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The 4% Rule, Explained Without Jargon

October 6, 2026 · 7 min read · By Prakash Damodaran

The 4% rule is the most quoted and least understood sentence in personal finance. It says you can spend 4% of your portfolio each year, adjusted for inflation, and your money should last about thirty years. That is roughly true. But the sentence hides a study, a set of assumptions, and a list of things the study never tested. This article opens all three.

Why does it matter? Because your FI number comes straight out of this rule: FI number = (recurring annual spending / chosen withdrawal rate) + planned one-time expenses. At 4%, that is 25 times your spending, plus the one-time costs added on top, never multiplied. If you misunderstand the rule, you misunderstand your number. So let us get it right.

What did the Trinity study actually do?

In 1998, three professors at Trinity University in San Antonio, Philip Cooley, Carl Hubbard, and Daniel Walz, asked a practical question: how much can a retiree withdraw each year without running out of money? They tested withdrawal rates from 3% all the way to 12%, running them against actual US stock and bond market returns from 1925 to 1995, over retirement periods of 15, 20, 25, and 30 years.

Their definition of success was simple: the portfolio still had money in it at the end of the period. That was it. No comfort margin, no dying rich, just not hitting zero. And the withdrawals were inflation-adjusted: in year one you spent the chosen percentage of the portfolio, and every year after you raised that dollar amount with inflation, no matter what the market did.

The headline result: withdrawal rates of 4 to 5% a year, adjusted for inflation, survived 30-year retirements in the vast majority of cases, as long as the portfolio held a healthy share of stocks, typically 50 to 75%. The conservative end of that range, 4%, became the famous rule. That is the whole study in one paragraph.

Where does the 4% come from?

It comes from inflation-adjusted math, so say it plainly: year one, spend 4% of your portfolio. Year two, spend that same dollar amount plus inflation. Year three, the same again, plus inflation. Keep going for thirty years. Across nearly every 30-year stretch of US market history from 1925 to 1995, a portfolio with half or more in stocks survived that pattern.

And here is where your FI number enters. If you can live on 4% of your portfolio per year, your portfolio needs to be 25 times your annual spending, because 1 divided by 0.04 is 25. That is the origin of the 25x shorthand. But 25x alone is never the whole answer: it covers recurring annual spending only, and planned one-time expenses get added on top, never multiplied. The full formula, in today's dollars: FI number = (recurring annual spending / chosen withdrawal rate) + planned one-time expenses.

What do 3% and 5% buy you?

The withdrawal rate is a choice, and 3%, 4%, and 5% are three different deals with safety. At 3%, your FI number is 33x your annual spending: the most expensive target, the widest margin, the right choice for very long retirements or anyone who wants maximum cushion. At 5%, your number is 20x your spending: cheaper to reach, thinner margin, defensible if your spending is flexible or your horizon is shorter.

Four percent, the 25x middle, is a reasonable starting point for a retirement of about thirty years, which is what the study tested. Retiring at 40 with fifty years ahead of you? The study has nothing to say about that, and 4% starts to look aggressive. Consider 3 to 3.5% instead, which is 33x to 29x your spending. The rate you pick should match your horizon and your nerves, not someone else's blog post.

What does this look like in dollars?

Take a household spending $60,000 a year on recurring costs. At 4%, the FI number is $60,000 divided by 0.04, which is $1,500,000. At 3%, it is $60,000 divided by 0.03, which is $2,000,000. That half-million-dollar difference is the price of the wider safety margin: real money, and real years of additional saving.

Now add a planned one-time expense: $150,000 for a child's college, coming in a few years. It goes on top, never multiplied: $1,650,000 at 4%, $2,150,000 at 3%. Notice what did not happen: nobody inflated anything. Everything is in today's dollars, because the rule's inflation adjustments already handle rising prices. Your portfolio, growing in real terms, is measured in today's dollars too.

What does the rule NOT cover?

Fees. The study assumed a portfolio with no advisory fees and no fund expenses. If you pay a 1% annual fee and spend 4%, your portfolio is effectively paying out 5% a year. Fees come straight out of your safety margin, which is why low-cost investing is not a preference. It is part of the math.

Taxes. The study ignored taxes entirely. Your 4% has to cover everything you spend, including the taxes you owe on withdrawals and income. So the "annual spending" in your FI formula is gross spending, taxes included, not just the fun part of the budget.

Horizons longer than 30 years. The longest period the study tested was thirty years. A 40-year-old retiree faces fifty years, and the study offers no evidence there. Longer horizons mean more chances for a bad decade early on, and early crashes do the most damage. This is called sequence-of-returns risk: a crash in your first five years of withdrawals hurts far more than the same crash twenty years in. A lower withdrawal rate is the simplest defense.

Two more honest footnotes. The study used US market history only, and the future may not cooperate the same way. And "success" meant the portfolio did not hit zero; some surviving paths ended with almost nothing left. The rule is a planning tool built on history, not a guarantee from the universe.

Has the rule been re-tested since 1998?

Many times, against more years of data and other countries' markets, and the broad finding has held up: withdrawal rates around 4% survived thirty-year retirements in most historical periods. Critics argue that today's high stock valuations and lower bond yields could make the future worse than the past, and they have a point worth hearing. The honest response is not to abandon the rule but to remember it was always a starting point: if the future is worse than the past, the answer is a lower rate and a bigger number, chosen deliberately, not panic.

There is also a quieter criticism, and it cuts in your favor: the study assumed you never adjust spending, even in a crash. Real retirees cut back in bad years, which improves real-world outcomes beyond what the study measured. The rule is pessimistic about your behavior and optimistic about history. Knowing both biases is what lets you use it wisely.

How should you actually use it?

Pick your horizon honestly. If it is around thirty years, 4% is a defensible starting point. If it is much longer, start at 3 to 3.5%. Compute your FI number in today's dollars: recurring annual spending divided by your chosen rate, plus planned one-time expenses on top. Then leave the number alone for a year and revisit it.

The rule exists to give your planning a spine, not to be recited like a spell. Understand what it tested, respect what it did not, and treat 4% as a starting point. Adjust your rate, not your hope.

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