The 4% rule is the most famous guideline in retirement planning: withdraw 4% of your portfolio in your first year of retirement, give yourself an inflation raise every year after, and history says your money should outlast a 30-year retirement. It’s also the engine inside every FIRE number and every Coast FIRE calculation — including our Coast FIRE calculator. Here’s what it actually says, where it came from, and what the latest 2025–2026 research changes (and doesn’t).
The Definition, Precisely
The rule is more specific than most people realize:
- Year 1: withdraw 4% of your starting portfolio balance
- Year 2 onward: withdraw last year’s dollar amount adjusted for inflation — not 4% of whatever the portfolio happens to be worth
So with a $1,000,000 portfolio, you withdraw $40,000 in year one. If inflation runs 3%, you withdraw $41,200 in year two, $42,436 in year three, and so on — regardless of whether the market rose or crashed. That inflation-adjusted consistency is the whole point: your lifestyle never has to shrink.
| Portfolio at retirement | Year-1 withdrawal (4%) | Monthly equivalent |
|---|---|---|
| $750,000 | $30,000 | $2,500 |
| $1,000,000 | $40,000 | $3,333 |
| $1,500,000 | $60,000 | $5,000 |
| $2,000,000 | $80,000 | $6,667 |
Where It Came From: Bengen and the Trinity Study
1994 — William Bengen. A financial planner (and former MIT-trained engineer), Bengen tested every historical 30-year period of US market data against an inflation-adjusted withdrawal plan, using a portfolio of roughly 50% large-cap US stocks and 50% intermediate-term government bonds. The highest withdrawal rate that survived every historical period — including retiring right before the 1970s stagflation — came out to 4.15%. He called it the “SAFEMAX.” The financial press rounded it down, and the “4% rule” was born.
1998 — The Trinity Study. Three professors at Trinity University extended the analysis across multiple stock/bond mixes and success-rate thresholds. Their finding: portfolios with 50–75% stocks sustained a 4% inflation-adjusted withdrawal for 30 years with a 95%+ success rate in historical data. The study gave the rule academic credibility, and it has been the default planning assumption ever since.
Flipped around, the rule gives you the Rule of 25: if 4% withdrawals are safe, you need 100 ÷ 4 = 25 times your annual spending invested to retire. That inversion is the foundation of every FIRE number — we cover it in depth in our Rule of 25 guide.
The 2026 Update: 4.7% vs 3.9%
Here’s where it gets interesting. The two most important recent pieces of research point in opposite directions:
Bengen’s 2025 revision — up to 4.7%. In his August 2025 book A Richer Retirement, Bengen re-ran his analysis with a broader portfolio (seven asset classes including small-cap, international stocks, and T-bills, roughly 55% stocks / 40% bonds / 5% cash). The historical worst-case SAFEMAX rose to 4.7%. His striking finding: 4.7% was the floor — only the single unluckiest retiree in ~400 historical scenarios needed to withdraw that little, and the average historically safe rate was around 7%. His caveat for early retirees: for 50-year horizons, the safe rate drops to roughly 4.1%.
Morningstar’s 2025 research — down to 3.9%. Morningstar’s annual State of Retirement Income study takes the opposite approach: instead of assuming the future will resemble the past, it uses forward-looking return forecasts (which currently reflect high equity valuations and modest expected returns). Its base case for new retirees: 3.9% for a 30-year horizon with a 90% probability of success — up slightly from 3.7% the year before.
Why they differ: Bengen asks “what survived the worst of history?” Morningstar asks “what survives the returns we actually expect from today’s prices?” Neither is wrong; they’re answering different questions. Bengen’s number is a floor tested against a century of actual markets, while Morningstar’s is a forecast that moves with valuations and bond yields each year. It’s also worth noting what Morningstar’s own research found alongside its cautious base case: retirees willing to be flexible with spending could start at nearly 6% — flexibility, it turns out, is worth more than precision.
What a regular person should do:
- Retiring at a traditional age (60s), 30-year horizon: 4% remains a fine benchmark, with Bengen’s work suggesting it carries real margin
- Retiring early with a 40–50+ year horizon: plan on 3.25–3.5%, and be honest that you’re choosing safety over spending
- Either way: treat the rate as a starting assumption you revisit annually, not a law
Sequence-of-Returns Risk: The Rule’s Weak Point
Two retirees can earn the exact same average return over 30 years and have opposite outcomes — the only difference being the order of good and bad years. That’s sequence-of-returns risk.
