The 4% Rule: The Simple Formula That Changed Retirement Planning

In 1994, a financial planner named William Bengen published a 10-page paper that would eventually let millions of people retire decades earlier than they planned. He wasn't trying to start a movement. He was trying to answer a simple question his clients kept asking: how much can I safely withdraw from my portfolio without running out of money?

Bengen's answer was 4%. If you have $1,000,000 invested, you can withdraw $40,000 in year one, increase that amount with inflation every year after, and historically your money would have lasted at least 30 years — even through the worst market conditions in modern US history. That's the whole rule. It's simple, it's been stress-tested against a century of market data, and it's been argued about ever since — which means it's worth understanding properly, not just repeating.

The origin story: one planner's 10-page paper

Bengen was a financial planner in Southern California facing a question with no rigorous answer: how much of a client's savings could they spend each year in retirement? The conventional wisdom was "somewhere around 4–6%," untested against real historical data. So he tested it. In 1994 he published "Determining Withdrawal Rates Using Historical Data" in the Journal of Financial Planning — a dense, unglamorous paper that quietly became one of the most influential documents in personal finance. He built a 50% stock / 50% bond portfolio and ran it against every rolling 30-year period of US market returns going back to 1926, testing withdrawal rates from 3% up to 10%.

The worst-case retiree in his dataset started withdrawals in 1966 — walking straight into a decade of stagflation and some of the poorest real stock returns in US history. Even in that scenario, a 4% initial withdrawal rate, increased with inflation every year after, lasted at least 33 years before the portfolio ran dry. Every other historical starting year did better. That's the entire origin of "the 4% rule": the rate that survived the worst year on record.

Bengen's paper covered one portfolio mix. Four years later, in 1998, three researchers at Trinity University — Philip Cooley, Carl Hubbard, and Daniel Walz — published a follow-up called "Retirement Savings: Choosing a Withdrawal Rate That Is Sustainable" in the AAII Journal. This became known as the Trinity Study, and it's the paper most people actually mean when they cite "the 4% rule" today. Cooley, Hubbard, and Walz tested multiple stock/bond allocations, not just 50/50, and found that a 100% stock portfolio with a 4% withdrawal rate succeeded in roughly 95% of historical 30-year periods — a result that made "4%" the mainstream shorthand.

Bengen kept working on the question himself. In a 2006 update, he revised his own recommendation upward to approximately 4.5%, after adding small-cap stocks to the portfolio mix alongside large-caps and bonds — their higher historical returns nudged the safe rate up. The FIRE community never fully adopted the higher number. 4% remained the standard, partly because it's simpler to communicate, and partly because it leaves a small margin for fees, taxes, and the fact that no future looks exactly like the past.

The number has held up reasonably well since. A 2011 update covering 1926–2009 (which includes the 2008 crash) still found roughly a 96% success rate over 30 years with a 75% stock allocation. But "held up for 30 years" is doing a lot of work in that sentence — and it's the detail most 4% rule articles skip past.

The math behind the rule: why "25x" matters

The 4% rule and the "25x rule" are the same statement written two different ways. If you can safely withdraw 4% of your portfolio each year, your portfolio needs to be 25 times your annual spending — because 100 ÷ 4 = 25. This is the single most useful shortcut in FIRE planning: multiply your annual spending by 25 to find your target number.

Monthly spending	Annual spending	FIRE number (25×)
$3,000	$36,000	$900,000
$5,000	$60,000	$1,500,000
$7,500	$90,000	$2,250,000
$10,000	$120,000	$3,000,000

The "adjusted for inflation" part of the rule means the dollar amount you withdraw grows every year, even though the percentage is fixed against your original portfolio value, not your current balance — more on exactly how that works below.

Historically, at roughly 7% average annual returns and 2.5–3% inflation, a portfolio withdrawing on this schedule has survived 30+ years in the vast majority of historical scenarios — on the order of 95–96%, per the studies above. The failures cluster in a small number of genuinely bad historical windows, like the 1966 cohort, where high inflation and weak returns hit at the same time. It's a small tail risk, but it isn't zero — which is exactly why early retirees, facing much longer horizons than the original studies tested, need to think about this differently.

The crucial thing FIRE people often miss

The Trinity Study modeled 30-year retirements. If you retire at 65, a 30-year horizon makes sense. If you retire at 40, you're looking at a 50-year retirement — and the data becomes less favorable at that timeframe.

