What the 4% rule actually means

The 4% rule comes from William Bengen's 1994 study, which tested every rolling 30-year retirement window in US market history and asked: what fixed initial withdrawal rate, adjusted for inflation, would have survived every one of them?

Bengen's answer was roughly 4%. A retiree starting with $1,000,000 could draw $40,000 in year one, increase that figure by inflation each subsequent year, and never run out across any historical 30-year window in the dataset.

The rule rests on a 60/40 stock/bond mix and assumes the retiree rebalances annually. Change the asset mix or the rebalancing rule, and the safe rate changes too.

Sequence-of-returns risk is the reason a single average return is misleading. A retiree who hits a bad bear market in years one and two is in a permanently worse position than one who hits the same bear market in years 15 and 16 — even if both have the same long-run average return.

A historically-tested withdrawal rate is not a guarantee about the future. It is the rate that worked across every historical sample examined. The next 30 years can be different. Inflation regimes, interest-rate paths, and equity valuations are not stationary.

Monte Carlo simulations attempt to answer the 'what about futures we haven't seen' question by generating thousands of plausible return sequences and reporting what fraction of them survive the withdrawal schedule.

A 95% Monte Carlo success rate does not mean a 95% chance of surviving — it means 95% of the generated paths survived under the simulation's assumptions. Garbage assumptions in, garbage probability out.

More recent research (Pfau, Kitces, others) suggests the 4% figure is conservative in some historical regimes and aggressive in others. Today's starting valuations and yield environment matter to where the right number for you sits on that spectrum.