The 4% Rule: Why 25× Your Spending Is the Magic FIRE Number

Where the 4% Rule Comes From

In 1994, financial advisor William Bengen set out to answer a simple question: what is the maximum percentage of a retirement portfolio you can withdraw each year without running out of money over a 30-year retirement? Using historical US stock and bond market data going back to 1926, he found the answer was 4%.

This was validated and popularized by the 1998 Trinity Study, which tested withdrawal rates from 3% to 12% across portfolios of various stock/bond mixes over rolling 30-year periods. A portfolio of 50–75% stocks withdrawing 4% annually succeeded in 95%+ of historical scenarios.

The Simple Math

The 4% rule gives us the famous "25× rule": your FIRE number is 25 times your annual spending. This is just the inverse of 4% (1 ÷ 0.04 = 25).

Each year you increase your withdrawal by inflation (historically ~3%/yr) to maintain your purchasing power. The portfolio's investment returns cover this and preserve the principal.

Want to see your personal FIRE number? The FIRE Calculator runs the real math including taxes, account type, and your specific spending, not just a simple 25× estimate.

Why Taxes Change Everything

Here's what most 4% rule explanations skip: the spending number in the formula must be your after-tax spending, but what actually matters to your portfolio is your gross withdrawal, what you pull out before taxes.

If you need $50,000/year to live on and your money is in a traditional 401(k), you might need to withdraw $65,000–$70,000 to net $50,000 after federal, state, and FICA taxes. That means your real withdrawal rate is closer to 5–5.5%, not 4%. The research says 5% portfolios fail in roughly 20% of historical scenarios.

This is why account type matters so much. A dollar in a Roth IRA is worth more than a dollar in a traditional 401(k) because Roth withdrawals are tax-free. A dollar in a taxable brokerage account falls somewhere in between, you owe capital gains tax, but at a lower rate than ordinary income.

The FIRE Calculator on this site adjusts your targets based on account mix. If you have a mix of Roth, traditional, and taxable accounts, the real math gets complicated quickly, and the simple 25× rule will give you the wrong number.

The 4% Rule and the FIRE Timeline

One thing that surprises people is how dramatically your savings rate, not your income, determines how long it takes to reach your FIRE number. Here is why: a high savings rate does two things simultaneously. It increases how fast your portfolio grows, and it decreases the portfolio size you need (because you are spending less).

Someone saving 10% of their income needs roughly 40 years to reach FIRE. Someone saving 50% needs around 17 years. The 4% rule is the finish line, your savings rate is how fast you run toward it.

The 4% Rule's Weaknesses

The rule was designed for a 30-year retirement. If you retire at 40, you may need your money to last 50+ years, a scenario the original research didn't fully model. For longer retirements, many FIRE practitioners use a more conservative 3–3.5% withdrawal rate (the Fat FIRE and FIRE tiers on this site use 3% and 3.33% respectively).

Other caveats to keep in mind:

• Sequence of returns risk, retiring into a market crash is far more dangerous than the long-term average suggests. A 30% drawdown in year 1 of retirement is devastating. Read more about sequence of returns risk here.
• US-centric data, the research is based on US markets. Other developed markets have had lower long-run returns.
• Valuation sensitivity, some researchers argue that starting withdrawals during periods of high market valuations warrants extra caution.
• Flexibility helps, retirees who can trim spending by 10–20% during downturns dramatically improve their success odds.
• The bridge problem, if you retire before 59.5, you cannot touch your 401(k) without a 10% penalty. You need a bridge strategy using taxable accounts or a Roth conversion ladder to fill the gap.

Variations: Lean, FIRE, Fat FIRE, and Coast

The 4% rule is just one point on a spectrum. Different FIRE strategies use different withdrawal rates depending on how much cushion you want:

• Lean FIRE, uses a 5% withdrawal rate (20× spending). Higher risk, smaller portfolio needed, frugal lifestyle required.
• FIRE, the classic 4% / 25× rule. Solid historical success rate over 30-year periods.
• Fat FIRE, uses 2.5–3% withdrawal (33–40× spending). Built for long retirements and lifestyle flexibility.
• Coast FIRE, a different approach entirely. Instead of targeting a withdrawal number, you target the amount needed today so compound growth reaches your FIRE number by retirement age, with zero additional contributions. Calculate your Coast FIRE number here.

The Bottom Line

The 4% rule is a powerful heuristic, not a guarantee. Used as a starting point, combined with tax-aware planning, flexible spending, and a diversified portfolio, it has held up remarkably well across a century of market history. The key is to use the right gross withdrawal number, not just your after-tax spending target, and to plan for a longer runway than the original research assumed.

Use the FIRE Calculator to find your real number, one that accounts for taxes, account types, and your specific situation rather than a back-of-the-envelope 25× estimate.

My Take

One downfall of the 4% rule is that it is usually built for a 30 year outlook or so, someone at traditional retirement age drawing down a portfolio they spent 40 years building. With FIRE you have to plan further ahead and make sure there is enough bridge money to fill in from a taxable brokerage until you actually hit retirement age and can pull from a 401k without penalty. The rule still works, you just have to solve the bridge problem first.