A large number of baby boomers have saved far less than needed for a secure retirement, according to a study released Wednesday by Bankers Life Center for a Secure Retirement.

Middle-income boomers feel $500,000 in investable assets is the threshold for retirement security, according to the study, The Middle-Income Boomer Retirement Gap: Savings, Education and Advice. The survey included 1,000 Americans aged 50 to 68 with annual household incomes between $25,000 and $100,000.  Of the respondents, 45 percent are already retired.

Most middle-income boomers have not saved $500,000, the survey says. Only 13 percent of boomers have investable assets of $500,000 or more.  Even more concerning, according to Bankers Life, more than half (54 percent) have less than $100,000, and one-third (34 percent) report investable assets of less than $25,000.

Partly because of this lack of savings, 94 percent of baby boomers, who are still working, plan to retire someday, but only 35 percent expect to retire at or before age 65, according to the study.

Boomers are concerned about meeting their financial needs, including health-care costs, in retirement, Bankers Life says. Only 38 percent feel very or extremely confident about their retirement savings, while 37 percent have some doubts and one quarter are not too confident or not at all confident.

One explanation for low investable assets may be that boomers are heavily invested in their homes, Bankers Life says.  While their median investable assets are $25,000 to $100,000, the median home equity value reported by survey participants was $100,000 to $250,000.