The 4% rule, examined: where it came from and where it breaks

The 4% rule is the number the entire FIRE movement leans on, which is exactly why it deserves a hard look rather than a nod. In the opening post we used it as a rough plank — draw 4%, save 25× — and promised to put it on trial. This is the trial. The good news for its fans is that it is far more robust than the “the 4% rule is dead” headlines claim. The bad news is that it was never designed for the job FIRE hands it, and pretending otherwise is how people run out of money at seventy-two. So let us read the actual papers, because almost nobody quoting the rule has.

What Bengen actually did in 1994

The rule has an author, and he was not a professor. William Bengen was a California financial adviser who got tired of colleagues pulling safe withdrawal rates out of thin air, so in October 1994 he published “Determining Withdrawal Rates Using Historical Data” in the Journal of Financial Planning.

His method was refreshingly concrete. Take a retiree with a portfolio split 50/50 between US stocks and intermediate-term Treasury bonds. Have them withdraw some percentage in year one, then increase that pound amount by inflation every year regardless of what markets do. Now run that plan through every rolling 30-year period in the historical record going back to 1926 — including retirees unlucky enough to start just before the 1929 crash, the 1970s stagflation, and every other horror. What is the highest starting withdrawal rate that survived all of them, even the worst?

The answer came out at about 4.15%. Bengen called it the “SAFEMAX” — the maximum historically safe rate — and the industry promptly rounded it to a clean 4%. That rounding is the whole rule. Flip it over and 4% becomes the 25× multiplier: if 4% of your pot covers a year, then 25 years of spending is your target pot. The FIRE number you have heard a hundred times is just Bengen’s SAFEMAX standing on its head.

What the Trinity Study added in 1998

Four years later, three finance professors at Trinity University in Texas — Philip Cooley, Carl Hubbard and Daniel Walz — published “Retirement Savings: Choosing a Withdrawal Rate That Is Sustainable” in the AAII Journal. This is the paper everyone actually cites, usually as “the Trinity Study,” and it did something Bengen’s single SAFEMAX did not: it built a whole table of success rates.

Instead of reporting one magic number, they ran many withdrawal rates and stock/bond mixes across rolling periods from 1926 to 1995 and reported the probability each combination survived. The headline result is the one worth memorising: a stock-heavy portfolio (50/50 or 75/25) withdrawing an inflation-adjusted 4% survived roughly 95–100% of historical 30-year windows. Push to 5% and the success rate fell to around 83%. Push to 6% and it dropped to about 68% — a one-in-three chance of running dry. The lesson is not “4% is safe and 5% is reckless.” It is that safety is a dial, not a switch, and small turns of the dial move the odds a lot.

The three cracks FIRE has to worry about

Now the fine print the beach photos skip. The 4% rule has real limits, and early retirees walk straight into all three.

It was built for a 30-year retirement. Bengen’s retiree was sixty-five. A FIRE retiree might be forty, staring down fifty or sixty years. Stretch the horizon and the safe rate falls, because there is more time for a bad sequence to catch you — many analyses put a very-long-horizon safe rate closer to 3.25–3.5%. Borrow a 30-year rule for a 50-year retirement and you have quietly borrowed someone else’s risk tolerance.

It is US data, on a good century. The 1926-onward record is the track of history’s most successful large economy over its most successful period. Studies that widen the lens to other developed countries — which had wars, hyperinflations and lost decades the US mostly dodged — find materially lower safe rates. “It worked in America” and “it works everywhere” are not the same claim.

Its real enemy is timing, not average returns. The rule survives or dies on the order in which returns arrive, not their long-run average. A crash in your first few retired years, while you are selling to eat, does damage a later crash never could. That is called sequence-of-returns risk, it is the single most important idea in decumulation, and it gets its own post because it deserves one.

Why “the 4% rule is dead” is also wrong

Having scared you, let me un-scare you, because the doom headlines overcorrect. The rule’s own author thinks it is, if anything, too conservative.

Bengen never stopped researching. Once he added more asset classes — small-cap stocks especially — his safe rate rose. He revised it up to about 4.5%, and in more recent work, examining the full sweep of history, he has argued the figure is nearer 4.7%. Remember what SAFEMAX means: it is the rate that survived the single worst starting year in history. In the average historical retirement, a 4% retiree didn’t just survive — they died with a bigger pot than they started with, because they were withdrawing far below what their good years could have supported.

And the rule assumes something no real human does: that you will robotically raise your spending with inflation while watching your portfolio halve, never once tightening your belt. Nobody behaves like that. In a bad year real retirees skip the big holiday and delay the new car — and even modest flexibility of that kind, formalised in dynamic-withdrawal schemes like the Guyton-Klinger “guardrails,” lifts the safe starting rate considerably. The rigid 4% rule is a worst-case floor with the human being sawn off. You are not that rigid.

How to actually use it

So treat the 4% rule as neither gospel nor a corpse. It is a superb planning tool and a lousy autopilot. Use it the way it earns its keep: as the back-of-envelope that turns your spending into a target — 25× your honest annual costs — so you have a finish line to run at. Then, as you approach it, tighten the assumptions to your real life: a longer horizon nudges you toward 3.5%, a willingness to flex your spending buys you back room toward 4.5%.

Every one of those adjustments needs the same raw inputs: what you truly spend in a year, and what your invested pot is genuinely worth across every account. This is where a rule of thumb meets reality and usually loses, because those two numbers are scattered and stale for most people. Strata’s planning tools take a swing-free version of this — a FIRE number computed from your spending and a withdrawal rate you choose, plus a Monte Carlo test that reports your odds across thousands of market paths rather than a single tidy percentage. That is the honest version of the 4% rule: not one number pretending to be certain, but a range of outcomes you can actually see. Next we start touring the variants, beginning with the gentlest one — coast FIRE. Or, if you’d rather pin down your own 25× target than read about withdrawal rates in the abstract, you can build your portfolio and run the numbers on your real book.