The 4% Rule Explained: Trinity Study Made Simple

Last updated: June 18, 2026 · 6 min read

What Is the 4% Rule?

The 4% rule is the most widely cited guideline in retirement planning. It states that you can safely withdraw 4% of your investment portfolio in the first year of retirement, then adjust that dollar amount for inflation every year thereafter, and your money should last at least 30 years with a very high probability of success.

For example, if you have a $1,000,000 portfolio, you would withdraw $40,000 in year one. If inflation is 3% that year, you would withdraw $41,200 in year two. And so on — the dollar amount keeps pace with inflation while the withdrawal rate stays at 4% of the original balance.

The beauty of the rule is its simplicity: it gives you a single, actionable number. Your retirement number is simply 25 × your annual expenses. Need $50,000 per year? Aim for $1,250,000.

The Trinity Study: Where the 4% Rule Comes From

The 4% rule originates from the Trinity Study, published in 1998 by three finance professors at Trinity University: Philip L. Cooley, Carl M. Hubbard, and Daniel T. Walz. The study's formal title is "Retirement Savings: Choosing a Withdrawal Rate That Is Sustainable."

The researchers back-tested portfolios against historical US stock and bond market data from 1926 to 1995. They tested withdrawal rates from 3% to 12% across various portfolio allocations (stocks vs. bonds) and retirement horizons (15, 20, 25, and 30 years).

Their key finding: a portfolio of 50-75% stocks and 25-50% bonds, with a 4% initial withdrawal rate adjusted for inflation annually, had a 95-98% success rate over 30-year retirement periods. At 3%, success was essentially 100%. At 5%, failure rates became unacceptably high.

William Bengen: The Original 4% Discovery

Before the Trinity Study, financial planner William Bengenpublished a seminal paper in 1994 in the Journal of Financial Planning titled "Determining Withdrawal Rates Using Historical Data." Bengen arrived at the same 4% conclusion independently, using similar back-testing methodology.

Bengen tested every 30-year rolling period from 1926 onward. The worst-case scenario was someone retiring in 1966, just before a brutal bear market and high-inflation period. Even that retiree, withdrawing 4% and adjusting for inflation, did not run out of money until year 33 — just past the 30-year mark.

This is why the 4% rule is sometimes called the Bengen rule. Bengen himself has since updated his research and now suggests that 4.5% may be safe for traditional retirees — but remains cautious for early retirees.

Is 4% Safe Enough? Critiques and Adjustments

The 4% rule is not without its critics. Several important limitations have been raised:

• It was designed for 30-year retirements. If you retire at 35 and expect to live to 95, that is a 60-year horizon. The 4% rule's success rate drops significantly for periods longer than 30 years.
• It assumes US historical returns. Future returns may be lower than the exceptional 20th-century US market. Many analysts predict lower forward-looking equity returns of 4-6% real.
• It assumes rigid spending. In reality, retirees can cut spending during market downturns. Dynamic withdrawal strategies (spending more in good years, less in bad ones) significantly improve success rates.
• High valuations matter. The Shiller CAPE ratio at the start of retirement strongly predicts sustainable withdrawal rates. At today's elevated CAPE levels, some researchers suggest 3.0-3.5% as a safer starting point.

4% vs 3.5%: What Early Retirees Should Know

For the FIRE community — where retirement horizons of 40, 50, or even 60 years are common — a growing consensus recommends a 3.5% or even 3.0% safe withdrawal rate. This provides an additional margin of safety against:

• Sequence of returns risk (bad returns early in retirement)
• Longer-than-expected lifespans
• Healthcare cost surprises
• Lower future equity returns

Using a 3.5% withdrawal rate instead of 4% means you need about 28.6× your annual expenses instead of 25×. On a $50,000 annual spend, that is the difference between $1,250,000 (4%) and $1,428,000 (3.5%) — an extra $178,000 in savings required. It is a bigger hill to climb, but the peace of mind is worth it for many.

The Role of Inflation in the 4% Rule

Inflation is the silent killer of retirement plans. The 4% rule's inflation adjustment is critical: without it, $40,000 of purchasing power in year one would be worth about $22,000 after 20 years at 3% inflation. The rule explicitly accounts for this by increasing your dollar withdrawal each year by the inflation rate.

However, the rule assumes that your portfolio returns outpace inflation by enough to cover the increasing withdrawals. In a high-inflation environment like the 1970s — when inflation averaged 7%+ and stocks went sideways — even a 4% withdrawal could be dangerous. This is another reason the early retirement community tends toward the more conservative 3.5% rate.

Disclaimer: This article is for educational purposes only. It does not constitute financial advice. Past market performance does not guarantee future results. Please consult a licensed financial advisor before making investment decisions.