How long will my business cash last?
The formula everyone uses flatters you, because real revenue doesn't arrive in a straight line. Here is the honest way to work out your runway — and a simulation you can reproduce yourself in two minutes.
The straight-line answer (and why it lies)
The formula in every textbook is runway = cash ÷ (monthly costs − monthly revenue). If you have £80,000, spend £18,000 a month and bring in £12,000, you're burning £6,000 a month, so the answer is "about 13 months". Simple.
The problem: that calculation assumes next month's revenue equals this month's, every month, forever. Small-business revenue rarely swings less than 10–15% month to month. Volatility is not noise that averages out — a bad quarter early can end the business even when the averages say you were fine. Straight-line runway is the best case wearing a disguise.
What survival probability actually looks like
The honest question isn't "how many months of average burn do I have?" but "what fraction of the plausible futures keep my cash above zero for the horizon I care about?" That needs a simulation: thousands of possible revenue paths with your stated growth and volatility, counting how many survive.
A real worked example (run on this site's engine — deterministic, so you'll get the identical result):
cash £80,000 · revenue £12,000/mo · costs £18,000/mo · growth 2%/mo · volatility 15%/mo · horizon 12m
Straight-line says ~13 months. The simulation says the survival probability over 12 months is 60–70% — roughly one in three plausible futures runs out of cash — and the plausible runway range is 9 to 13 months. Same numbers, very different decision.
The three numbers that move the answer most
- Monthly costs. In the example above, ±20% on costs swings survival by more than any other input. Costs are also the number you control most directly — which is why cost discipline beats revenue optimism in a cash crisis.
- Revenue volatility. Two businesses with identical averages and different volatility have very different survival odds. If one customer is more than ~20% of revenue, your true volatility is lumpier than your spreadsheet shows.
- Growth assumptions. Compound monthly growth above ~5% is very hard to sustain for a year. If your survival verdict flips when you halve your growth assumption, the decision is resting on hope.
The environment you're operating in
For context, not destiny: 39.4% of UK businesses born in 2018 were still trading five years later (ONS Business Demography 2023). Roughly three in five don't make it to year five — which is exactly why watching your cash honestly matters more than feeling optimistic about it.
What no calculator can tell you: whether a specific customer will churn, whether that invoice will pay on time, or whether you'll land the contract. A good runway model doesn't predict these — it shows you how exposed you are if they go wrong, and refuses to answer at all when the inputs are guesses. LEXUN's engine does exactly that: it declines to run on unknowns rather than inventing values, and it shows every result as an honest range, never a false point.
Work out yours
The LEXUN runway analysis runs entirely in your browser — nothing leaves your device. Enter your cash, revenue, costs and volatility, mark which are facts and which are assumptions, and get a survival band, the drivers that move it, a stress scenario, and a frozen criterion so you can come back and check whether we were right.
Run the runway analysis →