Retirement Monte Carlo Simulator
Stress-test your retirement plan against hundreds of simulated market histories. Instead of assuming one smooth average return, the simulator draws a different random sequence of good and bad years for every path and reports how often your pot survives the full horizon.
New to safe withdrawal rates? Read the 4% rule guideYour numbers
Your invested pot on day one of retirement.
A fixed real (inflation-adjusted) amount taken at the start of every year.
After inflation. Long-run global equities have historically averaged roughly 5% real; that is history, not a promise.
Equity-heavy portfolios have historically sat around 10-18% annual volatility; bond-heavy mixes lower. Illustrative, not a forecast.
More paths smooth the percentile bands; the story rarely changes.
Same inputs, same simulation: identical settings always reproduce exactly the same paths, so your results are stable across reloads and shares. Re-roll draws a fresh set of random market histories.
What happens to my data?
Success rate
85.6%
Median ending balance
£1,047,823
10th percentile ending
£0
Worst decile runs dry by
Year 27
Across 1,000 simulated retirements, 144 ran out of money before year 30. The unluckiest 10% of paths had already failed by year 27. Figures are illustrative, not financial advice.
Balance fan chart
The spread of outcomes across every simulated path, year by year. Half of all paths end inside the dark band.
| Year | 10th percentile | Median | 90th percentile |
|---|---|---|---|
| 10 | £539,920 | £1,011,993 | £1,798,623 |
| 20 | £245,594 | £1,080,299 | £2,488,990 |
| 30 | £0 | £1,047,823 | £3,879,269 |
The average return is not the story. The order of returns is.
Two retirees can earn identical average returns and end up in completely different places, because the one who hits a crash in the first five years is selling cheap units to fund withdrawals that never get the chance to recover. That is sequence of returns risk, and it is exactly the spread this fan chart makes visible: every path here has the same expected return, and they still fan out into fortunes and failures. Our sequence of returns risk article walks through why the first decade of retirement carries most of the danger, and the 4% rule guide covers where the classic withdrawal rate came from and where it creaks.
What Monte Carlo does and does not do
This simulator draws each year's return independently from a normal distribution (i.i.d. normal real returns). Real markets are messier: crashes are more frequent and more violent than a normal distribution allows (fat tails), bad years cluster together, and returns partly depend on the valuations you start from. It also ignores fees, taxes, state or Social Security pensions, and the entirely human response of spending less in a crash. Treat the output as a stress test of a rigid plan, not a prediction. It is illustrative and it is not financial advice.
How the simulation works
Each simulated retirement starts with your pot, takes the full withdrawal off the balance at the start of every year, then applies one random real return drawn from a normal distribution with your chosen mean and volatility. If the balance cannot fund a withdrawal, that path has failed and stays at zero for the rest of the horizon.
The success rate is simply the share of paths that funded every withdrawal. The fan chart sorts every path's balance at every year and plots the 10th, 25th, 50th, 75th and 90th percentiles, so you can see the whole distribution rather than one trajectory.
Everything runs on a seeded random number generator, which is why the same inputs always produce the same simulation. The re-roll button steps the seed to draw a fresh set of market histories; if the success rate swings noticeably between rolls, increase the number of simulations.
Frequently asked questions
What does the success rate actually mean?
What return and volatility should I use?
Why do I get exactly the same result every time?
What does Monte Carlo simulation miss?
How does this relate to the 4% rule?
Do I need a 95% or 100% success rate?
Related reading
Sequence of returns risk
Why the order of returns, not the average, decides whether a retirement survives.
The 4% rule
Where the classic safe withdrawal rate came from, and the small print it carries.
Drawdown Calculator
The single-path version: model one retirement with inflation and state pension income.
FI Number Calculator
Work out the pot size you need before this simulator has anything to stress-test.