The 4% Rule: What Bengen Actually Found, and What It Does Not Promise

William Bengen tested withdrawal rates against every retirement cohort in U.S. market history and found 4% survived even the worst 30-year sequences. What the rule really says, its assumptions, and how to use it as a planning tool.

In 1994, financial planner William Bengen asked a sharper version of retirement's central question: not "what do markets average?" but "what withdrawal rate would have survived the worst retirement date in U.S. history?" Averages flatter; retirees live through sequences. Testing 30-year retirements starting every year through the twentieth century — including 1929, 1937, and the brutal 1966–1982 stretch — he found the answer clustered near 4%.

What the rule actually says

Withdraw 4% of the portfolio's value in year one of retirement, then adjust that dollar amount for inflation each year — ignoring the market thereafter. In Bengen's data (roughly half-stock portfolios), that schedule survived every historical 30-year cohort, including retirements that began at catastrophic moments. It is a worst-case-tested floor, not an average outcome: in most historical cohorts the retiree following it died with more money than they started with, often far more.

The assumptions worth naming

30 years. Longer horizons — early retirees, see the 25x framing — argue for lower rates or flexibility.
U.S. historical returns — a famously fortunate market. Future returns below history's worst would break precedent, which is not the same as impossible; many planners haircut toward 3.5% for conservatism, while Bengen's own later work, adding more asset classes, nudged the number up.
Rigid mechanical withdrawals — no human actually spends this way, and that is good news: modest flexibility (skipping inflation raises after bad years, guardrail rules) measurably strengthens survival in every study since.
Meaningful stock allocation — the rule fails with portfolios too conservative to outgrow inflation.

Using it without misusing it

The rule's best use is inverted, as a savings target: annual spending × 25 ≈ the portfolio that historically supported it. Its worst use is as an autopilot promise. Treat 4% as the planning anchor, plan to be flexible around it, and re-run the numbers as reality arrives. The FIRE calculator and retirement calculator both build on this framework — with your own numbers, and with the withdrawal rate as a dial rather than a decree, which is how Bengen himself talks about it.

The 4% Rule: What Bengen Actually Found, and What It Does Not Promise

What the rule actually says

The assumptions worth naming

Using it without misusing it

Run your own numbers

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