Monte Carlo Simulator

The Investment Return Calculator answers how much: one smooth projection at a fixed return. This one answers how likely: it runs 1,000 random markets at your return and volatility and shows the full range of outcomes. Same inputs, a fundamentally different question. The dashed line here is exactly what that calculator would draw.

Each iteration is one possible future, built month by month from random returns centred on your expected return with your volatility. With 1,000 of them, the share that hit a target, or run dry, becomes a meaningful probability, and the percentile bands show the spread. More paths give a smoother estimate; 1,000 is plenty for a stable read.

That gap is volatility drag. Compounding punishes a down year more than an equal up year rewards you, so once randomness is included the typical (median) outcome falls below the naive constant-return projection. The more volatile the portfolio, the wider that gap. It is the single most useful thing this tool shows.

A simulated market survives if the portfolio never hits zero before your horizon ends, after taking the monthly withdrawal you set. Survival probability is the share of the 1,000 markets that last the distance. We also show the year the portfolio runs out in the unlucky paths, so you can see how early failure tends to strike.

Neither. The simulation explores the consequences of your assumptions; it does not predict the future. Real markets can fall outside any model, returns are not perfectly random, and crises cluster in ways a simple model misses. Treat the probabilities as a structured way to compare plans, not a forecast.

The figures are nominal and before tax, like most projection tools. For after-fee, after-tax and inflation-adjusted modelling of a single path, pair this with the Investment Return Calculator. Here the focus is squarely on the range of outcomes that volatility produces.

100% is not on offer; chasing it just means oversaving and retiring later than needed. If you can trim spending in bad years, 85 to 90% is a sound target, and this tool colours results green from 85%. With a rigid budget or a very long horizon, aim for 90 to 95%. Below about 75% the plan is leaning on luck: lower the withdrawal, add contributions or extend the horizon. A failed simulation path is a plan adjustment in real life, not ruin.

Because randomness compounds: each year's surprise multiplies everything that came before, so uncertainty stacks instead of averaging out. The spread grows roughly with the square root of time, so ten years out it is about three times the one-year spread, not ten times. That is also why long horizons remain plannable at all: the range of final amounts widens in dollars, while the range of annualised returns narrows.

Yes. In retirement mode, enter $1,000,000, a $3,333 monthly withdrawal (4% a year), 30 years and a 7% return. At 12% volatility, roughly a 60/40 portfolio, survival probability comes out around 96%; at 16% volatility (all stocks) about 87%. Raising the withdrawal to 5% ($4,167) at 12% volatility drops it to about 86%. One caveat: the classic rule raises withdrawals with inflation, while this simulation holds them flat, so it flatters the result slightly.

For a broad stock portfolio, long-run nominal returns have averaged around 7 to 10% a year; the 7% default with 16% volatility is a reasonable planning stance. A 60/40 mix sits nearer 5 to 6% with 10 to 12% volatility. Always subtract your fund fees. If you would rather think in today's purchasing power, enter a real return instead, roughly 4 to 5% for stocks, and read every dollar on screen as inflation-adjusted.

Switch to Retirement mode, then enter your starting portfolio, expected return and volatility, the monthly withdrawal you plan to take, and the year it begins. The tool runs 1,000 markets and reports survival probability: the share in which the money lasts your full horizon without ever hitting zero. Because it draws a fresh return every month rather than a smooth average, a Monte Carlo retirement simulation captures sequence-of-returns risk, the danger that a slump in the first few years of withdrawals drains a portfolio that a calmer-ordered but identical-average market would have sustained. Watch the median depletion year on the failing paths to see how early trouble tends to strike, and if survival sits below about 85%, test a smaller withdrawal or a later start.

Yes to both. The simulator is completely free, needs no signup or download, and runs entirely online in your browser, so your figures never leave your device. Every mode, all 1,000 paths, the percentile fan chart and the probability read-outs are available with no paywall, trial or usage cap. Signing in is optional: it only prefills the inputs from your own accounts and is never required to run a simulation.

Treat it as a disciplined way to compare plans, not a forecast. Each percentile describes this model's assumptions, not a promise: the 5th is a bottom-5% run of the 1,000 simulated markets, the median a coin-flip outcome, the 95th a top-5% run. The results are only as trustworthy as the return and volatility you enter, and the model assumes both stay fixed and that each month is independent. Real crises cluster and arrive more often than a bell curve implies, so a genuinely bad decade can fall outside even the 5th percentile. Read the low percentiles as your planning margin, plan to the 10th or 25th rather than the median, and keep defences the model cannot see, such as a cash buffer and the willingness to trim spending in a downturn.