Cash runway calculator — with the odds, not just a number
A cash runway calculator normally does one division: cash in the bank divided by net monthly burn. Here is that number first, because it is what you came for. Then the same figures run through 5,000 simulated months, because the one number quietly assumes your burn rate never varies — and it always does.
The formula, and the number it gives
That is the whole of the plain calculation, and it is not wrong — it is incomplete. It treats burn as a constant. Real revenue lands late, dips in a quiet month, spikes on one invoice; costs step up when a supplier reprices. Run the same figures across 5,000 futures with month-to-month variation in them and the answer stops being a point and becomes a range: in this example the median future runs out in month 12, one future in ten by month 9, and the probability of still being above zero at twelve months is 45–55%. The 13-month figure was never the odds; it was one path out of five thousand. Already comfortable with distributions? The Monte Carlo cash flow simulator is the same engine with its assumptions in front; for a person rather than a business, the savings runway calculator asks the same question of a salary and a savings buffer.
Run it on your figures
GENUINELY UNCERTAIN
45–55%
probability the balance stays above zero over 12 months, across 5,000 simulated futures
- Months before cash runs out — 1 in 10 futures
- 9
- Median future
- 12
- Best 1 in 10
- 12+
- Scenarios — Expected
- Expected 48%
- Upside
- Upside 67%
- Downside
- Downside 28%
- Stress
- Stress 12%
- What moves it most
- Monthly costs
- Where the odds flip
- The median path reaches the horizon at or above 0.3%/month revenue growth; you stated 0%.
- Evidence quality
- moderate
Seed 564 · receipt rw_371c588a_234 · runway@1.0.1. Same figures, same seed, same answer — reproduce it or verify the receipt.
Worked example, computed by the engine at build time. Edit the figures and it recomputes here, in your browser.
The worked example above is the engine’s own output at seed 564, written into this page when the release was built; the receipt beside it lets anyone re-run the same figures and get the same answer. The full workspace at the decision simulator adds the things a landing page cannot: the fact-or-assumption labels on every input, the stress case, the sealed record and, later, the outcome. If you want the reasoning behind the model before you trust its number, read the cash runway guide; if you want every run kept and scored, see plans and pricing.
What the range means
Three percentiles and one probability. The 10th percentile is the month by which one in ten simulated futures had already run out of cash — your bad-but-plausible case, not a catastrophe. The median is the middle future. The 90th percentile is the best one in ten; “12+” means that future never ran out inside the horizon. The headline probability is simply the share of all 5,000 futures still above zero at the horizon. None of these is a forecast that something will happen; each is a count of what happened across the futures the model could generate from your figures, and the model’s blind spots — one revenue stream, independent months, no seasonality — are listed in the guide.
Questions people ask
How do you calculate cash runway?
The textbook formula is cash divided by net monthly burn. If you hold £130,000 and burn £10,000 a month, it returns 13 months. That figure assumes your burn never varies, which it always does.
Why does LEXUN give a range instead of one number?
Revenue and costs move month to month. LEXUN runs 5,000 futures with that variation included and reports the 10th, 50th and 90th percentile outcomes, plus the probability cash stays above zero.
Is the cash runway calculator free?
Yes. There is no account and no card. The simulation runs in your browser and your inputs stay on your device: this page has no Copilot box, so nothing you type here is sent anywhere.
What counts as a healthy cash runway?
There is no single healthy number, and this site does not publish one. Runway only means something against the horizon you have to survive: the next funding event, the next contract, the season. The useful question is the probability of hitting zero before that date, which a single-number calculator cannot show you.