Cash Flow Runway: The Number That Decides Whether Your Business Survives

Plenty of businesses that close down were profitable on paper the month they ran out of money. That sounds like a contradiction until you separate two things that get casually treated as the same: profit and cash. Profit is what you earned over a period once revenue and costs are matched up. Cash is what is actually in the account on the day a supplier, HMRC or your payroll provider expects to be paid. The gap between the two is timing — you invoice in March, get paid in May, but the wages, rent and software all go out in March and April regardless. A business can be growing and still lose that race. Runway is the number that tells you how much time you have before you do.

Burn rate: gross versus net

Burn rate is simply how fast cash leaves the business. It comes in two flavours and the difference matters.

Gross burn is everything that goes out in a typical month — salaries including your own drawings, rent, software subscriptions, contractors, loan repayments, the lot. It is the total outflow, ignoring anything coming in.

Net burn subtracts the revenue you actually collect that month. If you spend £12,000 and bank £4,000, your net burn is £8,000. Net burn is the figure that governs how long your balance lasts, because revenue genuinely received slows the rate the bank account drains even when it does not fully cover costs. The cash flow runway calculator works on net burn for exactly that reason. Watch gross burn too, though: it is your bare-survival number if revenue ever stopped tomorrow.

Calculating runway, with a worked example

The arithmetic is deliberately simple. There is no clever model hiding behind it, and you should be suspicious of anyone who makes it sound complicated.

Runway in months = cash on hand ÷ monthly net burn.

Take a small UK business with £30,000 in the bank, monthly expenses of £12,000, and £4,000 of revenue it reliably collects each month. Net burn is £12,000 − £4,000 = £8,000. Runway is £30,000 ÷ £8,000 = 3.75 months — roughly sixteen weeks. That is the whole planning horizon. Every decision about hiring, a new tool, a marketing push or raising money has to fit inside those sixteen weeks, or it has to be the thing that extends them.

One honest caveat: "cash on hand" means money you could genuinely spend today, not invoiced work you are owed. Invoiced is not collected. A £15,000 invoice sitting at 60 days does not extend your runway by a single day until it clears, and a customer going quiet can turn a comfortable position into an urgent one without anything else changing.

What a healthy runway actually looks like

Treat the common figures as heuristics, not laws. A widely used rule of thumb is to keep at least six months of runway, with twelve to eighteen months considered comfortable, especially for a funded business. Under three months is usually a signal to be making decisions now rather than next quarter.

The reason these are heuristics is that the right floor depends on how predictable you are. A business on recurring contracts with steady costs can operate safely on less than one with lumpy project income, seasonal swings or a single large customer. Volatility is the variable the rules of thumb cannot see, and only you can. Knowing your fixed-cost base — which the break-even calculator helps you pin down — tells you how hard the floor really is.

The levers that extend it — and their costs

There are only four ways to buy more runway, and each has an honest trade-off.

• Cut burn. The fastest and most reliable lever, because it is entirely within your control. The cost is real: cuts to people or marketing can slow the growth you need to escape the problem in the first place.
• Accelerate collections. Tighter terms, deposits, faster invoicing and chasing late payers convert money you have already earned into runway. It is often the cheapest lever and the most overlooked. The limit is that you cannot collect revenue that does not exist yet.
• Grow revenue. The healthiest fix long term, but the slowest and least certain. It rarely arrives fast enough to solve a short runway on its own.
• Raise financing. A loan or investment buys time, but a loan adds fixed repayments that increase future burn — model them with the business loan repayment calculator before counting on it. Funding only counts once the cash is actually in your balance, not when it is promised.

The limits of a steady-burn model

A runway figure assumes burn is roughly constant. Reality is lumpier: a quarterly VAT bill, an annual software renewal, a seasonal dip or a one-off equipment purchase will all shorten real runway below the headline number. The model is a planning tool, not a guarantee. Re-run it whenever your monthly pattern changes, and stress-test it by asking what happens if your largest customer pays a month late.

Next steps: work out your own number with the cash flow runway calculator, then re-run it with a pessimistic revenue figure to see your true floor. None of this is financial advice — it is a way to make the decision with clearer eyes.