Nearly 60 percent of Americans say they plan to retire by age 65, but nearly the same percentage fear they'll never save enough to do it. Forty-one percent of those surveyed believe they are saving less than the average person their age.

Most Americans are feeling stretched financially, which adds to the challenge of planning and saving for retirement. The survey found that financial stress is keeping three out of four non-retired Americans (75 percent) up at night. The leading cause of sleepless nights for most non-retirees (34 percent) is supporting children and saving for college, while retirement is a top concern for only 13 percent. 

Among non-retirees, 61 percent of women and 52 percent of men are concerned they may never save enough for retirement.

One quarter of non-retired men report they are not at all concerned with the prospect of not having enough to retire, and employed men tend to save a higher percentage of their income for retirement than women (an estimated 7.2 percent vs. 5.6 percent, respectively). 

Non-retired Americans ages 45-64 are more likely to be concerned they'll never save enough for retirement (at 63 percent), while younger generations (ages 18-34) are less likely (at 52 percent). 

Working Americans indicate they know they aren't saving as much for retirement as they should, with nearly a quarter of employed adults not actively contributing.

Employed Americans ages 35-64 are more inclined to believe they should save more than 10 percent of their income for retirement than those ages 18 to 34.

Among employed adults, the average percentage of their income saved for retirement increases with household income, ranging from an estimated 3.7 percent (less than $35,000 a year) to 10.4 percent ($100,000 or more). When asked how much the average 45-year-old person needs to save to retire at age 65 and have an after-tax income of $50,000 a year, slightly more than half of non-retirees said less than $1 million is needed.

"When determining how much to save for retirement, there are a number of questions to ask and options to consider. For starters, it's important to understand your time horizon, risk tolerance and goals – do you plan to move, would you like to travel, or take up new hobbies? You should also prepare for unexpected and rising costs, like healthcare," said Greenshields. "Retirement looks very different for different people, and it's important to enter that phase of life with eyes wide open."

Capital One ShareBuilder is an online investing site. It is a subsidiary of Capital One Financial Corp.