The Trinity Study Explained

Every FIRE calculation traces back to one academic paper: the Trinity Study. But most FIRE practitioners have never read it — and what it actually says is more nuanced than "withdraw 4% forever."

What the Trinity Study Is

Full title: "Retirement Savings: Choosing a Withdrawal Rate That Is Sustainable" Authors: Philip L. Cooley, Carl M. Hubbard, and Daniel T. Walz Published: 1998, AAII Journal University: Trinity University (hence the name)

What It Actually Studied

The researchers tested withdrawal rates from 1% to 11% across rolling historical periods from 1926 to 1995. They tested different portfolio allocations and retirement horizons (15, 20, 25, and 30 years).

Key findings for a 30-year retirement with a 50/50 to 75/25 stock/bond portfolio:

Withdrawal Rate Success Rate
3% 100%
4% 95-98%
5% 85%
6% 68%
7% 50%

What "Success" Means

The Trinity Study defined "success" as not running out of money within the specified period. It did NOT require the portfolio to maintain its value — ending with $1 after 30 years was counted as a success. This is why calculating your own FIRE number requires adapting the Trinity findings to your specific retirement horizon.

Critical Limitations Everyone Ignores

  1. US-only data. The study only used US stock and bond returns from 1926-1995. International markets have had lower safe withdrawal rates.

  2. 30-year maximum. The study only tested up to 30 years. A FIRE retiree at 40 needs 50+ years.

  3. No fees. The study assumed 0% investment fees. A 1% advisor fee drops the safe withdrawal rate to ~3%.

  4. Static withdrawals. The study assumed you withdraw the same inflation-adjusted amount every year regardless of market performance. Real retirees adjust.

  5. Historical, not predictive. The study said "this worked in the past" — not "this will work in the future."

Modern Updates

Researcher Finding Year
Pfau (2010) International data: SWR as low as 0.5% (Japan) 2010
Kitces (2008) CAPE-based dynamic withdrawals can allow 4-5% 2008
Guyton-Klinger (2006) Decision rules increase SWR to 5-5.5% 2006
Big ERN (2018) 3.25-3.5% for 60-year FIRE retirements 2018
Morningstar (2021) 3.3% is the new 4% in low-yield environment 2021

What This Means for Your FIRE Number

  • For a 30-year traditional retirement: 4% is still reasonable
  • For a 50-year FIRE retirement: 3.25-3.5% is safer
  • For a Fat FIRE with flexibility: 4% is fine (you can cut spending)
  • For a Lean FIRE with no margin: 3-3.25% buffer recommended

These targets illustrate the 25x rule in action — higher withdrawal rates correspond to lower multipliers and less margin for error.

Bottom Line

The Trinity Study was groundbreaking — but it's not scripture. Use the 4% rule as a planning benchmark, not a guarantee — see modern updates to the 4% rule for current research. For early retirees with 40-50 year horizons, target 3.25-3.5% withdrawal rates and build in flexibility to reduce spending in down markets.

Safe Withdrawal Calculator FIRE Number Calculator

Sources