In 1994, financial planner William Bengen published a study in the Journal of Financial Planning that would become the most referenced piece of research in retirement income planning. His finding: a retiree who withdrew 4% of their initial portfolio in year one, then adjusted that dollar amount annually for inflation, had never run out of money over a thirty-year retirement across any historical sequence going back to 1926.
That finding became the 4% rule. It was immediately misapplied. Bengen's research was descriptive, not prescriptive. It identified a historically safe starting withdrawal rate for a specific portfolio, time horizon, and set of historical conditions. It was not a universal prescription that every retiree with any portfolio size can safely spend 4% per year indefinitely.
Thirty years of subsequent research has refined, challenged, and contextualized Bengen's original work. Some researchers argue the safe rate is now lower given current valuations. Others argue that dynamic withdrawal strategies make the specific starting rate less important. What everyone agrees on is that retirement spending decisions are among the most consequential a retiree makes, and that the 4% rule, understood properly, is a useful starting point and a dangerous ending point.
This paper covers how Bengen's research actually works, what the subsequent research says, why current conditions complicate the standard application, the alternatives to a rigid withdrawal rate, and how to think about sustainable spending in the context of your specific plan.
Retirement spending decisions are irreversible in a way that investment decisions are not. You can change your asset allocation next year. You can't un-spend money from last year. A retiree who sets their initial withdrawal rate too high early in retirement, before the portfolio has demonstrated whether it can sustain that rate, is making a commitment that compounds badly if markets disappoint.
The conditions that made 4% historically safe, moderate starting valuations, strong bond returns, and a thirty-year horizon, are not uniformly present today. As of the mid-2020s, the cyclically adjusted price-to-earnings ratio on U.S. equities has spent extended periods at levels historically associated with lower forward returns. The bond market went through its worst decade in modern history in the 2010s followed by a sharp repricing in 2022. Long-term real bond yields have recovered but remain uncertain.
None of this means the 4% rule is wrong. It means the 4% rule is a historical observation about a specific set of conditions, and those conditions vary. Understanding how withdrawal rate sustainability actually works, rather than applying a single number reflexively, is what separates thoughtful retirement planning from retirement planning by shortcut.
Bengen's original 1994 research examined fifty-year rolling historical periods starting in 1926. For each starting year, he tested what initial withdrawal rate, as a percentage of the starting portfolio, could be sustained for at least thirty years without depleting the portfolio. The portfolio assumed a mix of approximately 50% stocks and 50% intermediate-term government bonds.
The worst historical starting year in Bengen's analysis was 1966, which coincided with the beginning of a period of high inflation, poor equity returns, and weak bond performance. Even in that worst-case scenario, a 4% initial withdrawal rate survived the full thirty years. No historical starting year produced a failure at 4%. At 5%, some historical sequences failed. This established 4% as the floor of historical safety across all tested sequences.
Bengen himself has noted in subsequent publications that his research was intended to identify a conservative floor, not an optimal rate. In most historical sequences, a retiree starting at 4% ended with substantially more than they started with. The 4% rate was not the expected outcome. It was the minimum that survived the worst historical case.
In 1998, Cooley, Hubbard, and Walz at Trinity University published what became known as the Trinity Study, extending Bengen's analysis to examine withdrawal rate sustainability across different portfolio allocations and time horizons. Their work produced success rate tables showing the probability that different withdrawal rates would survive different holding periods across historical sequences.
The Trinity Study confirmed Bengen's finding for thirty-year horizons and 50/50 portfolios but also showed that success rates declined materially at longer horizons, higher withdrawal rates, and more conservative portfolios. A 4% withdrawal rate from a 100% bond portfolio, for example, had historically failed in a significant percentage of thirty-year sequences because bond returns were insufficient to sustain the inflation-adjusted withdrawals.
The sustainable withdrawal rate is not independent of portfolio allocation. An all-bond portfolio cannot sustain the same withdrawal rate as a balanced or equity-heavy portfolio because bonds don't provide the long-term growth needed to offset inflation-adjusted withdrawals over decades. But an all-equity portfolio introduces sequence of returns risk that can devastate a plan if a bear market arrives in the first few years of retirement.
The historically optimal allocation for maximizing sustainable withdrawal rates has generally been in the 50-75% equity range for thirty-year horizons. More conservative allocations reduce the upside that makes high withdrawal rates possible. More aggressive allocations increase sequence risk that can produce catastrophic early-retirement losses.
The original Bengen research and the Trinity Study both used thirty-year horizons, appropriate for someone retiring at 65 in 1994 with an expected lifespan to 95. But thirty years is increasingly inadequate as life expectancy has extended and early retirement has become more common. A couple retiring at 60 may need a forty-year plan. The success rate at 4% over forty years is meaningfully lower than over thirty years, which means the appropriate starting rate for a long horizon is somewhat below 4%.
The Society of Actuaries publishes longevity data showing that a 65-year-old woman today has a median life expectancy to approximately 87, with a 25% probability of reaching 94. For planning purposes, using a horizon of 90 or 95 is appropriate for most healthy retirees. The calculator at plan.johnkoyle.com allows you to set your planning horizon explicitly and see how the success rate changes at different horizons.
