How long do you think you'll live? Tough question for sure, but it's a critical piece of your retirement planning that helps let the implications of aging sink in regarding your finances and lifestyle. If you're like many people, however, you suffer from two common misunderstandings about your longevity. Fortunately, a new tool developed by the Society of Actuaries (SOA) and American Academy of Actuaries (AAA) helps clear up the confusion.

First, the misunderstandings:

You focus on average life expectancies as reported in the media.

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You might hear that the average American lives until their late 70s, but that's the wrong number for aging baby boomers to focus on. That's the average life expectancy at birth, which factors in everybody who dies in childhood and early to mid-adulthood. But if you've lived to your 50s or 60s, you're in a more select group of people who've been healthy enough -- or lucky enough -- to make it that far.

Many people make this common mistake of confusing life expectancies at birth with remaining life expectancies at their current age, such as 65. Instead, you'll want to focus on the remaining number of years you can expect to live, given your current age. Such a calculation will usually show you might live until your 80s or 90s.

You think your life expectancy is your destiny.

If you're in your 50s or 60s and use one of the available calculators to determine your life expectancy, you might find that you have another 20 or 30 years on average to live -- or more. But it's important to understand that your estimated life expectancy isn't your destiny.

A life expectancy is just an average period of time you might expect to live based on assumptions about mortality rates. It's entirely possible that you could live well beyond your calculated life expectancy -- or fall well short. Using statistical terms, the "standard deviation" around calculated life expectancies is quite large.

Clearing up the confusion

To counter these common misunderstandings, the SOA and AAA recently released the online Actuaries Longevity Illustrator, which estimates the possible range of lifespans for individuals or couples, based on four inputs:

Age

Gender

Whether you smoke

Self-reported health as poor, average or excellent

Research shows these four factors are reasonable predictors of your longevity. For instance, suppose you're a 65-year-old woman who doesn't smoke and self-reports average health. According to the Longevity Illustrator, you have a 50/50 chance of living at least another 23 years, to age 88. But you also have a 25 percent chance of living at least another 29 years, to 94. Those one-out-of-four odds are the same as drawing a diamond out of a deck of cards.

On the other hand, the odds are also one in four that you'll pass away before 81, living 16 years or less. The range between these two possible one-in-four lifespans is 13 years, illustrating the uncertainty surrounding how long you might live.

Given these same factors, you'd also have a 10 percent chance of living at least another 34 years to 99, and a similar 10 percent chance of passing away by 74, living only another nine years or less.

Now suppose you're a 65-year-old man who doesn't smoke and self-reports average health. The Longevity Illustrator shows that you have a 50/50 chance of living at least another 20 years to 85. The remaining lifespans with one-in-four odds range from passing away by 78 (only 13 more years) to living at least until 92, another 27 years. This results in a "25 percent uncertainty spread" of 14 years.

The Actuaries Longevity Illustrator also allows a couple to see how long their money might need to last -- which should be as long as one of them is still alive. So if our 65-year-old man and woman were married, the one-in-four remaining lifetimes for them range from 22 to 32 years. And there's a 50/50 chance at least one of them will be alive in 27 years, surviving to 92.

It's just one of life's realities that you could live for a long time -- or not very long at all. It's simply not possible to know for sure. Nevertheless, it's a good idea to learn about your potential longevity because it gives you a better idea of how long your money might need to last.

It's also a good idea to develop reliable sources of retirement income that can last the rest of your life, no matter how long you live. Such sources include Social Security, a pension if you have one, a payout annuity from an insurance company and living on just interest and dividends from invested assets. With these sources, you'll want to build a diversified portfolio of retirement income.

Given the uncertainty regarding how long you could live, you'll want to plan your finances -- and your life -- to enjoy today while also being prepared to live a long time.