The 4% rule, and what it is not

Where the 4% withdrawal shorthand comes from, the horizon it was studied on, and why a calculator that uses it is not a guarantee.

Multiply annual spending by 25, or divide by 0.04. That is the 4% rule as people use it on a napkin. The research behind the napkin is narrower than the napkin.

What was actually studied

William Bengen's 1994 paper looked at historical U.S. returns and asked how high an initial withdrawal could be, increased with inflation, without running out of money over 30 years. A mix around half to three-quarters stocks supported something near 4% in the worst starting years he studied. The Trinity study later asked a similar historical question and reported success rates for several mixes and withdrawal rates.

Success in those studies means the portfolio lasted the horizon in the historical window they had. It does not mean the next 30 years will rhyme. It does not include advisor fees, fund fees, or taxes. It assumes you actually rebalance into the mix that was tested, and that you do not spend a large extra amount in year three.

How this site uses it

The FIRE and Coast calculators use your withdrawal rate only to turn spending into a portfolio size. They do not simulate 1929 or 1966. The 'how long will my money last' page does a different, simpler thing: one constant return, withdrawals that rise with inflation. If you set the return equal to inflation plus a bit, money lasts a long time. If you set a low return and a high withdrawal, it does not. That page will tell you it ignores sequence risk, because a constant return cannot show a bad decade at the start.

Levers that matter as much as the rate

Flexibility. Spending that can drop after a bad market year is not the same product as a fixed inflation-adjusted paycheck.
Other income. Social Security, a pension, or an annuity changes how much the portfolio must pay, and when.
Horizon. Forty years is not thirty. Early retirement is a longer test than the one Bengen published.
Costs. One percent a year in fees is a large share of a 4% withdrawal.

Use 4% as a starting label so two people can talk. Do not use it as a guarantee that a portfolio will last.

Questions

Does the 4% rule mean I can spend 4% forever?

The original research asked a different question: in past U.S. markets, what initial withdrawal, raised with inflation each year, survived a 30-year retirement across the historical starting dates that were tested? Four percent was a cautious answer for a particular stock-and-bond mix over that horizon. It was not a proof that 4% works for 50 years, for every country, or after a large fee.

Should I use 3% or 5% instead?

A lower rate buys a longer or harder horizon at the cost of a bigger portfolio and more working years. A higher rate does the opposite and fails more often in the historical tests. This site lets you type the rate. It does not pick one. Early retirements and high stock valuations are reasons researchers argue about whether 4% is thick enough. The argument is not settled by a form field.