A simplified example: two portfolios start at $1,000,000 withdrawing $40,000/year. Portfolio A loses 20% in year one ($1,000,000 → $800,000 → $760,000 after withdrawal). Portfolio B gains 20% ($1,200,000 → $1,160,000 after withdrawal). Same average market over time, but Portfolio A is digging out of a hole while selling shares cheap — early losses plus withdrawals lock in damage that later gains can’t fully repair. This is why the first 5–10 years of retirement matter more than all the rest, and why inflexible inflation-adjusted withdrawals are the riskiest version of the strategy.
What the 4% Rule Doesn’t Cover
Before betting your retirement on any withdrawal rate, know what’s outside the model:
- Taxes. The original research tested withdrawals from tax-advantaged accounts without modeling the tax bill. Money coming out of a traditional 401(k) or IRA is taxed as ordinary income — a $40,000 withdrawal might net you $34,000–$36,000 after federal and state taxes. Roth withdrawals are tax-free, which is one reason planners love Roth conversions in low-income years.
- Fees. The studies assume index-like returns. Paying 1% annually in fund fees and advisory costs can knock a full percentage point off your safe withdrawal rate.
- One-time shocks. The model assumes smooth, inflation-adjusted spending. Real retirements include roof replacements, family help, and long-term care — expenses the steady-withdrawal plan has no line item for.
- Non-US investors. The rule is built on US market history, one of the strongest records anywhere. Researchers testing other countries’ data often find lower safe rates.
- Early retirement horizons. The rule was validated over 30 years. A 45- or 50-year retirement isn’t just “30 years plus more” — the failure modes change, which is why longer horizons demand lower rates (Bengen himself pegs 50-year SAFEMAX near 4.1%).
Flexibility Strategies That Raise the Safe Rate
The research is consistent on one point: rigid spending is expensive. Retirees willing to adjust can safely start meaningfully higher — Morningstar found flexible systems support starting rates approaching 6% in some configurations. Common approaches:
- Guardrails: set a band (say, withdrawals stay within 20% of the planned rate); if the portfolio falls and your effective rate breaches the ceiling, cut spending temporarily; if it surges past the floor, give yourself a raise
- Skip the inflation raise after down years: a simple rule with an outsized effect on portfolio survival
- Cash buffer: hold 1–2 years of expenses in cash or short-term bonds so you never sell stocks during a crash — you spend the buffer and refill it in good years
- Part-time income floor: even $10,000–$15,000/year of early-retirement income dramatically reduces withdrawal pressure (this is the entire logic of Barista FIRE)
How This Maps to Your Coast FIRE Number
Every Coast FIRE calculation starts here. The two-step formula (detailed in our Coast FIRE formula guide) is:
- FIRE number = annual spending ÷ withdrawal rate
- Coast FIRE number = FIRE number ÷ (1 + real return)^years until retirement
The safe withdrawal rate input on our calculator is literally the 4% rule — default 4%, adjustable. Try it: switching from 4% to 3.5% raises a $40,000-spending FIRE number from $1,000,000 to about $1,143,000, and your Coast FIRE number rises by the same ~14%. Watching the number move as you drag the SWR input is the fastest way to internalize what this article just explained.
Two final calibration notes. First, the 4% rule assumes portfolio-only income — if you’ll collect Social Security or a pension, subtract that guaranteed income from your spending before applying the rule, and your required portfolio shrinks substantially (see Coast FIRE with Social Security for the full method). Second, the rule is a planning heuristic built on US historical data, not a guarantee — stress-test your plan at 3.5% and 3% before betting a decades-long retirement on any single number.
The Bottom Line
The 4% rule has survived three decades of scrutiny remarkably well: its creator now says it may be too conservative for 30-year retirements, while forward-looking models counsel a touch more caution. The sane middle path: use 4% as your benchmark, 3.5% for long horizons, stay flexible in downturns, and layer guaranteed income on top.
Then put it to work: enter your numbers in the Coast FIRE calculator, and see exactly how much you need invested today — at whatever age you are now — to let compound growth carry you the rest of the way.