Monte Carlo Investment Simulator

This tool projects how a recurring investment plan might play out by running many randomized market simulations instead of one fixed-rate guess. Set a starting balance, monthly contribution and its annual growth rate, an expected return and volatility, and how many years you plan to contribute. You can optionally add a drawdown phase, withdrawing a set amount each year for retirement or another goal, to see how likely the money is to last. Toggle "Compare a second scenario" to run a second, independent set of assumptions side by side against the first.

Each run simulates monthly returns drawn from a normal distribution built from your expected return and volatility, compounding the balance and adding contributions (or subtracting withdrawals) month by month across the full timeline. It repeats this hundreds or thousands of times, then reads off the 10th, 25th, 50th, 75th and 90th percentile balance at every year to build the shaded fan chart, and buckets the ending balances into a histogram. For a drawdown phase, it also tracks what share of runs never hit zero before the withdrawal period ends.

The model assumes returns are independent and normally distributed year to year, which is a simplification: real markets show clustering, fat tails, and periods of sustained under- or over-performance that this cannot reproduce. It also ignores fees, taxes, and any changes to your contribution or withdrawal amounts beyond the fixed growth rates you set. Treat the results as a way to compare assumptions and stress-test a plan, not as a forecast of what any specific portfolio will actually do.