Typical 401(k) growth: 8.7% a year

Highway sign with the words Retirement-Next Exit

That growth is based on the average balance for accounts that existed from 1999 through 2006 and shows how well employees investing in 401(k) plans can do despite the stock market's ups and downs during that time.

It counts contributions and earnings, but subtracts any loans. (Nearly one out of every five 401(k) participants eligible to borrow against their retirement plans has done so.)

The average account balance rose from $67,760 to $121,202, according to the Benefit Research Institute and the Investment Company Institute.

The median account balance (meaning half were worth more and half were worth less) increased from $24,898 to $66,650.

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