This is the single most important limitation of directly applying the 4% rule to early retirement. The longer your retirement, the lower the safe withdrawal rate. Here's what the research suggests by retirement length:

Retirement length	Retire at age	Suggested rate	Confidence
30 years	65	4.0%	Very high
35 years	60	3.7%	High
40 years	55	3.5%	Moderate-high
45 years	50	3.3%	Moderate
50+ years	40–45	3.0%–3.25%	Conservative

These are not official published rates — they're synthesized from multiple studies including the Trinity Study updates, BigERN's Safe Withdrawal Rate series, and Pfau research. The key point: retiring early means using a more conservative withdrawal rate, which means a larger required portfolio.

Success rates if you stick with a flat 4%

Another way to look at the same problem: instead of asking "what rate should I use for my timeline," ask "what happens if I just use 4% no matter how long I'll live." Based on historical Monte Carlo analysis across different horizon lengths, here's roughly how often a flat 4% withdrawal rate has held up:

• 30-year retirement: approximately 96% historical success
• 40-year retirement: approximately 87% historical success
• 50-year retirement: approximately 80% historical success

An 80% success rate means roughly 1 in 5 historical 50-year retirees using a rigid 4% withdrawal would have run out of money before the end. That's a real number, not a scare tactic — and it's the reason a 30-something retiree needs a meaningfully different plan than a 65-year-old retiree, even though both might describe themselves as "using the 4% rule."

What FIRE retirees actually use

In practice, the FIRE community has converged on three common approaches rather than one universal number:

• Conservative: 3.5% (28.5× annual expenses) — the standard choice for anyone retiring before 45 with no other income sources.
• Standard: 4% (25× annual expenses) — reasonable for retirements around 50+, or for anyone planning to stay flexible on spending.
• Optimistic: 4.5% (22× annual expenses) — used mainly by people expecting meaningful other income, like a pension, rental income, or a spouse who keeps working.

Sequence of returns: the real threat

Average returns aren't what breaks a retirement plan. When the bad years happen is what matters. A 30% market drop in year 3 of retirement does far more damage than the same drop in year 25, because you're selling depressed assets to fund living expenses, locking in losses the portfolio never recovers from. This is sequence of returns risk, and it's the biggest reason a static "average return" projection is misleading for anyone planning a 40–50 year retirement.

It's also why a single projection line on a spreadsheet is close to useless here. MyFIRE runs 1,000 Monte Carlo simulations using real historical S&P 500 returns from 1928 to 2023, so you can see how your specific plan would have held up starting in every one of those years, including the genuinely bad ones like 1929, 1966, 2000, and 2008 — a very different, more honest picture than "7% average return, therefore you're fine."

The Social Security offset

Most FIRE retirees will eventually collect Social Security, typically starting somewhere between 62 and 70. When it arrives, it directly reduces how much you need to pull from your portfolio — which meaningfully improves your long-term survival odds, especially since it arrives well after the highest-risk early years of retirement have passed.

Example: a 50-year-old retiree spending $90,000/year draws the full $90,000 from their portfolio for the first 17 years. At 67, Social Security starts paying $50,000/year, dropping the portfolio draw to $40,000/year for the rest of retirement. Their effective withdrawal rate falls sharply right when their portfolio has had the least time to recover from any early bad sequence — which is exactly when the extra cushion matters most.

How inflation adjustment works

The rule isn't "withdraw 4% every year." It's "withdraw 4% in year one, then increase that dollar amount by inflation each year." So if you have $1,000,000 and you withdraw $40,000 in year one, and inflation is 3%, you withdraw $41,200 in year two. Year three, $42,436. And so on. Your portfolio may grow faster or slower than this, but you keep taking the inflation-adjusted amount regardless.

This matters because many people intuitively imagine taking 4% of their portfolio's current value each year — which would give you variable income and actually protect the portfolio better. The traditional rule uses a fixed real withdrawal, which gives you stable income but can erode the portfolio in bad sequences.

Calculate your number

How to apply this to your own plan

Step 1: calculate your FIRE number. Monthly spending × 12 × 25 (or × 28.5 for the more conservative 3.5% rate). Be honest about the spending figure — include housing, food, transportation, healthcare (budget $8,000–$15,000/year for a household before Medicare), travel, and irregular costs like car replacement or home repairs. Most first-time calculations underestimate real spending by 20–30%, usually because they forget the irregular stuff.

