The 4% Rule Explained

In 1994 the financial planner William Bengen looked at every 30-year retirement window in US market history and asked a simple question: what is the highest starting withdrawal rate that never ran out of money, including for someone who retired at the worst possible moment?

The answer was about 4%. The Trinity study a few years later reached similar conclusions with a portfolio of roughly 50 to 75% stocks and the rest bonds. Both were historical studies of what would have survived, not predictions of what will.

The mechanic that people most often get wrong is the inflation adjustment. You take 4% of the portfolio once, in year one. After that you adjust that dollar figure for inflation, regardless of what the portfolio does.

On a $1,000,000 portfolio, year one is $40,000. If inflation runs 3%, year two is $41,200, whether the portfolio rose to $1.1M or fell to $800,000. You are not taking 4% every year; you are taking a fixed, inflation-adjusted income stream and asking whether the portfolio can sustain it.

The safe withdrawal calculator runs this out year by year, and the FIRE calculator works backwards from a target income to the portfolio it requires.

Here is why the safe rate is 4% and not the 7% or so that historical average returns might suggest. When you are drawing an income, theorder of returns matters enormously, even though it makes no difference at all while you are still saving.

Two retirees can experience identical average returns over thirty years. The one who gets the bad years first sells shares into a falling market to fund spending, and those shares are gone when the recovery arrives. The one who gets the good years first is drawing from a portfolio that grew before the drawdown came.

It also explains why the first decade of retirement carries so much more weight than the last. A portfolio that survives its first ten years intact is very likely to survive the full thirty.

It assumes rigid spending. No real retiree keeps spending the same inflation-adjusted amount through a 40% drawdown. Modest flexibility, skipping one inflation adjustment or trimming 10% after a bad year, raises success rates substantially.

Spending is not flat anyway. Real retirement spending tends to be high early, lower through the middle years, then higher again late as health costs arrive. A flat inflation-adjusted line describes almost nobody.

It ignores taxes and fees. The studies modelled gross portfolio returns. Withdrawals from a traditional account are taxable, and a 1% advisory fee comes straight off the sustainable rate.

It is US data. The 20th-century US was one of the most successful equity markets in history. Studies using international data generally find lower safe rates, which is a reason for humility rather than panic.

Use 4% to size the target, because 25 times spending is a clear and reachable goal. Then plan to be flexible about the withdrawal itself, since that flexibility is worth more than any adjustment to the starting percentage.

If you are retiring early, start lower, nearer 3.25 to 3.5%, because a 50-year horizon was never what the rule tested. If you have Social Security or a pension arriving later, you can often withdraw more before it starts and less after, which no single fixed rate captures.