How much a retirement portfolio can pay out each year without running dry. In 1994 a financial planner named William Bengen tested every 30-year stretch since 1926 and found that a first-year withdrawal of 4% of the portfolio, raised each year for inflation, had survived all of them, including the stretch beginning in the late 1960s. That's the origin of 'the 4% rule.' It's a historical finding about a 50/50 stock-bond mix, not a law.