The 4% Rule, Actually Explained
The most quoted number in retirement is also the most misquoted. Here is what the research actually says, what it assumes, and how to stress-test it yourself.
The 4% rule is usually stated in one breath — "withdraw 4% of your portfolio in year one, adjust for inflation, and you're safe for 30 years." The short version hides three load-bearing assumptions. Pull them out and you can use the rule intelligently instead of superstitiously.
Assumption 1: the horizon is 30 years
The research behind the rule tested 30-year retirements. If you retire at 65, that's about right. If you retire at 45, it isn't — you need the money to last twice as long, and longer horizons fail more often at any fixed rate. For 40+ year horizons, 3–3.5% is the more common planning band in the FIRE community.
Assumption 2: the portfolio is roughly half stocks
The Trinity study used a 50/50 stock/bond portfolio. All-stock portfolios failed more often in bad sequences (the crashes are deeper); all-bond portfolios failed more often quietly (inflation eats them). The rule is a property of a balanced portfolio, not of "the market."
Assumption 3: history is representative
The rule is backtested on US data from 1926 onward — a period in which the US was one of the world's best-performing markets. That's a strong bet on one country's history, and a fair criticism of any backtested rule.
The arithmetic underneath
Strip the sequence risk out — assume a steady 5% return with 2% inflation — and the math is a plain annuity:
sustainable rate = r ÷ (1 − (1 + r)⁻ⁿ), where r = (1+return)/(1+inflation) − 1, n = yearsPlug in 30 years and the answer is 5.16% — noticeably above 4%. The 4% figure is deliberately conservative: it prices in the fact that real returns don't arrive in a straight line. A crash in year two, when your balance is largest relative to withdrawals, does damage that a steady average never shows.
| First-year withdrawal | After 30 years (steady 5%) | Verdict in this simulation |
|---|---|---|
| 4% ($40,000) | $974,501 | Ends with more than it started |
| 6% ($60,000) | depleted in year 24 | Fails even with steady returns |
How to actually use it
Treat 4% as the middle of a band, not a switch. Compute your target at 3%, 4%, and 5% (the FIRE calculator does exactly that against your spending), and let the difference between them tell you how much precision your plan needs. The wider the band you can survive, the less any single number matters — and the less a bad decade can hurt you.