What Monte Carlo actually tells you
A single baseline projection uses one assumed return path — useful for cash-flow storytelling, weak for uncertainty. Monte Carlo redraws market returns and inflation many times, runs the same household spending plan through each draw, and counts how many of those simulated paths funded spending through your planning age (age 95 on this free page).
The free calculator runs 100 traditional paths and highlights weaker (P10), median (P50), and stronger (P90) outcomes. Phrasing matters for YMYL trust: we report the share of modeled paths that reached the planning age — not that “your retirement is X% likely to succeed.”
Monte Carlo does not answer “how long will my money last under one return?” or “how much can I spend under fixed assumptions?” Those are Money Longevity and Spending Capacity. Use those first to pick a candidate lifestyle, then come here to pressure-test it.
What this calculator assumes
Means, volatility, floors/caps, and the success definition come from the same Monte Carlo engine used in the account tools (defaults such as roughly 8% mean market return with material volatility, and inflation noise around a 2.5% mean — always confirm on the methodology page if parameters change). This public page keeps a simplified household: three investment buckets, illustrative Social Security, contributions until retirement, and plan through age 95. See complete Monte Carlo methodology.
Traditional mode samples independent yearly shocks. Account tools can also bootstrap historical years or blocks; those modes are reserved for the logged-in experience so this page stays fast and clearly scoped.
Reading P10 / P50 / P90 without overconfidenceing
P10 is a weaker ending portfolio among the saved paths; P90 is stronger; P50 sits in the middle. A plan can show a high path-success share and still have an uncomfortable P10 ending balance — or the reverse. Look at both the success count and the path trio. If P10 fails early while the headline share looks fine, spending or retirement age may need another pass through Spending Capacity or Longevity before you trust the stress test.
How this page differs from our other free tools
- Historical calculator — same household idea, but real past sequences instead of random draws.
- Sequence of returns — teaching demo of order risk; not a full plan engine.
- 2-Minute Stress Test — baseline + Monte Carlo + historical together with a broader verdict.
- In-app Monte Carlo — 500 / 5,000 runs, advanced parameters, bucket toggles, fuller path explorer.
Important limitations
One hundred paths is a sketch, not a research paper. Simplified Social Security is illustrative. Taxes are not modeled. Results depend on the return model you did not personally calibrate on this page. Educational what-if projections only — not advice.
Tour Jim & Susan’s sample plan to see Monte Carlo in a full household context, or create a free account to run higher path counts on your actual accounts.