What the 4% rule does and does not claim

The 4% rule says that a retiree can withdraw 4% of their portfolio in the first year, increase that dollar amount with inflation each subsequent year, and be reasonably confident the money lasts thirty years. It is a useful anchor. It is also narrower than its reputation suggests.

What the original research established

The rule descends from work in the 1990s examining historical US market data, most notably William Bengen's 1994 study and the subsequent Trinity Study. Both asked a specific question: what withdrawal rate would have survived every historical 30-year period, for a portfolio held in a mix of US stocks and bonds?

The answer was around 4%. Note carefully what that sentence contains: historical, US, 30 years, stocks and bonds. Change any of those and the number moves.

The four things that break it

1. A retirement longer than thirty years

The rule was calibrated to a 30-year horizon. Someone retiring at 45 may need the portfolio to last fifty years or more, and success rates fall as the horizon lengthens. Early retirement is precisely the case where the rule is most cited and least applicable.

2. Sequence-of-returns risk

This is the subtle one. Two retirees can experience identical average returns over thirty years and end up in completely different places, purely because of the order in which the good and bad years arrived. Poor returns in the first few years, while withdrawals are being taken from a shrinking portfolio, do damage that later good years cannot repair — the shares sold cheaply to fund early withdrawals are not there to recover.

This is why the average return is the wrong statistic for anyone drawing down, and why a constant-return projection is systematically optimistic about a retirement's early years.

3. Fees

The historical studies used index returns. A portfolio paying 1% a year in fund and advisory fees is not earning index returns; it is earning index returns minus one percent, every year, including the bad ones. That reduces the sustainable withdrawal rate materially.

4. Starting valuations

Historical success rates blend periods that began at cheap valuations with periods that began at expensive ones. Withdrawal rates that began at high valuations fared worse. This is one of the main arguments made by those who favour 3.25% to 3.5% as a starting point today.

What to use instead

Nothing replaces it cleanly, but three adjustments make it more robust.

Start lower and stay flexible. A 3.5% starting rate with the willingness to trim spending in bad years is far more durable than a rigid 4%. Most of the failure scenarios in the historical data involve continuing to withdraw the full inflation-adjusted amount into a deep bear market.

Count your other income. Social Security, a pension, or part-time work reduces what the portfolio must cover, and it reduces it permanently. That is usually a larger effect than any tuning of the withdrawal rate.

Hold a cash buffer. One to two years of spending in cash lets you avoid selling into a decline, which directly addresses the sequence risk that causes most failures.

The honest summary

The 4% rule is a good first approximation and a bad plan. Use it to size the problem — it tells you a portfolio of roughly 25 times annual spending is the neighbourhood you are aiming for — and then plan for a lower rate, more flexibility and more margin than the arithmetic demands.