Simulador de Monte Carlo
Run 1,000 possible markets instead of one tidy projection. See the full range of where a portfolio could land, the odds of reaching a goal, or whether a retirement survives. The probabilistic companion to our Investment Return Calculator.
1The market
$
% / yr
%
years
2Contributions
$
Added at the end of each month
Median outcome after 25 years1,000 simulations
872.781
Half of the 1,000 simulated markets ended above this, half below. Your total contributed was 325.000.
Worst case - 5th336.439
Poor - 10th382.793
Strong - 90th2.024.833
Best case - 95th2.676.389
Range of outcomes over time
5th-95thMedianNaive projection
$2.78M
$2.09M
$1.39M
$696k
$0
Now4y8y12y16y20y24y25y
Volatility drag
A standard calculator, with no volatility, would project 1.087.765 (the dashed line). Once the swings of a real market are included, the median outcome is 872.781, about 20% lower. That gap is the cost of risk a single-line projection hides.
Total contributed325.000
Embed on your site Last updated: June 2026
How to use this calculator
From a question to a range, in four steps.
1
Pick a question
Choose a mode at the top: project growth, test a retirement withdrawal, or find the odds of hitting a target. The mode decides which extra inputs appear and what the headline answer means.
2
Set the market
In card 1, enter your starting portfolio, the expected annual return, the annual volatility, and your time horizon. Volatility is the dial that widens or narrows the cone of outcomes.
3
Add the cashflows
In card 2, set the contributions, withdrawals or target for your chosen mode. We prefill these from your accounts when you are signed in.
4
Read the range
The fan chart shows the spread of 1,000 simulated markets. Read the headline probability or median, scan the best and worst case percentiles, and hover the chart for any year.
Key concepts
Reading a distribution of futures.
Why a range, not a number
A normal calculator assumes your return arrives smoothly every year. Real markets do not: the same average can produce wildly different paths. A Monte Carlo simulation runs thousands of those paths so you see the spread of what could actually happen, not one tidy line.
Geometric Brownian motion
Each simulated month applies a random return drawn from your expected return and volatility, compounding on the last balance. Run it 1,000 times and the outcomes fan out into a distribution. It is the standard model for how asset prices wander.
Volatility drag
Because losses hurt more than equal gains help when compounding, the median outcome sits below the naive projection at the same average return. The dashed line on the chart is that naive projection; the gap to the median band is the drag your calculator never showed you.
Percentiles, best to worst
The 95th percentile is a top-5% market; the 5th is a bottom-5% one. The bands show where the middle 50% and 80% of outcomes land. Planning to the median alone ignores how bad the unlucky paths get, which is exactly what the percentiles reveal.
Sequence-of-returns risk
In retirement, the ORDER of returns matters as much as the average. A crash early in withdrawals can drain a portfolio that an identical-average but better-ordered market would have sustained. Survival probability captures this; a single projection cannot.
Survival vs success probability
In retirement, survival probability is the share of markets where your money lasts the full horizon without hitting zero. In goal mode, success probability is the share where you finish at or above your target. Both turn a vague hope into a concrete number.
Tips for using the odds
Turning probabilities into plans.
Plan to a low percentile
Building a plan that works at the 10th or 25th percentile, not the median, is what keeps it intact when markets disappoint. The median is a coin-flip; the lower bands are your margin of safety.
Mind the volatility input
Volatility moves the answer more than people expect. A 60/40 portfolio (about 11%) gives a far tighter cone than an all-stock one (about 17%) at the same average return. Set it to match what you actually hold.
Stress-test retirement early
Run retirement mode with a withdrawal that starts in year one and watch survival probability. If it is below about 85%, a lower withdrawal or a later start usually buys a large jump in safety.
Contributions beat timing
Raising the monthly contribution lifts the entire fan, including the worst-case bands. It is a far more reliable lever than hoping for a higher return, which simply widens the cone.
Re-run as life changes
A simulation is a snapshot of today's assumptions. Re-run it when your portfolio, horizon or spending shifts, rather than trusting a number you generated years ago.
Perguntas Frequentes
How is this different from the Investment Return Calculator?+
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.
What do the 1,000 iterations actually do?+
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.
Why is the median below the dashed line?+
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.
What counts as survival in retirement mode?+
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.
Are these returns guaranteed or predictions?+
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.
Does it account for inflation or tax?+
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.
Each of the 1,000 paths applies monthly returns drawn from your expected return and volatility (geometric Brownian motion), compounding on the running balance, with contributions or withdrawals applied monthly. Results are nominal and before tax, use a fixed seed for reproducibility, and assume returns are independent month to month. Markets are not perfectly random and the future may differ; treat the probabilities as illustrative, not financial advice.
1,000-path geometric Brownian motion at your return and volatility - nominal, pre-tax, illustrative, not advice