Monte Carlo FIRE Calculator
Our other calculators assume a steady, unchanging return. This one doesn't — it runs your plan through thousands of randomized market scenarios, covering both your saving years and your retirement withdrawal years, to show your actual probability of success.
How this simulation works
Each of 3,000 simulated scenarios draws a different random annual return (based on the average return and volatility you set) for every year of your plan. During your working years, your monthly contributions are added on top of that year's return. Once you hit your retirement age, contributions stop and your annual spending is withdrawn instead — if a scenario's balance ever hits zero during retirement, that scenario counts as a failure. Your probability of success is the share of scenarios that never ran out of money. See our methodology page for where the default return, inflation, and volatility assumptions come from.
FAQ
What is a Monte Carlo simulation?
Instead of assuming a single fixed annual return every year, a Monte Carlo simulation runs your plan thousands of times, each time drawing a randomly varying return for every year based on typical market volatility. Some of those runs look like a great market decade, some look like a crash-heavy decade, most land somewhere in between. The percentage of runs where your money lasts is your "probability of success."
Why is this probability different from the other FIRE calculators on this site?
Our Coast, Barista, Lean, and Fat FIRE calculators all assume a steady, unchanging annual return — useful for a quick, simple estimate, but it hides "sequence of returns risk": a market crash early in retirement does far more damage than the same crash late in retirement, even if the average return ends up identical. This simulator models that risk directly by running many randomized scenarios covering both your saving years and your retirement withdrawal years.
What counts as a "safe" probability of success?
There's no universal cutoff, but many planners treat 85-95% as a reasonably safe target, with anything below ~70% worth revisiting (lower expenses, more savings, working longer, or a lower withdrawal rate). A 100% success rate isn't realistic to expect or require — it usually just means you're oversaving.
What return and volatility assumptions does this use?
By default, a 7% average annual real return with 15% standard deviation, roughly in line with long-run historical stock-heavy portfolio statistics — but these are modeling assumptions, not guarantees. Use "Show advanced options" on the return field to set your own nominal return and inflation assumptions, adjust volatility for a more bond-heavy mix, or see our methodology page for the full explanation of where these defaults come from.