What a Monte Carlo retirement simulation actually tells you

Each trial draws a sequence of yearly investment returns (and often inflation) from a statistical model calibrated to asset behavior, then plays your entire plan through it: contributions, retirement date, Social Security, taxes, RMDs, spending. One trial might retire you into a 2008; another into a roaring decade. Repeating this 1,000 times maps the distribution of outcomes your plan could face — which is exactly what a single 'average return' projection hides.

Quality varies enormously between tools, and the differences are mostly invisible: whether taxes are modeled year-by-year or as a flat haircut, whether Social Security rules are real or approximated, whether returns are drawn with realistic volatility. Two tools can give the same portfolio success rates 15 points apart. Ask what's inside — a good tool will tell you (deterministic engines with published assumptions exist for exactly this reason).

'Failure' in a simulation rarely means destitution — it means the modeled plan needed adjustment before the horizon: trimming spending, downsizing, or leaning harder on Social Security. A 90% success rate is better read as 'in 1 of 10 futures, you'd need a mid-course correction.' Real retirees correct course; simulations that assume you'd rigidly spend into ruin overstate true risk.

That's also why chasing 99% is usually a mistake: the last few points of modeled certainty are bought with permanently lower spending, guarding against futures that adaptive behavior would handle anyway. Many planners consider 75–90% a healthy operating range, treated as a dashboard gauge — not a one-time grade.

A success rate compresses everything into one number; the value is underneath:

• How much can I safely spend? — the spending level that keeps success in your comfort band (guardrails turn this into a live monthly range).
• What drives my risk? — sensitivity analysis reveals whether your plan hinges on returns, spending, longevity, or claiming age.
• When would trouble show up? — fan charts show whether weak futures fail at 78 or 92, which changes how you'd respond.
• What does the fix cost? — the point of simulation is comparing interventions: retire 9 months later, trim $300/month, claim at 68 — each with its own success delta.