The first time I sat in on a board meeting for a startup I was working with, someone asked how much runway they had left. The founder said "about eight months." The CFO said "five." They were both looking at the same bank balance but calculating burn rate differently. The founder was using gross burn — total cash out — against the cash on hand without adjusting for the revenue already coming in. The CFO was using net burn, subtracting monthly revenue from monthly costs. The difference was three months of runway, which in startup terms is the difference between making it to the next fundraise and not. Understanding which number you are talking about matters before you start any conversation about cash.
Gross Burn vs Net Burn — Not the Same Thing
Gross burn is total cash outflow each month: salaries, rent, software subscriptions, contractor payments, marketing spend, and every other cost. If the business spends £80,000 per month on operations, gross burn is £80,000. Net burn is gross burn minus cash revenue — use a burn rate calculator to track both figures side by side as costs change each month. If the same business is bringing in £30,000 per month from customers, net burn is £50,000. Runway calculations should use net burn: if there is £400,000 in the bank and net burn is £50,000, the runway is eight months. Using gross burn in this example would give five months — an accurate picture of how fast cash is leaving, but not a useful measure of how long the business can sustain itself, because it ignores the revenue that partially offsets costs.
Runway: What It Tells You and What It Does Not
Runway is the number of months of cash remaining at the current burn rate — a startup runway calculator lets you model how headcount changes or revenue growth shifts the timeline before those decisions are made. It answers a specific question — how long before we run out of money if nothing changes — but it is a static calculation in a dynamic situation. A startup growing revenue quickly will see its net burn decline over time (and eventually flip to positive if revenue exceeds costs). A startup that recently expanded headcount will see burn accelerate in coming months even if the current number looks manageable. Runway should be calculated not just on the current monthly burn but on the expected burn in two or three months' time, once known upcoming costs are incorporated. Hiring two engineers next month, for example, increases next month's burn immediately — the runway calculation should reflect that rather than extrapolating from this month's cost base.
Why Burn Rate Needs to Be Justified by Progress, Not Just Managed
Burn rate is not inherently bad. Spending money is how a startup invests in growth: hiring product people to improve the product, hiring sales people to acquire customers, spending on marketing to build awareness. The relevant question is not whether burn is high but whether it is producing results commensurate with its cost. A startup spending £100,000 per month and adding £40,000 in new monthly recurring revenue each month is deploying capital effectively. A startup spending the same amount and adding £5,000 in MRR has a problem that is not really about the burn rate — it is about the efficiency of the spending. Investors reviewing burn look at growth metrics alongside the number: burn per new customer acquired, cost to acquire £1 of ARR, and the payback period on customer acquisition spend.
The Most Common Mistake: Assuming Next Round Is Closer Than It Is
Fundraising takes longer than founders expect. A seed-to-Series A process that founders budget six weeks for often takes four to six months from first meeting to money in the bank. This has practical consequences for burn rate decisions. The conventional guidance is to begin fundraising when you have six months of runway remaining — not three. At three months, you are in a position where investors know you are running out of options, which affects the terms you can negotiate. At six months, you have genuine optionality: you can walk away from unfavourable term sheets, run a competitive process, and choose your investors rather than accepting whoever will move quickly. The burn rate decisions you make today — on headcount, office space, and marketing spend — determine when you hit that six-month threshold.
Reducing Burn Without Destroying Progress
When burn needs to come down, the temptation is to cut proportionally across every department. In practice, this tends to slow progress in every direction simultaneously without fixing the underlying issue. A more effective approach is to identify which spending is directly attributable to revenue growth and which is not. Headcount in sales and product development is usually growth-linked spending; office space, management overhead, and discretionary tooling often are not. Software subscriptions accumulate in early-stage companies faster than their actual usage justifies — a periodic audit of active subscriptions against actual usage typically surfaces meaningful savings. Paid marketing spend is easier to reduce quickly than headcount, though the growth impact of cutting it depends heavily on the channel's efficiency and lead time to results.
Burn Rate and the Pressure It Creates on Decision-Making
There is a psychological dimension to runway that is worth acknowledging. When burn rate is high and runway is short, decisions get made faster than they should. A founder with three months of cash remaining will accept terms from investors they would have rejected at six months. They will hire quickly to fill gaps rather than waiting for the right person. They will launch before the product is ready. Low runway compresses time horizons and produces short-term decisions in situations that require long-term thinking. Managing burn rate conservatively in the early stages is partly a financial strategy and partly a strategic one: keeping runway long preserves the ability to make decisions from a position of optionality rather than desperation.