The 4% Rule Explained: Safe Withdrawal Rates and Your FIRE Number

The 4% rule is the single assumption sitting underneath almost every FIRE number. It says that if you withdraw 4% of your portfolio in your first year of retirement and adjust that amount for inflation each year after, your money has a high chance of lasting at least 30 years. Understand the 4% rule and you understand how the entire FIRE target is built.

Where the 4% rule comes from

The rule has two foundational sources. In 1994, financial adviser William Bengen tested historical US market data and found that a 4% initial withdrawal, adjusted for inflation, survived every rolling 30-year period he studied — including retirements that began just before major crashes. A few years later, three professors at Trinity University ran a broader study that became known as the Trinity Study, confirming similar success rates across different stock-and-bond mixes.

Both relied on the long-run returns of US equities — the S&P 500 has averaged roughly 7% real over the long term. You can read more about the original research behind the rule in the work of William Bengen.

The 25× shortcut

The 4% rule has a convenient inverse. If you can withdraw 4% per year, then the portfolio you need is 25 times your annual spending:

FIRE number = annual spending ÷ 4% = annual spending × 25

So $40,000 of annual spending implies a $1,000,000 target; $60,000 implies $1,500,000. This 25× shortcut is why FIRE math is so quick: pick your retirement budget, multiply by 25, and you have your number. Every calculator on this site, including the FIRE calculator, uses this relationship under the hood.

What can go wrong: sequence-of-returns risk

The biggest real-world risk to the 4% rule is not the average return — it's the order of returns. A market crash in the first few years of retirement, while you are also withdrawing, can permanently shrink the portfolio before it has a chance to recover. This is called sequence-of-returns risk, and it is why two retirees with the same average return can have very different outcomes depending on when the bad years land.

The 4% rule was designed to survive these bad sequences in the historical record, but it is a probability, not a guarantee. It also assumes a roughly 30-year retirement.

Why early retirees often use a lower rate

FIRE retirees frequently stop working decades before a traditional retiree, which means their money may need to last 40, 50, or more years rather than 30. A longer horizon raises the odds that a bad sequence eventually bites, so many FIRE planners adopt a more cautious withdrawal rate — often 3.5% — which corresponds to a larger 28–29× target.

A lower withdrawal rate is simply a more conservative assumption: it raises your FIRE number and your Coast FIRE number, buying a bigger safety margin. You can model any rate yourself — set your own safe withdrawal rate in the Coast FIRE number calculator and watch the target move.

How the rule drives your Coast FIRE number

Your withdrawal rate sets your FIRE number, and your FIRE number sets your Coast FIRE number — the amount you'd need invested today so compounding alone reaches that target by retirement. A more cautious 3.5% rate makes both numbers larger; a more aggressive 4.5% makes them smaller. Because the assumption flows through everything, it is worth choosing deliberately rather than defaulting to 4% without thought.

To put it to work, learn what Coast FIRE is, then calculate your own Coast FIRE number at the withdrawal rate you're comfortable with.

The takeaway

The 4% rule is a sturdy starting point, not a law of nature. It came from real historical data, it gives you the handy 25× shortcut, and it underpins every FIRE target — but it assumes a 30-year retirement and steady spending, and it can be strained by a bad run of early returns or a very long retirement. Treat 4% as your baseline, consider 3.5% if you're retiring young, and always set the rate consciously, because it quietly determines how big a number you're chasing.

This article is for educational purposes only and is not financial advice. Figures are illustrative and depend on assumptions that may not match your situation.