An organization we worked with was, on paper, in great shape. Revenue was strong and growing. If you did the math the way most organizations do it, cash in the bank divided by monthly spend, they had comfortable runway and nothing to worry about.
The problem was that none of that revenue was actually cash yet.
Their funding came through reimbursements, and reimbursements were tied to spend, not to the revenue they'd recognized. Before they could submit for a single dollar, internal staff time and external expenses had to be coded correctly first. Then the request had to be submitted, reviewed, and paid. Start to finish, that cycle ran several weeks. So while the income statement looked healthy, the bank account was living well over a month in the past, and the runway number everyone was quoting was describing a company that didn't quite exist.
This is the thing about runway: the number is easy to calculate and easy to get wrong. The math takes ten seconds. The assumptions behind it are where founders fool themselves.
Here's how we think about it.
The Simple Version (and When It's Enough)
The basic runway formula is exactly what you've seen everywhere:
Runway (in months) = Cash on hand ÷ Net monthly burn
Two things worth getting right even in the simple version. First, use net burn, not gross. Net burn is what you're actually losing each month after revenue, not just what you're spending. Second, use a real average, not your best month. Three months of actual net burn averaged together beats a single cherry-picked figure.
The simple version is genuinely enough in one situation: when your cash conversion is fast and predictable. If you're a SaaS business collecting on subscriptions, money hits your account close to when you recognize it, and there's no meaningful lag between "we earned it" and "we have it." In that world, cash on hand divided by net burn tells you something true.
If that's you, run the simple math, keep an eye on it monthly, and don't overcomplicate it.
The Real Version (and Why Most Companies Need It)
The simple formula quietly assumes that revenue equals cash. For a lot of businesses, that assumption is false, and the gap between the two is exactly where runway gets misjudged.
The fuller version doesn't just divide two numbers. It builds forward, month by month:
- Starting cash for the month
- Plus cash actually collected, not revenue earned, but money that will genuinely land in the bank that month, timed to when you'll collect it
- Minus cash actually going out, including the lumpy, easy-to-forget items: annual software renewals, quarterly tax payments, that one big contractor invoice
- Ending cash, which becomes next month's starting point
You do that across the next 6 to 12 months, and your runway is the month where ending cash crosses zero. It's more work, but it's the version that reflects reality when there's any lag between earning money and holding it. Our 13-week cash flow forecast template is the starter version of this same model, rolling weekly instead of monthly.
You need this version if any of the following is true: you invoice clients and wait to get paid, your revenue is seasonal or lumpy, you depend on reimbursements or milestone payments, or you have large periodic expenses that don't show up in a smoothed monthly average. In other words, most companies that aren't pure subscription.
The reimbursement case above is the extreme version, but the same principle applies to a professional services firm waiting on net-60 invoices or an ecommerce brand with cash tied up in inventory. Recognized revenue and available cash are not the same thing, and runway is a cash question.
The Two Ways Founders Inflate Their Own Runway
Almost every "we have more time than we do" miscalculation we see comes down to one of these, often both at once.
1. Being too bullish on revenue projections
Optimism is a founder's job in most contexts, but it's a liability in a runway model. When the forecast assumes deals close on schedule, growth continues on trend, and nothing slips, the runway looks great, right up until the quarter comes in light and the number you were relying on turns out to have been a hope, not a plan. Build the runway model on conservative, defensible revenue, then stress-test it: what does runway look like if revenue comes in meaningfully below plan?
2. Counting revenue before it's collected
This is the one that got the organization above, and it's more common than people think. Booking revenue when it's earned is correct accounting, but runway doesn't spend accrued revenue, it spends cash. If you're treating a signed contract, an issued invoice, or an approved reimbursement as though the money is already available, you're overstating your runway by however long your collection cycle takes. For some businesses that's a couple of weeks. For the organization above it was well over a month.
Put those two mistakes together and you get a company that thinks it has twelve months and actually has six.
What to Do When the Runway Is Shorter Than You Thought
The instinct, when the real number comes in lower than expected, is to start cutting immediately. That's usually the wrong first move. Here's the order we actually work through.
1. Fix the forecast before you cut anything
You can't make good decisions off a number you don't trust. Before touching costs, get to a runway figure that reflects real collection timing and honest revenue assumptions. Cutting based on a bad model can do as much damage as the shortfall itself, you might slash something that mattered to solve a problem that was partly a measurement error. With the organization above, the first thing we did was build a more robust projection model so everyone was working from reality, not from the rosy version. This is exactly the work our FP&A services are built for.
2. Attack collections before you attack costs
Very often the cash is already yours, it just hasn't arrived yet. Before cutting anything, look at what's sitting in accounts receivable and how fast you can pull it in. For the organization above, the highest-leverage move wasn't spending less; it was submitting for reimbursement more often. We increased how frequently they drew down funds and stood up a cross-functional process so expense coding, the program side, and finance were aligned and reimbursements could go out faster. Same revenue, dramatically better cash position, without cutting a thing. If you invoice clients, the equivalent is tightening payment terms, invoicing on completion instead of monthly, or simply chasing what's overdue. Tight AR discipline usually starts with a clean accounting function.
3. Then cut, specifically, not across the board
If you still need to reduce burn after the forecast is honest and collections are tightened, cut with a scalpel, not a hatchet. Across-the-board percentage cuts feel fair and are almost always the wrong tool: they starve the things that are working to protect the things that aren't. Look at spend line by line, and cut what isn't driving the business forward while protecting what is. We wrote a longer piece on this called How to Reduce Cash Burn Without Killing Growth.
One Last Thing
Here's the part most founders don't want to hear: your runway number is only as good as the day you last updated it. It isn't a figure you calculate once at a board meeting and quote for the next quarter, every closed deal, every slipped payment, every new hire moves it. The founders who get surprised aren't bad at math. They calculated runway correctly in March and then kept trusting that number in June, after the company underneath it had changed.
Treat runway as something you revisit every month against what actually happened, not what you hoped would. The math takes ten seconds. Doing it honestly, on a real cash basis, every month, that's the discipline that keeps you from waking up with half the runway you thought you had.
The Bottom Line
At Finative, building projection models and cash forecasts that founders can actually trust is a core part of what we do. Our fractional CFO and FP&A teams live inside this problem. If your runway number feels more like a guess than a plan, get in touch and we're happy to help you get to one you can stand behind.