Although it seems to be headed back down the other way lately, the life expectancy of Americans was on the rise. However, according to a study from the Society of Actuaries, workers haven't been accurately figuring their life expectancy. In fact, the study says, 51 percent of the U.S. population incorrectly estimates their life expectancy by at least five years, going either too high or too low. And that could be letting them in for some unpleasant surprises come the time they finally do retire. When trying to calculate how much money they'll need for retirement, the study finds, people are either overestimating or underestimating their life expectancy even given considerations about their health — and it can be surprising which demographic groups are going the wrong way with their estimates. Along with mis-estimating their life expectancy, people didn't necessarily judge the state of their health accurately either. Many considered themselves healthy even though their answers to questions about lifestyle and other factors influencing lifestyle indicated otherwise. "We know for example that while 70 percent of the sample is overweight (as calculated using self-reported height and weight), only 57 percent of all respondents considered themselves to be overweight," said the report. The study compared current actuarial estimates, based on a general population's estimated life expectancy based on current age and sex, as well as self-reported smoking habits and health status, with how individuals viewed their own life expectancy and survival probabilities. It then measured changes in the respondents' perceptions after they have been exposed to a series of health and lifestyle questions regarding factors known to impact longevity. While overall people were slightly more likely to underestimate their longevity, by a median of 2.0 years, that doesn't mean that they were in the clear as far as retirement planning goes. And considering that 23 percent overestimated and 28 percent underestimated their projected life expectancy by five or more years, that means that a lot of people are heading for retirement poorly prepared in one way or another. Those who expect to live longer may be planning on working longer and even postponing their retirement savings, which could bring them up short when they do retire—especially if they are unable to work as long as they'd planned. In addition, they may not be saving enough to meet the need of any financial dependents if they die earlier than they expect. Those who think they'll be on the short end of the scale might not believe they have to save as much, when in truth they might need considerably more than they think—particularly if they're prey to chronic health conditions that could eat up their savings more quickly than they expect. In fact, medical progress could mean that they live longer, even though dealing with health conditions such as diabetes, cardiac issues or mobility problems. READ MORE:

• Nearing retirement but not ready to fund it: Baby Boomers
• Top 5, bottom 5 countries for life expectancy and where the U.S. stands
• Retirement is changing: Is it time for a new social contract?