How to Calculate Startup Runway and Burn Rate (UK Guide)

Knowing how to calculate startup runway is the single most useful piece of financial hygiene an early-stage founder can have. Runway tells you how many months of cash you have left at your current spending, and burn rate tells you how fast that cash is leaving. Get both right and you can time a fundraise, cut costs before it is too late, and answer the first question almost every UK investor asks. This guide shows the calculations, a worked example, and the benchmarks investors expect in 2026.

What runway and burn rate actually mean

Burn rate is how much cash your company spends each month. Runway is how long that cash will last before you run out. They are two sides of the same coin: runway is simply your cash balance divided by your monthly burn. Both are measured in real cash leaving the bank, not accounting profit, because a startup can be loss-making on paper for years yet stay alive as long as there is money in the account.

How to calculate your burn rate

There are two versions, and the difference matters:

Gross burn is your total monthly operating spend: salaries, software, rent, marketing, everything going out.
Net burn is gross burn minus the cash revenue you bring in. It is the figure that actually shrinks your bank balance.

For runway, always use net burn. A simple way to calculate it is to take your cash balance at the start of a month and subtract the balance at the end; the fall is your net burn for that month. Because a single month can be lumpy (an annual software bill, a one-off invoice), most founders average the last three months to get a stable figure.

How to calculate startup runway

The formula is straightforward:

Runway (months) = current cash balance ÷ average monthly net burn.

If you hold £300,000 and burn £25,000 a month net, you have twelve months of runway. The discipline is in keeping the inputs honest: use the cash actually in the bank (not committed grants or unsigned term sheets), and use a net burn that reflects your real recent spending rather than an optimistic budget. If your revenue is growing fast, you can model a declining burn, but treat that as a scenario, not the headline number you rely on.

A worked example

Say a London SaaS startup has £480,000 in the bank. Over the last three months its gross burn averaged £70,000 and it collected £22,000 a month in customer cash. Net burn is £70,000 minus £22,000, or £48,000 a month. Runway is £480,000 divided by £48,000, which is ten months. That ten-month figure changes everything: it means the founders should be opening fundraising conversations now, because a UK seed round routinely takes three to six months to close, and you never want to be raising with only weeks of cash left. If instead they grew revenue to £40,000 a month, net burn would fall to £30,000 and runway would stretch to sixteen months.

What counts as a healthy runway

The rule of thumb most investors use in 2026:

Below 6 months: danger zone. You are either raising now or cutting hard.
12 to 18 months: comfortable working runway; enough to hit milestones and raise without panic.
18 to 24 months: what a fresh seed or Series A round should buy you. Investors size a round to give roughly this long.

The concept of being “default alive” is worth internalising: a company is default alive if, on its current growth and burn, it would reach profitability before the cash runs out. If you are default dead, you are relying on the next raise to survive, and that dependence weakens your position in every negotiation.

How to extend your runway

When runway is tight, the levers are limited but reliable:

Cut net burn: the fastest lever is usually the biggest cost, which for most startups is payroll, followed by paid marketing that is not converting.
Pull revenue forward: offer annual contracts paid upfront, or incentivise quarterly-in-advance billing, to bank cash sooner.
Delay non-critical spend: defer hires and tooling that do not move your key milestone.
Use non-dilutive finance carefully: R&D tax credits, grants and revenue-based finance can bridge a gap without giving away equity. The UK government’s business finance support pages list schemes worth checking.

Every month of runway you buy is a month of leverage in your next raise, which is why founders track it weekly, not quarterly.

Runway metrics investors actually check

Beyond the headline number, sophisticated investors look at efficiency. The burn multiple, net burn divided by net new annual recurring revenue added in the same period, has become a standard test: under 1 is excellent, 1 to 1.5 is good, and above 2 signals you are spending a lot to grow a little. Pair your runway with a clear view of the milestone it buys (a revenue target, a product launch, a key hire), because runway with no plan attached does not reassure anyone. For the wider set of numbers investors expect, see our guide to the SaaS metrics founders should track, and how these feed into startup valuation at pre-seed and seed.

Frequently asked questions

How do you calculate startup runway?

Divide your current cash balance by your average monthly net burn. For example, £300,000 in the bank with a £25,000 monthly net burn gives twelve months of runway. Use cash actually in the account and an average of the last three months of burn.

What is the difference between gross burn and net burn?

Gross burn is total monthly spending. Net burn is gross burn minus the cash revenue you collect. Net burn is the number that reduces your bank balance, so it is the one to use for runway.

How much runway should a startup have?

Most investors want to see 12 to 18 months of working runway, and a seed or Series A round is usually sized to provide 18 to 24 months. Below six months is a danger zone that forces you to raise or cut quickly.

What does “default alive” mean?

A startup is default alive if its current growth and spending would carry it to profitability before the cash runs out. If it would run out first, it is default dead and depends on raising again to survive.

What is a good burn multiple?

Burn multiple is net burn divided by net new annual recurring revenue added. Under 1 is excellent, 1 to 1.5 is good, and above 2 suggests you are spending heavily for modest growth.

How often should founders track runway?

Weekly or at least monthly. Cash position and burn can move quickly with new hires, big invoices or a slow sales month, so checking often gives you time to act before runway becomes critical.