The 4% rule is the single most important idea in early retirement planning. It tells you how much you can spend from your portfolio each year without running out of money — and therefore how much you need to retire. Here's everything you need to know.
If your invested portfolio is large enough that 4% of it covers your annual expenses, you are financially independent. You can stop working. Your money will sustain you indefinitely.
Imagine you have £1,000,000 invested in a diversified portfolio of stocks and bonds. Each year, you withdraw £40,000 to live on (4% of £1,000,000). Meanwhile, your portfolio continues to grow through investment returns. Historically, this 4% withdrawal has been sustainable indefinitely — the portfolio grows enough to replenish what you take out.
The practical formula is the inverse:
The 4% rule has two origins.
William Bengen, a financial planner, published research in 1994 analysing historical US market returns from 1926–1992. He found that a retirement portfolio of 50% US stocks and 50% US bonds could sustain a 4% annual withdrawal for at least 30 years in every historical scenario — even starting at market peaks or entering recessions.
Professors at Trinity University expanded Bengen's work, analysing withdrawal rates from 3% to 12% over 15, 20, 25, and 30 year periods. They found 4% had a success rate of 98% over 30 years with a 75% stock / 25% bond portfolio. This became the definitive reference for the FIRE movement.
The original study modelled 30-year retirements. If you retire at 40 and live to 90, your portfolio needs to last 50 years — a scenario the Trinity Study didn't specifically address. Most early retirement researchers recommend 3–3.5% for very long retirements.
The 4% rule has been questioned in recent years for several reasons:
The original study used historical bond yields that were much higher than today's. With bonds yielding less, the traditional 60/40 portfolio may return less than historically assumed. Some researchers suggest a 3.3% withdrawal rate for today's environment.
The study used US stock and bond data. Global markets have historically returned less than the US — investors in other countries using a pure domestic portfolio may face lower success rates.
The original study assumed fixed inflation-adjusted withdrawals every year — in practice, most retirees spend less in bad market years and more in good ones. This flexibility dramatically improves success rates. Real-world early retirees rarely follow a rigid 4% rule mechanically.
Conclusion: The 4% rule remains a useful planning tool, but treat it as a starting point rather than gospel. A 3.5% rate gives meaningful additional margin for long early retirements.