Safe Withdrawal Rate

A safe withdrawal rate is expressed as a percentage of the portfolio's value on the day you retire. You take that amount in year one and then increase the dollar figure by inflation each year afterward — you do not recalculate the percentage against a changed balance. Confusing those two is the most common misuse of the concept.

The familiar 4% figure comes from William Bengen's 1994 study and the Trinity study that followed, both of which tested historical US stock and bond returns against 30-year retirements. The finding was that a 4% initial rate with a portfolio of roughly 50% to 75% stocks survived every historical starting year in the data, including retirements beginning at the worst possible moments.

The assumptions behind that number are narrower than its reputation. It rests on one country's history over a period of exceptional equity returns, assumes a 30-year horizon, and ignores investment fees and taxes. Someone retiring at 40 is planning for fifty years rather than thirty, which is why early retirees commonly work from 3% to 3.5% instead.

The rate is a planning heuristic, not a spending rule anyone follows literally. A retiree who mechanically raised withdrawals through a 40% market decline would be doing exactly what the historical worst cases warn about, and the same studies show that modest flexibility — skipping an inflation increase after a bad year, trimming discretionary spending — improves survival more than any allocation change.

Read in reverse, it is also a savings target. Dividing 100 by the withdrawal rate gives the multiple of annual expenses to accumulate: 25 times at 4%, 33 times at 3%. That inversion is what makes the figure central to retirement and FIRE planning alike.