Step 2: account for taxes. A FIRE number built on 25× spending assumes you can withdraw and spend all of it. If most of your portfolio sits in a traditional 401(k) or IRA, withdrawals are taxed as ordinary income — add roughly 15–25% to your target, or build a Roth-heavy withdrawal strategy that minimizes the tax hit in the first place.

Step 3: model it against real market history, not a straight line. A static 4%-of-$X calculation doesn't account for sequence of returns, the years before Social Security starts, or the flexibility to spend less during a downturn. The 4% rule gives you a target; whether your specific plan actually survives that target under real historical conditions is the question that actually matters.

When the 4% rule is conservative enough

There are several reasons the historical success rate might actually understate your real-world safety:

• Most people flex their spending. Retiring into a market crash, most people naturally cut back on discretionary spending. A rule with some flexibility ("we'll travel less this year") has much higher real success rates than a rigid withdrawal rule.
• Social Security helps. Most early retirees in the US will eventually receive Social Security. Even a modest $1,500/month at 67 meaningfully reduces pressure on your portfolio in the later decades when sequence-of-returns risk has already passed.
• Part-time income is common. Many early retirees do some paid work — consulting, hobby income, seasonal work. Even $10,000–$15,000 per year dramatically improves portfolio longevity.

The debates worth knowing about

The FIRE community debates withdrawal rates endlessly, and some of these debates are worth understanding:

Three researchers worth knowing by name

Wade Pfau (American College of Financial Services) argued in 2013 that 4% might be too generous given the era's low bond yields, which undercut a key assumption behind Bengen's original 50/50 portfolio. He suggested rates closer to 2.8–3% for retirees starting out in that environment.

Michael Kitces, a widely-read financial planning researcher, pushes the other direction: his analysis shows the 4% rule rarely comes close to failing, and in most historical scenarios the portfolio actually grows substantially rather than depleting. His point is that 4% functions less like a tightrope and more like a floor.

Karsten Jeske, known in the FIRE community as Big ERN, has published the most detailed withdrawal-rate research aimed at early retirees — a Safe Withdrawal Rate series running well past 100 posts on his site, Early Retirement Now. His conclusion: 4% is well-supported for a 30-year retirement, but a 50-year FIRE retirement is safer around 3.25–3.5%.

The 3.5% camp: Modern research using current valuations (notably high stock prices relative to earnings) suggests future returns may be lower than the historical average. Some researchers therefore recommend a 3.3%–3.5% rate even for 30-year retirements. This is the more conservative, less-wrong-if-wrong position.

The "it doesn't matter" camp: Since most FIRE retirees will have some flexibility in spending, part-time income, and Social Security, the difference between 3.5% and 4% may not meaningfully change real outcomes. A 50-year retirement with flexibility and Social Security may be safer at 4% than a rigid 30-year retirement at 3.5%.

The variable withdrawal camp: Instead of a fixed real withdrawal, some advocate taking a percentage of current portfolio value each year. This naturally reduces withdrawals in bad markets and increases them when the portfolio grows. It gives you variable income but near-zero risk of portfolio depletion. Many financial planners now recommend this approach.

The 4% rule doesn't cover your bridge years

Here's something almost no article about the 4% rule mentions: the rule assumes you can actually get to your money. If you retire at 50, most of your retirement savings likely sits in a 401(k) or traditional IRA — and you can't touch that money penalty-free until 59½.

That means an early retiree actually needs two separate numbers, not one:

1. Your FIRE number — 25× (or 28.5× at 3.5%) your annual expenses, sized for the full retirement that starts once your tax-advantaged accounts are accessible.
2. Your bridge fund — money held outside retirement accounts, in a taxable brokerage, sized to cover spending from your retirement date until age 59½.

The bridge fund isn't governed by the 4% rule at all — it's a straightforward depletion problem, not a perpetual-withdrawal problem. A simple starting estimate: annual spending × years until 59½. Retire at 50 spending $60,000/year, and you're bridging 9.5 years — roughly $570,000 needed in taxable accounts, before accounting for any growth the bridge money earns while it's drawn down, which typically lowers the amount actually needed since the balance isn't sitting idle.

The payoff of getting this right: while the bridge fund is spent down, the 401(k) keeps compounding, untouched. Many early retirees are surprised to find their tax-advantaged accounts are larger at 59½ than they were on the day they retired. This is the phase most retirement calculators skip entirely, since they're built for people retiring at 65 who don't have a bridge problem in the first place — and it's exactly the kind of thing worth modeling precisely rather than estimating.

References and further reading

Frequently asked questions