You run the simulation, and a number appears: 85% success. Now what? Is that good? Should you be celebrating, or worried? And what exactly is the other 15%?
A Monte Carlo result is one of the most useful numbers in retirement planning, but only if you know how to read it. Here is what the score actually means, why landing in the 80s is usually the goal rather than 100%, and how to treat the whole thing as a living plan you steer over time instead of a one-time verdict.
A Monte Carlo simulation runs your retirement plan through hundreds or thousands of possible futures. Each run uses a different randomized sequence of market returns and inflation, drawn from the statistical behavior of historical markets. In some runs the market is kind to you early on. In others it drops 30% the year you retire and limps along afterward.
The success rate is simply the percentage of those runs in which your money outlasted you. An 85% result means that in 85 out of 100 simulated lifetimes, you still had money left at the end of your plan. In the other 15, the portfolio ran dry before the finish line.
That framing matters. The number is not a grade on a test and it is not a prediction of what will happen. It is a measure of how much margin your plan has against bad luck. If you want the full mechanics, our Monte Carlo explainer walks through how the thousands of runs are generated.
The one-sentence version: Your success rate is the share of simulated futures in which you do not run out of money. It measures resilience, not destiny.
The instinct is to chase 100%. More is better, right? Not here. A score in the 80s, roughly 80% to 90%, is what most planners consider the healthy target, and there is good reasoning behind it.
To reach a 99% or 100% success rate, the simulation has to survive even the worst sequences in tens of thousands of runs, including crashes deeper than anything in recorded history. The only way a plan clears that bar is by spending very little, retiring very late, or dying with a large unspent portfolio. A 100% score is usually a sign that you are underspending your own retirement. You traded years of travel, time, and freedom for a safety margin you will almost certainly never need.
When your plan shows 85%, the 15% of runs that fail are generally the genuinely awful scenarios: a severe crash in your first few years combined with stubbornly high inflation and below-average returns for a decade. These are real risks worth respecting, but they are also the scenarios where you would obviously change your behavior in real life. Nobody keeps spending on autopilot while their portfolio is cut in half. Which leads to the most important point in this article.
A plan in the 80s is strong enough to weather ordinary volatility but honest enough that it is not built on extreme self-denial. It assumes you will make small course corrections along the way, which is exactly what real retirees do. That is the difference between a static forecast and a living plan.
| Success Rate | What it usually signals |
|---|---|
| 95–100% | Likely underspending. Room to spend more, retire earlier, or give/leave more. |
| 80–90% | The healthy target. Resilient to bad markets, with built-in room to adjust. |
| 65–80% | Workable but watch closely. Small changes now meaningfully improve the odds. |
| Below 65% | The plan needs real adjustment to spending, timing, or savings. |
Here is the mental shift that makes Monte Carlo genuinely useful: the success rate is not a final score stamped on your retirement. It is a snapshot of one set of assumptions on one particular day. Markets move, your spending changes, tax law shifts, and your goals evolve. The plan is meant to move with them.
Think of it the way a pilot thinks about a flight plan. The route is set before takeoff, but the pilot is constantly making small adjustments for wind, weather, and traffic. The destination does not change; the path bends to stay on course. A retirement plan works the same way. You set it, then you revisit it, ideally once a year and after any major financial change, and you nudge the inputs to reflect reality.
This is why the same plan can read 85% today, 78% after a rough year, and back to 84% the following year without anything going wrong. The number breathes because your life and the markets breathe. The goal is not to lock in a perfect score once. It is to keep the plan in a healthy range as conditions change.
The most common fear is a long stretch of below-average returns early in retirement. This is the real danger, often called sequence-of-returns risk, and it is exactly the scenario hiding in that bottom tail of failed runs. But a living plan has an answer that a static withdrawal rule does not: it adjusts.
If weak returns persist and your success rate starts drifting down, you have several levers, and you rarely need to pull more than one or two:
The point is not that you will definitely have to do these things. It is that the option exists, and the plan can show you in advance how much each lever is worth. When you model a temporary spending cut and watch the success rate climb back into the 80s, you are seeing the plan do its real job: not predicting the future, but showing you the steering wheel.
This is the whole idea behind the comparison and insights tools in the calculator. You can build a second scenario, dial spending down or push a retirement date, and watch the two success rates side by side. You can also ask the tool to solve for you: the maximum spending that keeps you in your target range, or the earliest retirement age that still clears it. Instead of guessing how much a change is worth, you see the points it adds or removes. That turns a vague worry about bad markets into a concrete, ranked list of moves.
The success rate is the headline, but the fan chart underneath it tells the fuller story. The wide band of outcomes shows the range of portfolio paths across all the runs: the optimistic top edge, the median line down the middle, and the worrying bottom edge. Two plans can share the same 85% score while having very different shapes. One might cluster tightly around a comfortable median; another might have a high median but a long, thin tail of severe failures.
Pay attention to the median path, the line where half of outcomes land above and half below. That is a more realistic picture of your likely retirement than either the rosy top or the scary bottom. And remember that a single bad early run on the chart is not a forecast. It is one of the futures your plan is being stress-tested against, which is exactly the point.
A Monte Carlo success rate is a measure of resilience, not a prophecy. Aim for the 80s, not 100%, because a score in that range means your plan is strong enough to handle bad markets without forcing you to underspend the life you saved for. Most important, treat the number as living. Revisit it each year, and when returns disappoint, pivot with one or two small, deliberate adjustments to bring the odds back up.
If you want a starting point for the spending side of this, our guide to the 4% rule and safe withdrawal rates pairs naturally with the simulation. Then open the calculator, run your own numbers, and try moving a lever or two to see how your plan responds. That, more than any single score, is how you build a retirement plan you can trust.
Run a Monte Carlo simulation on your real numbers, then adjust spending, timing, and Social Security to watch your success rate move.
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