This calculator is for informational purposes only and does not constitute financial advice. Results are estimates based on inputs you provide. Consult a qualified financial advisor before making financial decisions.
Retirement Planner
Visualize your 401k growth and plan for financial freedom.
S&P 500 average is ~10%
Historical average is ~3%
Total Savings (65)
$0
Total Interest
$0
Wealth Growth Projection
The Power of Time
Notice how the "Total Balance" curve (purple) starts to separate exponentially from "Your Contributions" (green) over time? That is compound interest working for you. Starting just 5 years earlier can double your retirement nest egg!
Retirement planning is fundamentally a question of probability, not certainty. Historical market returns vary widely year to year, so a single-return projection can be wildly optimistic or pessimistic. A Monte Carlo simulation runs thousands of scenarios and tells you the realistic probability that your savings last as long as you do.
How Monte Carlo and the 4% rule work together
The 4% rule says you can withdraw 4% of your starting portfolio in year one of retirement, then adjust for inflation each year, with a high probability of not running out over 30 years. The number comes from the Trinity Study, which back-tested historical sequences. Monte Carlo extends this idea: instead of assuming average returns, it randomly samples from historical volatility, runs the scenario 1,000+ times, and reports the percentage of trials in which your money lasts.
When to use this calculator
Use it any time you're about to make a major retirement decision: choosing a target retirement age, deciding whether to take Social Security at 62 vs 67, evaluating whether you can afford to retire now. A success rate above 85% is generally considered safe; below 75% means you're under-saved or planning to over-spend, and you have decades of compounding to fix that if you act early.
Worked example
A 40-year-old with $200,000 saved, contributing $1,500/month, planning to retire at 65 with $80,000/year of expenses (in today's dollars). Monte Carlo with a 60/40 portfolio shows roughly an 85-90% success rate over 30 years of retirement. Drop the contribution to $750/month and the success rate falls below 70% — a meaningful gap that's invisible in a single-return projection.