Wade Pfau has produced some of the most cited contemporary research on safe withdrawal rates, updating Bengen's analysis with current market conditions and forward-looking return assumptions. His work consistently finds that the safe withdrawal rate is sensitive to starting valuations, and that a retiree beginning retirement at historically elevated valuations should plan conservatively, perhaps at 3% to 3.5%, rather than assuming the full 4% is historically appropriate for their situation.
Pfau's research on this topic was published in the Journal of Financial Planning and updated through Advisor Perspectives and his book Retirement Planning Guidebook. His valuation-adjusted analysis shows that the safe withdrawal rate in high-CAPE environments has historically been meaningfully lower than 4%, reflecting the lower forward returns that elevated starting valuations tend to predict.
Michael Kitces published research in 2015 on what he called the ratcheting safe withdrawal rate, a strategy that allows retirees to increase their withdrawal rate when portfolio performance significantly exceeds what was required to sustain the plan. His work showed that retirees who started at a conservative rate and systematically increased withdrawals when portfolio milestones were hit achieved higher lifetime income than those who locked in a single starting rate and never adjusted.
Kitces's ratcheting approach reflects a broader principle: the initial withdrawal rate decision is important but not immutable. A retirement plan that monitors portfolio performance and adjusts withdrawals in response to actual outcomes, rather than rigidly adhering to a predetermined schedule, is more resilient and often generates more lifetime income.
Jonathan Guyton and William Klinger published research in the Journal of Financial Planning in 2006 on decision rules for dynamic withdrawal management. Their guardrails approach established specific triggers for reducing withdrawals in poor market conditions and increasing them in good conditions. Their research showed that these dynamic rules allowed initial withdrawal rates meaningfully higher than the static 4% rule without sacrificing plan sustainability.
The key insight from Guyton and Klinger is that the 4% rule's conservatism is largely a function of its rigidity. By allowing the withdrawal amount to flex modestly in response to market conditions, a retiree can start at a higher rate while maintaining the same probability of plan success. The tradeoff is income variability, which some retirees can tolerate and others cannot.
Morningstar's retirement research team published a widely cited 2021 analysis finding that, given then-current market conditions and lower expected bond returns, the safe initial withdrawal rate for a thirty-year retirement was approximately 3.3% rather than 4%. This research was updated in subsequent years as interest rates rose and market conditions changed. The core finding, that the appropriate starting rate depends on current conditions and should not be mechanically applied from decades-old research, is consistent with the broader body of literature.
The 4% rule is a historical observation about a specific portfolio, horizon, and set of market conditions. It is not a guarantee. Applying it without understanding its assumptions, or without accounting for how current conditions differ from the historical average, is planning by shortcut rather than by analysis. A retiree in a high-valuation environment with a forty-year horizon who starts at 4% is taking meaningfully more risk than a 1994 retiree with the same starting rate.
The 4% rule specifies a 4% initial withdrawal that is then adjusted annually for inflation. The inflation adjustment is not optional. A retiree who takes 4% in year one but holds the dollar amount flat in subsequent years is actually running a declining real withdrawal rate, which is more conservative than the rule implies. Conversely, a retiree who takes 4% and increases the nominal amount by more than inflation is running a higher effective withdrawal rate than the rule's historical safety record covers.
The withdrawal rate in Bengen's research measures the percentage of the portfolio withdrawn, not total retirement income as a percentage of pre-retirement income. A retiree with $1,000,000 in a portfolio and $40,000 per year in Social Security income needs the portfolio to generate much less than 4% to meet total spending needs. The sustainable withdrawal rate applies to the portfolio draw, and any guaranteed income source that reduces the portfolio draw effectively reduces the sequence risk and extends the portfolio's longevity.
The 4% rule tells you what a historical portfolio could sustain. It doesn't tell you what you need to spend. Some retirees need less than 4% to live well. Others need more. The right starting point is an honest assessment of actual spending needs, not a calculation of what 4% of your portfolio equals. If your actual spending need requires more than 3.5% of your portfolio and you're retiring into an elevated valuation environment, you have a planning problem that the rule doesn't solve.
Build your retirement income plan from the bottom up. What does your life actually cost? Break spending into fixed expenses, housing, insurance, utilities, food, healthcare, and discretionary expenses, travel, entertainment, hobbies, gifts. The fixed expenses define your floor. The discretionary expenses define your flexibility. A plan that distinguishes between these two categories is more resilient than one that treats all spending as equally committed.
Map your guaranteed income sources, Social Security, pension, and any annuity income, against your spending plan. The gap between guaranteed income and total spending is the portfolio draw. That gap, divided by the portfolio value, is your effective withdrawal rate. If the gap is large relative to the portfolio, you have sequence risk. If the gap is small, you have resilience.
A retirement spending plan should include explicit rules for what you'll do if the portfolio underperforms. Will you cut discretionary spending by 10% in a year when the portfolio declines significantly? Will you delay a major purchase? Will you reduce travel temporarily? These decisions, made in advance and documented, are worth more than a marginally lower starting withdrawal rate because they give the plan real flexibility that Monte Carlo models typically don't credit.
