Editor's pick: Originally published June 15.

Almost a quarter of future retirees who expect to receive Social Security either guess or don't know how much their benefits will be. At the same time, nearly three in 10 current retirees say their Social Security benefits ended up being less than they expected. David Giertz, president of distribution and sales at Nationwide, said this should not be the case.

"The Social Security handbook has thousands of rules and regulations so it's very complex," said Giertz. "Nevertheless, there are online tools and advisors to help people find out."

According to the Nationwide survey, future retirees say they expect to get $1,610 in monthly Social Security benefits. While recent retirees report their actual monthly benefit is about $1,378, those who have been retired longer reported receiving just an average of $1,185 per month.

According to the third annual Nationwide Retirement Institute survey conducted online by Harris Poll among 909 U.S. adults age 50 or older living in or approaching retirement, about one in four recent retirees would change when they started drawing Social Security to a later age. Of those recent retirees who wouldn't change, 39% say they were compelled to draw when they did by a life event.

The survey also showed that more than a third of current retirees say health problems keep them from living the retirement they expected -- and 80% of recent retirees say those health problems came earlier than expected. Health care expenses specifically keep one in four current retirees from living the retirement they expected, according to the survey.

Complicating matters more is that 86% of future retirees cannot correctly identify the factors that determine their Social Security benefit amount. As a result, many retirees do not know how to maximize their Social Security and cannot accurately determine their benefit amount.