Understand and apply the 4% rule to set your FI number

Your FI number is 25 times your annual spending — the level at which historical markets support indefinite withdrawal.

Why it works

The 4% rule comes from the Trinity Study (Bengen 1994 and subsequent analyses), which found that a portfolio invested in equities and bonds historically survived 30-year withdrawal periods at a 4% annual withdrawal rate in most scenarios. Multiplying your annual spending by 25 gives the portfolio size that makes you FI — it is a rough, evidence-grounded target, not a guarantee.

How to do it

1. Calculate your current annual spending across all categories.
2. Multiply by 25 to find your rough FI number.
3. Note that early retirees with 40+ year horizons may want a 3–3.5% withdrawal rate for a larger safety margin.

Evidence

Bengen (1994) found that a 4% withdrawal rate from a stock/bond portfolio survived 30-year periods in all historical scenarios using US market data. Subsequent analyses with global data show the rule is more fragile outside US historical returns. (observational)

The 4% rule is calibrated to 30-year retirements using US historical returns; FIRE participants with 50+ year horizons and in low-return environments may find 4% too aggressive. It is a starting point, not a guarantee.

Sources

• Bengen (1994), determining withdrawal rates using historical data, Journal of Financial Planning
• Pfau (2010), safe savings rates — challenges the 4% rule under current market conditions, Journal of Financial Planning

Common mistake

Treating the 4% rule as a precise guarantee rather than a historical guideline, and failing to adjust for early retirement horizons, international market returns, or low-yield environments.