The 4% Rule: Where It Came From and Whether It Still Works

What Bengen actually wrote

In October 1994, a financial planner named William Bengen published a paper in the Journal of Financial Planning asking a deceptively simple question: how much can a retiree safely withdraw from a 50/50 stock/bond portfolio every year, with the withdrawal indexed to inflation, without running out of money in 30 years?

He used US data from 1926 onwards. For every starting year, he simulated a 30-year retirement, varying the initial withdrawal rate. He found:

• 4% survived every 30-year period he tested, including the worst — retirees starting in 1966.
• 5% failed in roughly a quarter of the periods.
• 4.5% survived all 30-year periods up to and including the 1973 stagflation cohort.

The 4% number wasn't a recommendation. It was the worst-case-survivable rate for a 30-year retirement using stock/bond data going back to 1926. Bengen himself later argued the right number is closer to 4.5% once you allow for international diversification.

What the Trinity Study added

In 1998, three professors at Trinity University (Cooley, Hubbard, Walz) ran a similar study and popularised the framing as a "success rate" — the percentage of historical periods in which a given withdrawal rate would have survived for a given number of years. They confirmed Bengen's headline number: 4% had a 95%+ survival rate for 30-year horizons.

The Trinity Study is more widely cited than Bengen's original because it has a clean table format. The downside: people now treat "4% rule" as gospel without reading either paper.

Why it breaks down for FIRE

The 4% rule has two assumptions that don't hold for early retirees:

1. Horizon: 30 years. Someone retiring at 45 might need 50+ years. Survival rates fall sharply as horizon extends — at 50 years, the historical safe rate is closer to 3.5%.
2. No flexibility: it assumes a rigid inflation-adjusted withdrawal regardless of market conditions. Real retirees cut spending in down markets. That alone moves the safe rate up by 0.5–1%.

For a 50-year FIRE plan with no spending flexibility, the historical safe rate from our simulator is closer to 3.25–3.5%. With moderate flexibility (cutting spending by 10–20% in down years), it's closer to 3.75–4%.

What the actual number is for you

There's no single right answer. The right withdrawal rate depends on:

• Your horizon (years from now until end of plan)
• Your willingness to flex spending in bad markets
• Your equity/bond split
• Whether you're including a state pension or other income
• Your terminal-wealth preference — do you want zero left, or a buffer?

Our withdrawal survival tool lets you toggle each variable and see how the safe rate moves. The honest answer for most FIRE planners is somewhere between 3.25% and 4.25%.

How to use it sensibly

Use 4% as an anchor for estimating your FIRE number (annual expenses × 25). It's roughly right and it's easy to remember. Then, once you actually retire, use a more sophisticated framework — guardrails (see our guardrails article) or a flexible spending model — to actually run your withdrawals.

Don't treat 4% as a guarantee, and don't treat it as the maximum. Treat it as the conservative starting point of a much richer conversation.