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Where the 4 percent rule came from

The Trinity Study and William Bengen's earlier work examined US market history and asked what withdrawal rate, adjusted annually for inflation, would have survived every historical 30-year period. The answer was about 4 percent of the starting balance. It is a backward-looking result from one country's unusually strong century, not a law, and the people who produced it have consistently described it as a planning benchmark rather than a guarantee.

What the rule assumes

A 30-year horizon, a portfolio of roughly 50 to 75 percent equities, annual inflation adjustments, and US historical returns. Retiring at 50 rather than 65 stretches the horizon to 40 or 50 years, where the sustainable rate drops toward 3 to 3.5 percent. A more conservative portfolio lowers it further, because bonds alone have historically struggled to outpace inflation across long horizons.

Sequence of returns risk

This is the mechanism that actually causes failure. Two portfolios with identical average returns produce completely different outcomes depending on when the bad years land. A severe fall in the first few years, while you are withdrawing, permanently reduces the capital that would otherwise have compounded. The same fall twenty years later is largely survivable. Average returns are almost irrelevant; their order is decisive.

Flexibility is worth more than precision

The historical failures nearly all involve rigid inflation-adjusted withdrawals through a market crash. Reducing spending modestly during bad years, or skipping the inflation increase after a fall, improves survival dramatically. A retiree who can cut discretionary spending by 10 percent in a downturn can safely start at a meaningfully higher rate than one who cannot.

Treat the output as a range

Any single number here is false precision. Model 3, 3.5 and 4 percent and look at the spread of outcomes rather than the headline figure, and revisit the plan every few years against what actually happened rather than what was projected.