So, you have no idea how much you should save for retirement? Join the club. Eighty-one percent of Americans said they don't know how much money they'll need to fund their golden years, according to a new survey of retirement readiness from Bank of America Merrill Lynch. "It's a complex thing to calculate. You have to make four or five assumptions to get to an answer," said Geoff Sanzenbacher, an economist at Boston College's Center for Retirement Research. One of the biggest unknowns is determining how long you will live. Among the 4,854 people questioned in the survey in August, the average person said they expected to live until age 90. That's a 20-plus-year retirement if people leave work when they qualify for full Social Security retirement benefits. However, only 16 percent of those age 50 or older said they could fund a retirement lasting 20 years. That figure rises to just 27 percent if those 50 and older were to prepare for a 10-year retirement.

People generally said they know what they have to do to save for retirement. Roughly 66 percent of those surveyed said they know they ought to start saving early and live within their means while a majority said they understood the importance of contributing to retirement accounts regularly. However, obstacles abound. A lack of income (41 percent) and an abundance of debt (38 percent) were the top two reasons people gave for not saving enough.

A lack of retirement role models

Even among savers, there is a gap between intention and action. "People don't feel like they have strong financial role models," said Kevin Crain, head of workplace financial solutions for BofA Merrill Lynch. Role models are missing because people don't talk about money, and knowledge of personal finance is low, Crain said. For example, Merrill Lynch found that roughly 40 percent of people age 50 or older said they don't clearly understand the difference between 401(k) plans and IRAs. Only 17 percent of those near retirement were comfortable in their knowledge of Social Security. Crain said 401(k) plans and other workplace retirement plans that automatically enroll participants and increase their contributions annually can help people save more even if they don't know how much they will need. It's trickier to get people to understand their retirement savings needs. "You can't automate knowing. You can't automate people's engagement with their retirement plans," said Stephen Wendel, head of behavioral finance at investment research firm Morningstar. Wendel recommends that retirement plans and financial advisors present savings and investments in terms of the monthly income that money will provide when people retire so they can grasp what those sums actually mean to their lives. But that is cold comfort to the more than half of workers — roughly 55 million — who don't have access to a retirement plan on the job or the people who can't afford financial advice.

An easy way to estimate how much to save