Monte Carlo Retirement Planner

It actually does not change much. Monte Carlo runs thousands of random retirements; the percentage is just an average of how many succeeded. The difference between two runs of 5,000 simulations is typically less than 1%. If you want the most stable estimate, bump simulations to 10,000. If the number swings by 5%+ between runs, that's a sign your scenario is on a knife edge and the inputs need attention.

Most financial planners target 85-90%. Below 70% is widely considered "high risk." Above 95% means you are almost certainly underspending and could safely enjoy more of your money. The Trinity Study, which produced the 4% rule, used 95% as its success bar over 30 years: modern planning has generally relaxed to 85% to balance security against overshooting savings.

It's called sequence-of-returns risk. When you draw down a portfolio that just lost 30%, you sell more shares at the low to cover the same spending: permanently locking in losses that compounding would otherwise have recovered. The average return across your retirement could be identical; the order matters enormously. This is why the early years of retirement are the most fragile, and why bond ballast is more valuable then than later.

With caveats, yes: it's how institutional retirement planning software works. A fixed horizon assumes you live to the worst case in every simulation, which over-saves for the average person. The Gompertz curve here is calibrated to SSA-style life tables. Caveats: it does not adjust for your individual health, family history, or income (longevity correlates with both). If you have strong reason to expect a longer-than-typical lifespan, stay on fixed horizon and set the age higher.

Spending and Social Security inflate separately. The spending-inflation slider grows your annual expenses each year. The SS-COLA slider grows your benefit each year, typically slower. Over 30 years, even a 0.5% gap between the two compounds into a significant real-dollar drag on the portfolio. The default 3.0% spending vs 2.5% SS-COLA reflects historical Social Security underadjustment relative to broad inflation.

Target ending balance is the minimum portfolio you want at the end (horizon or death), in today's dollars (it is inflated with your spending-inflation rate before the comparison). Set $0 if you just need the money to last; set higher for a legacy or late-life-care cushion. Floor is much stricter: a run is marked a failure if the portfolio ever drops below this level at any point in retirement, even if it recovers. Use floor when you need a guaranteed minimum cushion (e.g., $50k always available for emergencies).

Several real-world effects: taxes (the portfolio is pre-tax; in reality, withdrawals from a 401k are taxed as income, and Roth/brokerage have different rules); required minimum distributions (RMDs from age 73+); healthcare shock costs (the spending curve is a smooth average, not the lumpy reality of a nursing-home year); long-term care insurance; home equity (HELOCs or reverse mortgages as backstops); part-time income in early retirement; and changing risk tolerance as you age. Use this as a stress test, not a withdrawal plan.