Firenum.com study finds 4% retirement rule success rate of 97.6%

A new backtest spreadsheet released by Firenum.com indicates that the 4% retirement withdrawal rule has a 97.6% success rate when applied to S&P 500 data dating back to 1872. Using a $1 million portfolio with a 3% annual inflation adjustment over 30 years, the study found that 122 out of 125 possible start years resulted in the portfolio remaining solvent.

The only three failures occurred during the Great Depression in 1928, 1929, and 1930. The analysis shows that adjusting the withdrawal rate significantly impacts survival: a 3.5% rate increased the success rate to 98.4%, while a 5% rate dropped it to 88.8%.

This data arrives amid ongoing debate regarding the ideal safe withdrawal rate. While William Bengen has proposed a 4.7% rate for a seven-asset portfolio, Morningstar’s 2025 report suggested a base case of 3.9%. The Firenum.com study notes that the results represent a ceiling, as the model assumes a 100% stock portfolio with no fees or taxes and reinvested dividends.