The 4% rule, explained
Where the most quoted number in retirement planning comes from, the math behind it, and the four ways it breaks — so you can use it as a planning input, not a promise.
Where it came from
In 1994, financial planner William Bengen published research asking a practical question: what percentage of a portfolio could a retiree withdraw each year — raising it with inflation — without running out of money over 30 years? Testing historical U.S. market data across rolling retirement dates, he landed near 4% for a stock-heavy portfolio. The 1998 Trinity study popularized the finding, and “the 4% rule” became shorthand for an entire retirement philosophy.
The math in one paragraph
Multiply desired annual portfolio spending by 25. That is the portfolio target: $60,000 a year ÷ 4% = $1.5 million. The 25x comes straight from the withdrawal rate (1 ÷ 0.04). The rule assumes you withdraw 4% in year one, then adjust that dollar amount for inflation each year after, over a roughly 30-year retirement, with the portfolio invested in a mix of stocks and bonds. It is a worst-case survival rule drawn from history, not an average outcome — most historical periods supported far more.
A retiree with $1.2 million withdraws $48,000 in year one. Inflation runs 3%, so year two’s withdrawal is $49,440, year three’s about $50,923, and so on. In the historical scenarios Bengen tested, portfolios surviving this pattern for 30 years defined the “safe” rate. Note what the example assumes: the retiree never earns another dollar, never cuts spending in a crash, and pays no advisory fees.
Four ways it breaks
1. Fees. The original research assumed no investment costs. A 1% annual advisory fee is effectively a 5% withdrawal, not 4%. 2. Longer retirements. Retiring at 50 means funding 40+ years, not 30 — the safe rate drops. 3. Low starting yields. When bonds yield little, the portfolio’s built-in cushion shrinks; Bengen himself later suggested the rate moves with market conditions. 4. Rigid spending. Real retirees cut back after a market crash. The rule’s pessimism comes largely from assuming they never do — flexible spending raises the sustainable rate substantially.
What Bengen says now
Bengen has revisited his own rule several times. His later research found that small tilts — adding small-cap and value stocks to the classic stock/bond mix — supported withdrawal rates around 4.5–4.7% in his testing framework. Other researchers reach different numbers depending on fees, asset mix, and whether retirees adjust spending after down years. The honest summary: there is no single safe number, only a range (roughly 3–5%) whose right answer depends on your flexibility, your costs, and how long the money must last. Anyone quoting exactly 4% as a guarantee is selling certainty the research never claimed.
How to use it anyway
Use 4% (or 25x spending) as a planning input: it converts a lifestyle into a portfolio target, which is exactly what the Coast FIRE calculator’s withdrawal-rate field does. Then stress-test it — try 3.5% and 4.5%, model a fee drag, and assume one bad decade early. The rule’s real value was never the number 4; it was proving that a simple, inflation-adjusted withdrawal plan survived the worst markets on record. Treat it as a starting line for your own scenario work, not a finish line.
Pick your withdrawal rate together with your spending estimate, then re-run the plan at a lower rate and a lower return. If the plan only works at exactly 4.0% and 5% real returns, it does not work.