway?

Every runway extension is one of three moves, and the sequencing matters:

Increase cash collected. Not just revenue — cash. Annual prepay incentives, tighter collection terms, and invoicing discipline extend runway even at constant ARR, because runway is a cash metric. Pricing is the most under-pulled lever here; if you haven't tested pricing in the last year, you're probably underpriced.

Decrease cash out. Run budget-versus-actuals monthly and investigate variances by asking why before cutting — an overage that's driving revenue is a reallocation candidate, not a failure. Cut the spend that isn't earning its return, and protect the spend that is.

Add capital. Equity, venture debt, or a credit facility. Each carries different dilution and covenant costs, and each gets cheaper the less you need it — which is the practical argument for starting the conversation at 12 months of runway rather than 6.

For companies operating on thin margins of error, the monthly runway calculation isn't enough resolution; a 13-week cash flow view shows the timing risk that a monthly average smooths over.

Burn Rate & Runway FAQ

Is burn rate the same as negative cash flow? Effectively yes — net burn is negative operating cash flow expressed as a monthly figure. The distinction that matters is that burn should exclude financing inflows: a fundraise raises your cash balance, but it doesn't mean your operations burned less.

Do pre-revenue startups have a net burn rate? Not meaningfully. With no cash inflows, gross and net burn are the same number, so runway is simply net cash divided by average monthly expenses. The gross/net distinction becomes useful the month collections begin.

How often should I recalculate runway? Monthly, as part of the close, using a trailing three-month average burn. Recalculate immediately after any step-change — a funding close, a major contract, a hiring wave — because runway is only as current as its inputs.

What's a "good" burn rate? There's no universal number — a good burn is one your runway supports and your growth justifies. A practical framing: with $600,000 in the bank and a 12–18 month target, your net burn ceiling sits between roughly $33,000 and $50,000 per month. Work backward from the cash you have, not forward from the spend you want.

Should I use my accountant's numbers or my bank balance? Both, and reconcile them. The bank balance tells you what happened to cash; accrual-basis books tell you why. When the two diverge, the difference is sitting in working capital — deferred revenue, receivables, payables — and understanding which one is driving it changes what you do next.

Getting the burn number right is bookkeeping. Knowing what it's telling you to do — when to cut, when to raise, when the model needs to change — is where a startup-focused finance partner earns its keep. If your P&L burn and your bank-balance burn don't match and nobody can tell you why, that's usually the place to start.