The 4% Rule and CoastFIRE aren't competing ideas. They're connected. The 4% Rule tells you how much you need to retire. CoastFIRE tells you how much you need today to get there. Understanding how they fit together makes both concepts much more useful.
This article walks through exactly how to connect them, when 4% is the right number, and when you should use something more conservative.
The 4% Rule comes from William Bengen, a financial planner who published the original research in 1994. He looked at historical US market returns and asked: "If you retired at various times in history and withdrew a fixed percentage of your portfolio each year, adjusted for inflation, how often would your money last 30 years?" The Trinity Study of 1998 is the paper most people cite, but it came later and largely confirmed what Bengen found.
Their answer: if you start with a 50/50 or 60/40 stock/bond portfolio and withdraw 4% per year (inflation-adjusted), your money survives 30 years in about 95% of historical scenarios.
Bengen's own term for this was SAFEMAX: the highest withdrawal rate that would have survived the single worst starting year in the historical record. That framing matters, and we'll come back to it.
Flip the 4% Rule around and you get the FIRE formula:
FIRE Number = Annual Expenses ÷ 0.04 = 25 × Annual Expenses
Want to spend $60K/year in retirement? You need $1.5M. Want to spend $80K? You need $2M. This is the Safe Withdrawal Rate (SWR) in action.
CoastFIRE builds on top of this. Once you know your FIRE number ($1.5M), CoastFIRE asks: "How much do I need today to grow into that $1.5M by my retirement age, without adding another dollar?"
The formula: CoastFIRE = FIRE Number ÷ (1 + r)^n
So if you're 35, want to retire at 65 with $1.5M, and assume 5% real return:
CoastFIRE = $1,500,000 ÷ (1.05)^30 ≈ $347,000
Here's the part most 4% Rule articles haven't caught up with. In August 2025, Bengen published A Richer Retirement and raised his own number from roughly 4.1% to 4.7%.
What changed wasn't the market. It was the portfolio. Bengen's original work assumed a plain 50/50 split of US large-cap stocks and bonds. The new research uses a broader mix: roughly 55% stocks, 45% bonds, 5% Treasury bills, with the equity side spread across US small-cap, micro-cap, mid-cap, and international. Adding US small-caps alone lifted the SAFEMAX from 4.15% to 4.5%.
So should you plan at 4.7%? For a 30-year retirement with a genuinely diversified portfolio, it's defensible. But two caveats matter for CoastFIRE specifically. First, the 4.7% assumes you actually hold that broader allocation; a three-fund portfolio of US total market, bonds, and nothing else doesn't earn the upgrade. Second, it's still a 30-year number. Bengen's own work shows stretching to 50 years pulls the rate down.
The practical takeaway is less "use 4.7%" than "the honest range is wider than you thought." Model your plan at 3.5%, 4%, and 4.7% and look at the spread. If your decision flips between those three, you don't have a plan yet, you have a coin toss.
The original research assumed a 30-year retirement. If you're retiring early, say at 50 instead of 65, you might need your portfolio to last 40 to 50 years. That changes the math.
Here's what research suggests for different retirement lengths:
| Retirement Length | Recommended SWR | FIRE Multiplier |
|---|---|---|
| 20 years (late retirement) | 5.0% | 20× |
| 30 years (standard) | 4.0% | 25× |
| 40 years (early retirement) | 3.5% | ~28× |
| 50 years (very early) | 3.0-3.25% | ~31× |
If you're planning to CoastFIRE by 40 and live to 90, use 3.5% as your SWR, not 4%. That changes your FIRE target significantly.
A growing number of researchers (notably Wade Pfau and Karsten Jeske of Early Retirement Now) argue that 4% is too optimistic for any early retiree because:
If you're risk-averse or planning a 40+ year retirement, using 3.25% or 3.5% as your SWR gives you a meaningful safety buffer with only moderately more required savings.
The CoastFIRE Calculator lets you set your SWR directly. Open the Advanced Settings and change the Safe Withdrawal Rate from 4% to whatever fits your situation:
You'll see both your FIRE target and CoastFIRE number update live.
Try Different SWRs in the Calculator →Not quite. You withdraw 4% in the first year, then adjust for inflation after that. If your portfolio drops, your withdrawal amount stays the same (in real dollars).
It's 95% safe in historical US data. That's very good, but not perfect. In a truly bad sequence (like retiring in 1966), the 4% Rule would have left you broke before 30 years. Flexibility helps enormously: being willing to cut spending in bad years dramatically improves success rates.
The 4% Rule applies to the portion of expenses your portfolio covers. If you have a pension and Social Security covering $30K of your $60K spending, your portfolio only needs to cover $30K, so you only need $750K, not $1.5M.
The 4% Rule and CoastFIRE work beautifully together. The 4% Rule (or whatever SWR you choose) sets your target. CoastFIRE tells you how early in life you can stop saving and let compounding finish the job.
Use 4% as a starting point. If you're retiring early or want more safety, dial it down to 3.25–3.5%. Run your numbers at multiple SWRs to see the range. Your true CoastFIRE number lives somewhere in that band, not at a single pinpoint.