Runway and burn rate: how UK startups should think about the numbers

Runway is the number of months a company can keep operating at its current rate of spending before it runs out of cash, and it is one of the most consequential numbers a founder tracks, because it determines how much time is available to hit the milestones a next funding round, or profitability, depends on. It is calculated by dividing the cash a company currently holds by its burn rate, the net amount of cash being spent each month, and while the arithmetic is simple, the judgement calls involved in getting a genuinely useful number are less so.

What is the actual formula, and what commonly gets it wrong?

Runway equals cash on hand divided by monthly net burn: a company with £300,000 in the bank and a net burn of £25,000 a month has roughlyundefinedmonths of runway. The most common way this goes wrong in practice is treating a single month’s figure, especially a recent unusually good or bad one, as representative, when spending and revenue both tend to be lumpy month to month, particularly around annual contract renewals, one-off costs such as equipment or legal fees, and seasonal patterns in revenue. It is worth calculating runway from an average of the last three to six months of actual cash movement, and checking it against a forward-looking budget rather than relying purely on the trailing average, since planned near-term costs, a new hire starting, a lease renewal, a tax payment, can materially shorten runway even if the trailing average looks healthy.

What is the difference between gross burn and net burn?

Gross burn is total monthly cash outgoings, payroll, rent, software, marketing, everything the company spends, without netting off any revenue. Net burn subtracts monthly revenue from gross burn to show the actual net cash outflow, and it is net burn, not gross burn, that should be used in the runway calculation, since it reflects what is genuinely happening to the company’s cash balance each month.

	What it measures	When it is the more useful number
Gross burn	Total monthly cash outgoings, before any revenue is netted off	Very early-stage companies with little or no revenue, where gross spend is the more honest picture of cost discipline
Net burn	Monthly outgoings minus monthly revenue	Companies with meaningful revenue, and the correct figure to use for calculating runway

For a pre-revenue or very early-revenue company, gross and net burn are close to the same number anyway, so the distinction matters more as a company starts generating meaningful revenue and net burn begins to diverge, hopefully favourably, from gross burn.

Are there UK-specific factors that complicate the simple formula?

Yes, a few. VAT is collected and paid on a cycle that does not always match when the underlying sales or costs hit the profit and loss account, so a VAT-registered company’s cash position in any given month can look better or worse than its underlying trading performance, and it is worth modelling VAT payments separately rather than assuming cash in the bank today reflects trading profitably. PAYE and National Insurance payments to HMRC, and corporation tax payments, are similarly periodic rather than smoothly spread, and can cause a noticeable cash dip in the specific months they fall due even when average monthly spending looks stable. R&D tax credit claims, where a company is eligible, can also distort the picture, since the cash benefit often arrives as a lump sum well after the R&D spend it relates to was incurred, meaning a company’s true underlying burn can look worse in the months before a claim is paid than the eventual full-year picture suggests. None of this changes the basic runway formula, but it means a single month’s cash movement, in the UK specifically, can be a misleading substitute for a properly modelled cash flow forecast.

How much runway should a company actually be aiming to have?

There is no universally correct number, since the right answer depends on the company’s stage, how predictable its revenue is, and how quickly it could realistically close a funding round if needed, but a widely cited rule of thumb among founders and investors is to start actively fundraising with at least six months of runway remaining, and ideally closer to nine to twelve months, rather than waiting until cash is critically low. Fundraising processes routinely take longer than founders expect, and negotiating from a position of genuine cash pressure tends to produce worse terms than negotiating with time to spare, so treating a specific runway threshold as the trigger to start a process, rather than deciding once the number is already uncomfortably low, is generally the safer approach.

Is a high burn rate automatically a bad sign?

Not necessarily. A high burn rate that is clearly buying measurable growth, more revenue, a larger customer base, better retention, is a very different situation from a high burn rate with little to show for it, and investors generally distinguish between the two rather than treating burn as inherently negative. The more useful question than “is burn too high” is usually “what is this spending actually buying, and is that trade genuinely worth it at this stage of the company’s life,” which is a judgement call rather than a fixed rule, and one that shifts as a company matures and is expected to demonstrate increasing capital efficiency alongside growth.

What should a founder actually track month to month?

Beyond the headline runway figure, it is worth tracking gross burn and net burn separately so that revenue growth or decline does not mask an underlying change in spending discipline, and tracking runway on both a trailing-average and a forward-budget basis so that planned near-term costs are reflected rather than only historical averages. It is also worth revisiting the runway calculation whenever a material change happens, a new significant hire, a large contract won or lost, rather than only reviewing it monthly on a fixed schedule, since a single large change can shift the picture meaningfully before the next scheduled review would have caught it.

How should burn rate be tracked alongside other startup metrics?

Burn rate and runway are rarely useful looked at in complete isolation from a company’s other core metrics, since a given burn rate means something very different for a company whose revenue is growing quickly and predictably than for one with flat or unpredictable revenue. It is worth tracking burn rate alongside the growth rate it is buying, sometimes summarised as a “burn multiple”, broadly how much net new cash is being spent for each pound of net new revenue generated, since a high burn rate paired with strong, efficient growth reads very differently to an investor than the same burn rate paired with slow or stalled growth. It is also worth distinguishing burn tied to genuinely fixed costs, such as core payroll and rent, from burn tied to more discretionary, scalable spending, such as paid marketing, since the latter can typically be throttled back relatively quickly if runway needs extending, while the former generally cannot without more disruptive decisions such as redundancies.

The practical takeaway

Runway and burn rate are simple in formula but easy to get wrong in practice if they rely on a single, unrepresentative month or ignore UK-specific cash timing quirks such as VAT and PAYE cycles. Because the consequences of running genuinely low on cash, a rushed, poorly negotiated fundraise or worse, are serious, it is worth building a proper rolling cash flow forecast, rather than a single trailing-average figure, and reviewing it regularly with an accountant or finance adviser as the company grows.