The retirement calculator at plan.johnkoyle.com allows you to enter your actual spending as a monthly budget, your income sources, your portfolio balance, and your retirement age. The Monte Carlo simulation then tests whether that spending level is sustainable across 500 possible return sequences and shows your success rate. Adjusting the monthly spending input shows directly how spending decisions affect plan resilience.
This is the baseline. Your advisor should be able to calculate your effective withdrawal rate from the portfolio and contextualize it against Bengen's research and current conditions.
A good advisor acknowledges the valuation sensitivity of withdrawal rate sustainability and either uses a conservative return assumption in the Monte Carlo or stress-tests the plan against a lower-return scenario.
This question tests whether there's a real dynamic spending plan or just a static assumption. The answer should include specific spending adjustments that would be made in response to a significant early-retirement loss.
Every dollar of guaranteed income reduces the portfolio draw and improves the plan's resilience. Your advisor should be able to show you the effective withdrawal rate net of all guaranteed income sources, not just the gross portfolio withdrawal percentage.
This is the reverse-engineering question. Rather than starting with spending and testing sustainability, you're asking what spending level is sustainable at a high confidence level over a long horizon. The answer gives you the upper bound of what the plan can confidently support.
The retirement planning calculator at plan.johnkoyle.com was built to model exactly the dynamics discussed in this paper. The retirement calculator at plan.johnkoyle.com places the spending decision at the center of the analysis. Enter your monthly budget in today's dollars and the calculator adjusts it forward for inflation across every Monte Carlo scenario. The Projection tab shows your spending relative to your portfolio and income sources at each age. The Monte Carlo tab shows your success rate across 500 scenarios at your current spending level. The Action Plan tab identifies the spending adjustments that would most improve your plan's resilience. Try adjusting the monthly budget input and watch how your success rate responds in real time.
John Koyle, AIF®, is the Co-Founder of Red Cedar Wealth Advisors, headquartered in Pocatello, Idaho. He holds the Accredited Investment Fiduciary (AIF®) designation, awarded by the Center for Fiduciary Studies (Fi360), which signifies completed coursework, a rigorous examination, and ongoing continuing education in fiduciary responsibility and prudent investment practices.
John serves individuals and families in or approaching retirement throughout eastern Idaho, the Pacific Northwest, and across the country via Zoom. Clients work directly with John, not a junior team. Red Cedar Wealth Advisors operates under Osaic Wealth, Inc. (Member FINRA/SIPC) and Osaic Advisory Services, LLC for investment advisory services.
Every client relationship is built on five integrated disciplines. The proportions shift with the client and the moment. The integration is constant. Most of the actual value sits in how these disciplines connect, not in any single one of them.
Retirement planning is a discipline that barely existed a generation ago. For most of modern history, people worked until they physically couldn't and then they died, usually fairly close together. The idea that an individual should spend decades saving, then spend decades drawing down those savings, with a plan that accounts for inflation, taxation, sequence risk, healthcare, longevity, and estate transfer, that's maybe forty years old.
Which means almost nobody your age grew up watching their parents do it properly. There's no cultural muscle memory. The advice industry backfilled that gap with rules of thumb that work sometimes and fail catastrophically the rest of the time. The 4% rule. 'You can take Social Security at 62.' 'Target-date funds will handle it.' These are not bad starting points. They are terrible ending points. Real planning is specific, personal, and built on principles that hold up across the full range of outcomes, not just the average one.
The tax code is a set of instructions Congress wrote to shape behavior. Aligning your financial life with those instructions is not aggressive planning. It is the planning. Roth conversion sequencing, capital gains compression on concentrated positions, charitable remainder trusts, Social Security timing, beneficiary coordination. None of these are obscure. They're all legitimate, all written into the code intentionally, and all under-used. Most CPAs handle compliance. The strategy work is a different discipline entirely.
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Every week, before any portfolio decision, John runs through four layers of market health: economic conditions, market internals, valuations, and sentiment. Each layer gets scored and those scores combine into a composite that maps directly to a portfolio posture, from aggressive on the positive end to defensive on the negative. The work is in the scoring. Once the scoring is done, the positioning follows. That removes one of the most dangerous things in investing: making it up as you go.
The protection gap quietly kills more plans than markets do. A significant net worth paired with inadequate liability coverage is a lawsuit away from a serious problem. Long-term care is more acute: a multi-year care event for one spouse can consume what was meant for the survivor. Most advisors relegate risk to a footnote because insurance conversations are uncomfortable. The math doesn't care. Coverage adequacy, umbrella sizing, long-term care planning, and life insurance structure sit alongside the portfolio in any complete plan.
These are the principles behind every plan John builds.
To begin a conversation, visit johnkoyle.com, use the retirement planning calculator at plan.johnkoyle.com, or reach John directly at john@redcedarwealth.com or (208) 915-8400. Initial consultations are complimentary and carry no obligation.