Understanding Volatility %

In the Monte Carlo section of The Path to Sound Retirement you will see an input labeled Volatility %. The default value is 15%. This single number has a large influence on the range of possible outcomes the simulation produces. Understanding what it represents helps you use the tool more effectively-and more honestly.

Volatility is not a prediction of next year’s market. It is a measure of how much returns are expected to bounce around an average. When you change it, you are changing how wide a set of futures the model explores. That choice is yours to make consciously.

What Volatility Actually Means

Volatility is expressed as a standard deviation of annual returns. In plain language:

A volatility of 15% means that in roughly two-thirds of years, the return will fall within ±15% of the mean return you assumed.
About 95% of years will fall within roughly ±30% of the mean (two standard deviations).

If you assume a 7% mean return and 15% volatility, many individual years might land between about −8% and +22%. Some will be worse; some better. The model uses that dispersion to generate thousands of different sequences.

Higher volatility creates wider swings-both good and bad years become more extreme. Lower volatility produces smoother, more predictable paths. Neither setting is “correct” in the abstract. The right question is whether the number roughly matches the kind of portfolio you actually hold.

Why the Default Is 15%

Historically, a diversified portfolio of U.S. stocks has shown annualized volatility in the 15-20% range over long periods. A 60/40 stock/bond mix has often landed lower, closer to 10-12% in many multi-decade windows. A concentrated or all-equity portfolio can run higher.

A 15% assumption is therefore a reasonable starting point for a stock-heavy retirement portfolio. If your portfolio is more conservative (higher bond allocation, more cash buffers), you might lower the number. If it is more aggressive or concentrated in a few sectors or individual stocks, you might raise it.

Do not lower volatility simply because you dislike seeing lower success rates. That is adjusting the thermometer instead of the temperature.

How Changing Volatility Affects Your Results

When you increase volatility in the calculator:

The success rate usually falls because more extreme bad sequences become possible
The gap between the 10th and 90th percentile outcomes widens dramatically
The sample paths chart shows much more dispersion

Lowering volatility has the opposite effect: higher success rates and tighter outcome ranges. This is why the tool lets you experiment-small changes in assumptions can reveal how sensitive your plan is to market turbulence.

Example: Suppose a plan shows a 92% success rate at 15% volatility. At 12% volatility the success rate might rise to 97%. At 18% it might fall to the mid-80s. Those differences are information about fragility, not a reason to pick the number that feels best.

Practical tip: Run your plan at 12%, 15%, and 18% volatility and compare the success rates and percentile outcomes. This sensitivity analysis is often more informative than any single number. Write down the results so you remember what the plan can tolerate.

Volatility and Sequence of Returns

Volatility and sequence-of-returns risk are related but not identical. Volatility describes how wide the year-to-year swings are. Sequence risk describes the damage that occurs when bad swings arrive early in retirement while you are withdrawing.

A higher volatility setting makes harsh early sequences more likely in the simulation. That is useful. It forces the model to test whether your spending level can survive the kind of markets that actually happen, not only the average path.

If your plan only looks strong at low volatility, you have learned something important: the plan is sensitive to turbulence. The responsible response is to examine spending, other income, cash reserves, or work horizon-not to permanently dial volatility down and declare victory.

Matching Volatility to Your Real Portfolio

Ask a few practical questions:

What is my approximate stock/bond mix?
Am I holding broad index funds or concentrated positions?
Do I have a multi-year cash or short-term bond buffer that reduces the need to sell stocks in a downturn?
How did my actual portfolio behave in 2008-2009, 2020, or 2022?

If you lived through a 30-40% drawdown with an all-equity portfolio and stayed the course, a 15-18% volatility assumption is not unrealistic for modeling. If you hold a large bond allocation and rebalance, a lower figure may fit better. The goal is congruence, not optimism.

A Simple Exercise in Accountability

Set aside fifteen minutes with the calculator:

Enter your best current estimate of portfolio, spending, other income, and years.
Set mean return to a moderate assumption (for example 6-7%).
Run Monte Carlo at 12% volatility. Record success rate and 10th-percentile ending value.
Run again at 15%. Record the same metrics.
Run again at 18%. Record them.
Look at the pattern. Does a modest increase in assumed turbulence collapse the plan, or does it hold?

If the plan holds, you have evidence of resilience. If it collapses, you have early warning while you still have levers: spending, savings rate before retirement, work horizon, or guaranteed income. Ignoring the warning is a choice. So is acting on it.

What Volatility Does Not Tell You

Volatility does not forecast the next crash. It does not measure your personal capacity to stay invested. It does not replace emergency savings, debt management, or a written spending plan. It is one input among several.

People sometimes treat a low-volatility assumption as a form of protection. It is not. It is a modeling choice. Protection comes from adequate savings, flexible spending, diversified holdings you understand, and the discipline to continue contributing through ordinary market cycles.

Volatility Is Not the Same as Risk You Cannot Survive

Portfolio volatility measures price fluctuation. Personal risk includes the chance that you sell at the bottom, stop contributing, or increase spending just as markets fall. A 15% volatility assumption in the model is useful only if your real behavior can tolerate the drawdowns that number implies.

If a 30% portfolio decline would cause you to abandon the plan, your true risk is higher than the simulation shows-because the simulation assumes you keep following the rules. Build cash buffers, keep spending flexible, and only hold an equity-heavy mix you can stick with. The model cannot supply courage; it can only show what courage (or its absence) would cost.

Closing

Volatility % is the dial that controls how stormy the simulated futures become. Use a number that roughly matches the portfolio you actually own. Then vary it deliberately so you can see how sensitive your retirement plan is to wider swings.

The teacher’s hope is not that you find a volatility setting that always produces green results. The hope is that you learn what your plan can and cannot tolerate-and that you take ownership of the adjustments that follow.

Peace is not the absence of volatility in markets. It is the presence of a plan that has been tested against it.

This article is for educational purposes only and is not financial advice. Always consult a qualified advisor for decisions about your personal situation.