Monte Carlo retirement simulation runs 1000 randomized scenarios using actual market volatility — not just a fixed average return. By sampling annual returns from a normal distribution, it shows the realistic spread: the optimistic 90th percentile, the median 50th percentile, and the cautious 10th percentile, so you can see what might happen if markets behave unusually well or poorly.
Simulation Parameters
Portfolio Value — Percentile Bands
| Year | Phase | 90th %ile | Median | 10th %ile |
|---|
How to Use the Monte Carlo Retirement Simulator
Most retirement calculators use a fixed average return — say 7% every year — which looks reassuring on paper but doesn't reflect reality. Real markets deliver 25% gains some years and 35% losses in others. The Monte Carlo retirement simulator accounts for this by running 1000 different market scenarios and showing you the range of outcomes, from the cautious 10th percentile to the optimistic 90th percentile.
Step 1: Enter Your Accumulation Phase Details
Start with your current savings — this is your starting portfolio value. Enter your monthly contribution, the amount you plan to add each month until retirement. Set the slider for years to retirement; if you're 45 and plan to retire at 65, that's 20 years. The more you accumulate before retirement, the wider your margin of safety in the simulation.
Step 2: Set Your Retirement Withdrawal and Duration
Enter your planned annual withdrawal — the amount you'll spend each year in retirement. A common rule of thumb is 4% of your final portfolio value, but you can enter any amount. Set the retirement duration slider; 30 years covers most retirements from age 65 to 95. Longer durations increase the risk of outliving your money.
Step 3: Calibrate Return and Volatility Assumptions
The expected annual return and volatility are the key parameters for the Monte Carlo simulation. The S&P 500 has historically returned about 10.5% annually with a standard deviation of 15-17%. After accounting for 3% inflation, real returns average around 7.5%. For a diversified 60/40 stock/bond portfolio, use 6-7% return with 10-12% volatility. The simulator uses the Box-Muller transform to sample returns from a normal distribution — each year's return is randomly drawn from that distribution across 1000 simulations.
Step 4: Read the Percentile Bands
The chart shows three bands: the 90th percentile (optimistic — only 10% of simulations beat this), the 50th percentile (median — exactly half did better), and the 10th percentile (pessimistic — 90% did better than this). The vertical dashed line marks your retirement year. The success rate stat shows what percentage of the 1000 simulations didn't run out of money — aim for 80-90%+ for a secure retirement plan.
Step 5: Toggle Inflation Adjustment and Export
Enable "Show real (inflation-adjusted) values" to see what your portfolio buys in today's dollars — this is more useful for retirement planning than nominal values. Use the Export CSV button to download the year-by-year percentile data for your own analysis or to share with a financial advisor. The collapsible table shows exact values for every year across all three percentiles.
FAQ
Is this Monte Carlo retirement simulator free?
Yes, completely free with no signup required. All 1000 simulations run locally in your browser using JavaScript — your financial data never leaves your device.
What is a Monte Carlo retirement simulation?
A Monte Carlo simulation runs thousands of retirement scenarios using randomly sampled market returns drawn from a normal distribution. Instead of assuming a fixed annual return, it accounts for the volatility of real markets — sometimes great years, sometimes terrible ones — and shows you the range of possible outcomes.
What does the 10th, 50th, and 90th percentile mean?
The 90th percentile shows the optimistic scenario — 90% of simulations did worse than this. The 10th percentile shows the pessimistic scenario — only 10% of simulations did worse. The 50th percentile (median) is the middle outcome. These three bands show the realistic spread of retirement outcomes.
What is the success rate percentage?
The success rate shows what percentage of the 1000 simulations still had money at the end of your retirement duration. A 90% success rate means 900 out of 1000 simulated retirements survived to the end without running out of funds.
What return and volatility values should I use?
The S&P 500 has historically returned about 10.5% annually with a standard deviation of about 15-17%. After inflation (3%), real returns average around 7.5%. For a diversified stock/bond portfolio, 7% return and 12-15% volatility are reasonable. More conservative portfolios (60/40) might use 6% return and 10% volatility.
How is the Box-Muller transform used?
The Box-Muller transform converts uniformly random numbers into normally distributed random numbers. Each year's simulated return is sampled from Normal(mean return, standard deviation), producing the realistic range of annual returns that reflects actual market behavior rather than a fixed average.
How does inflation adjustment work in this simulator?
When the nominal/real toggle is enabled, the chart shows real (inflation-adjusted) portfolio values alongside nominal values. Real purchasing power is calculated by discounting nominal balances by cumulative inflation each year, showing what your money will actually buy in today's dollars.
Does past S&P 500 performance guarantee future results?
No. The disclaimer shown in the tool is important: historical S&P 500 average return of ~10.5% does not guarantee future results. This simulator is an educational tool to understand the range of possible outcomes, not a prediction. Adjust your assumptions conservatively when planning.