The 4% Rule: Origins, Limitations, and What RMDs Do to It

Where the 4% Rule Came From

The "4% rule" originates from research published in 1994 by financial planner William Bengen, later expanded by Cooley, Hubbard, and Walz in what became known as the Trinity Study. Bengen analyzed historical U.S. stock and bond market data going back to 1926 and asked a simple question: what withdrawal rate from a diversified portfolio would have survived every 30-year historical period without running out of money?

His finding: a portfolio invested 50–60% in stocks and 40–50% in bonds could sustain a 4% initial withdrawal rate, adjusted upward each year for inflation, without being depleted in any 30-year historical period from 1926 through the 1990s. The worst periods — those beginning in the late 1960s and early 1970s, coinciding with high inflation and poor equity returns — were survived. 4% was the "safe" floor.

What the Rule Actually Says — and Doesn't

The rule has three specific assumptions that are frequently overlooked:

• 30-year horizon: The rule was designed for a 30-year retirement. A retiree who retires at 60 and lives to 95 needs a 35-year horizon. A retiree who retires at 55 needs 40 years. Success rates decline meaningfully as the time horizon extends beyond 30 years.
• Specific allocation: The research assumed roughly 50–75% equities. A more conservative allocation (e.g., 30% equities / 70% bonds) actually performs worse because bond returns are insufficient to offset the inflation-adjusted withdrawals, especially in low-rate environments.
• Historical U.S. returns: The study used historical U.S. equity and bond returns, which are among the highest in the world over that period. Applying the rule to a portfolio with international equity exposure or current market conditions requires acknowledging that future returns may be lower than the historical average used in the original study.

The Sequence of Returns Problem

The most dangerous risk for a retiree following the 4% rule is not average returns — it is the order of returns in the early years. A severe market decline in years 1–5 of retirement, combined with continued withdrawals, can permanently impair a portfolio in a way that later strong returns cannot fully repair. This is sequence of returns risk.

The 4% rule survived all historical 30-year periods, but barely in the worst cases. A retiree who retires at the beginning of a bear market — or who faces a 40% portfolio decline in the first few years — is living through the sequence-of-returns scenario the rule was tested against. The result is rarely catastrophic if spending is flexible, but it demands either a spending reduction or a willingness to adjust.

The Current-Environment Challenge

Research published in recent years — including updated analysis by Bengen himself and studies from Morningstar — has challenged whether 4% remains safe given current starting conditions. Key concerns:

• Valuation: When equity valuations are high (elevated price-to-earnings ratios), expected future returns are statistically lower. Starting a retirement withdrawal strategy from a highly valued market increases the probability of poor early returns.
• Lower bond yields (historically): When the original study was done in 1994, bond yields were much higher. A bond portfolio yielding 6–7% absorbs withdrawals differently than one yielding 2–3%. (Note: as of 2024–2025, bond yields have risen substantially, partially restoring this buffer.)
• Longer lifespans: Average life expectancy has increased since 1994. A 4% rate that survived 30 years is less reliably safe for a 35–40-year horizon.

Morningstar's 2023 research suggested a "safe" starting withdrawal rate for a 30-year horizon at 90% confidence was closer to 3.8% — not dramatically different, but notable for longer horizons or more conservative allocations, where figures in the 3.3%–3.5% range were cited.

What RMDs Do to the 4% Framework

RMDs introduce a structural complication that the 4% rule does not directly address: the IRS mandates minimum withdrawals regardless of what the retiree actually needs or what market conditions prevail.

The RMD Rate Accelerates Over Time

The RMD percentage is calculated by dividing the prior year's account balance by an IRS life expectancy factor. At age 73, the factor is approximately 26.5, producing an RMD rate of about 3.77%. By age 80, it is about 20.2 (RMD rate ~4.95%). By age 85, it is about 16.0 (~6.25%). By age 90, the rate exceeds 8%.

Age	IRS Life Expectancy Factor (approx.)	RMD Rate (% of prior year balance)
73	26.5	3.77%
75	24.6	4.07%
80	20.2	4.95%
85	16.0	6.25%
90	12.2	8.20%
95	8.9	11.24%

A retiree following a 4% rule from a traditional IRA will find that by their mid-80s, the mandatory RMD rate exceeds 4%. This means they are forced to withdraw more than the rule prescribes — whether they need it or not. If the excess income is not needed for spending, it sits in a taxable brokerage account, generating additional taxes each year. It also inflates MAGI, potentially triggering IRMAA and higher Medicare premiums year after year.

The "Phantom Spending" Problem

A retiree with a $2 million traditional IRA at age 85 faces an RMD of approximately $125,000 — even if their actual spending need is only $80,000. They are forced to recognize $45,000 in income they did not want, pay taxes on it, and either spend it (lifestyle inflation), reinvest it in a taxable account (ongoing tax drag), or donate it (requires charitable strategy). None of these are problems, but they are consequences of not managing the IRA balance earlier.

Alternative Withdrawal Rate Approaches

Dynamic Withdrawal ("Guardrails" Method)

Rather than a fixed inflation-adjusted withdrawal, the guardrails approach adjusts spending in response to portfolio performance. If the portfolio grows well and the current withdrawal rate falls below a lower guardrail (e.g., 3.5% of current portfolio), spending increases. If the portfolio declines and the withdrawal rate rises above an upper guardrail (e.g., 5.5%), spending decreases — typically by 10%. This dynamic adjustment dramatically improves portfolio survival probability because spending flexes with reality rather than assuming a fixed trajectory.

Floor-and-Upside / Floor First

Cover essential expenses (the floor) with guaranteed income — Social Security, pension, SPIA, QLAC — and use the portfolio only for discretionary (variable) spending. If guaranteed income covers 80% of essential needs, the portfolio's failure risk is tolerated because it is only funding non-essential spending. This approach inverts the 4% rule's emphasis: it starts with certainty, not probability.

RMD-Driven Withdrawal

Some financial planners recommend simply taking whatever the RMD requires each year and spending accordingly — letting the IRS formula effectively set the withdrawal rate. This produces a naturally declining percentage withdrawal as the retiree ages, aligned with shortened time horizon, and matches withdrawal to actual portfolio performance (RMD rises in good years and falls in bad years due to the prior-year-balance calculation).