Quick ReadA $1.6 million 401(k) grows to roughly $3 million by age 73, forcing RMDs that push a married couple's effective tax rate near 40%. Front-load conversions before Social Security starts at 70, and always pay the tax bill from a taxable account, never from the converted funds. A 62-year-old just retired with $1.6 million in a traditional 401(k), no pension, and plans to delay Social Security until 70. Ignored, the same $1.6 million compounds into a tax problem that eats into retirement income. Why the Bill Balloons at 73Left untouched at a 6% compound return, $1.6 million grows to roughly $3 million by age 73.