Monte Carlo simulation

Where the FIRE calculator plots a single deterministic line, Monte Carlo runs your plan thousands of times with randomised annual returns to give you a probability of success.

How to use it

1. Pick up the inputs from the FIRE calculator (or set them here).
2. Set the volatility (standard deviation of annual returns). 15% is a reasonable default for a global equity portfolio.
3. Optionally enable black-swan events — sporadic large drawdowns sampled from a fat-tail distribution. Use this if you want to stress-test the plan against a 2008-style year.
4. Set the number of runs. More runs = smoother distribution. 1000 is a good starting point; the math runs in the browser so feel free to push to 10,000+.

Reading the chart

• The shaded band is the spread between the 5th and 95th percentile outcomes.
• The center line is the median.
• The percentage at the top is the share of runs that ended at or above your FIRE target — your success rate.

What to look for

• A success rate below ~80% means your plan is fragile to bad sequences of returns. Lower your withdrawal rate, save more, or extend the horizon.
• A very wide band means your plan is highly sensitive to luck. Diversifying across asset classes (see the Asset allocation manager) is one way to narrow it.