How long does your retirement savings need to last? First, learn how long you might live.

One of the things that makes it so hard to save for retirement is not knowing how long it will last. You’re trying to amass a nest egg big enough to never run out of money while you’re alive — without knowing how long you’ll live.

But having a rough estimate can help you save smarter for your retirement years. 

A new study by the TIAA Institute-GFLEX Personal Finance Institute found that only 37% of U.S. adults have a solid understanding of how long they could potentially live. That means, the majority of the population is essentially aimlessly planning for retirement, not knowing how much to save or how long that savings needs to last.

“Longevity literacy is fundamental,” TIAA said. “Appropriate decision making is contingent upon understanding how long retirement may last.”

In the study, TIAA for the first time included a question to measure “longevity literacy”—or an understanding of how long men and women typically live. Men and women were asked about life expectancy at age 60 for men and women, respectively. Both versions of the question gave four response options—the correct answer, an overestimated response, an underestimated response, and a “don’t know” option.

The correct response was 85 years for women and 82 years for men. 

Read: Want to live to 100? Here’s what the latest longevity research says

A total of 37% respondents answered correctly, while 28% responded “don’t know” and were considered to have “poor longevity knowledge.” In addition, 25% chose the response that underestimated life expectancy for a 60-year-old, and 10% chose the response that overestimated life expectancy. 

“Considering those with poor longevity knowledge and those underestimating life expectancy together, more than one-half of U.S. adults (53%) are working with inaccurate information which can jeopardize their retirement preparedness,” TIAA said.

The importance of having strong longevity literacy is that it means people can plan better and save more appropriately for retirement.

“As with financial literacy, retirees with strong longevity literacy were more likely to plan and save for retirement while still working compared with those with poor longevity knowledge, and they tend to experience better financial outcomes in retirement,” TIAA said.

As with strong financial literacy, retirees with strong longevity literacy more typically planned and saved for retirement while still working and now tend to experience better financial outcomes in retirement, TIAA said.

Basically, those who demonstrated poor longevity knowledge reported that their lifestyle in retirement fell short of their preretirement expectations compared with those demonstrating strong longevity knowledge. 

TIAA found that the most striking finding of the study was that women tended to have greater longevity literacy than men—with 43% of women demonstrating strong longevity knowledge compared with 32% of men. This stands in marked contrast to financial literacy levels—financial literacy among women consistently lags that of men, the study said.

“While typically an overlooked factor, the importance of longevity literacy is not surprising since retirement income security inherently involves planning for the time that will be spent in retirement, which is uncertain,” TIAA said.

Do you have questions about retirement, Social Security, where to live or how to afford it at all? Write to [email protected] and we may use your question in a future story.