Use the inputs below to run a Monte Carlo simulation based on real (inflation-adjusted) returns. The results show the probability your portfolio lasts through retirement.

Assumes contributions and withdrawals occur at year end with real (inflation-adjusted) returns.

Success Probability --
Failure Probability --
Median Ending Balance --
5th / 95th Percentile --

Simulation Distribution

Each dot represents a Monte Carlo simulation and its ending portfolio value.

Y-axis:
Simulations Median 95th percentile

Historical Backtesting

How would your plan have performed if you started in different years of history? Each line shows your portfolio value over time for a different starting year (1871–present).

Historical Success Rate --
Periods Tested --
Worst Starting Year --
Best Starting Year --
Best case Median Worst case Retirement

Y-axis uses logarithmic scale to show the full range of outcomes. Uses 80% stocks / 20% bonds allocation with annual rebalancing. Historical data from Robert Shiller's S&P 500 dataset (1871–2023).

Assumptions & Notes

  • Monte Carlo simulation: This calculator runs 10,000 independent simulations, each generating a unique sequence of random annual returns. For each simulation, your portfolio grows during the accumulation phase (contributions + returns), then draws down during retirement (spending - returns). A simulation "succeeds" if your portfolio lasts through retirement; the success probability shows what percentage of simulations succeeded.
  • Returns are real (inflation-adjusted) and modeled as independent yearly draws from a normal distribution using the expected return and a fixed volatility.
  • Contributions and withdrawals are applied at year end.
  • Spending is constant in real terms during retirement.
  • Taxes, fees, and one-time cash flows are not included.
  • Volatility is set to 12% based on long-run historical data for a diversified stock-heavy portfolio and is not shown as an input.
  • Historical backtesting: Uses annual real (inflation-adjusted) stock and bond returns from 1871–2023, derived from Robert Shiller's S&P 500 dataset. The portfolio is modeled as 80% stocks and 20% bonds, rebalanced annually.
  • Sequence of returns risk: Historical backtesting captures how the order of returns matters—retiring just before a major crash (like 1929 or 1966) produces very different outcomes than retiring during a recovery.
  • Limitations: Past performance does not guarantee future results. These models assume you can stay invested through market crashes, don't account for behavioral factors, and ignore the impact of Social Security, pensions, or other income sources.

Disclaimer: This calculator is for educational and illustrative purposes only and does not constitute financial advice. Results are based on simplified models and assumptions that may not reflect your actual situation. Please consult a qualified financial advisor before making any investment or retirement decisions.