A rule of thumb used to determine the amount of funds to withdraw from a retirement account each year. The four percent rule seeks to provide a steady stream of funds to the retiree, while also keeping an account balance that will allow funds to be withdrawn for a number of years. The 4% rate is considered to be a "safe" rate, with the withdrawals consisting primarily of interest and dividends. The withdraw rate is kept constant, though it can be increased to keep pace with inflation.

The four percent rule helps financial planners and retirees set a portfolio's withdrawal rate. Life expectancy plays and important role in determining if this rate is going to be sustainable, as retirees who live longer will need their portfolios to last a longer period of time and medical costs and other expenses can increase as the retiree ages.