As people live longer, they spend more time in retirement, straining Social Security’s finances. This brief outlines the implications of three approaches to adjusting Social Security for longer lives: making no adjustment, which has applied over most of Social Security’s history; keeping constant the expected number of retirement years; and keeping constant the relative share of life in retirement. Compared to age 65 retirement in 1940, people under each rule would retire in 2100 at age 65, 79, and 76, respectively. The brief also shows how these calculations can be done under different assumptions.