Life expectancy is the average number of years a person can expect to live, calculated from age-specific death rates in a population during a given time period. It is a population-level statistic, not a prediction of how long any individual will actually live. The number you typically see reported, like the U.S. figure of 79.0 years in 2024, reflects a snapshot of current mortality conditions applied across all ages, from infancy through old age.
A Statistical Average, Not a Forecast
The most important thing to understand about life expectancy is what it isn’t. It does not tell you, personally, when you will die. It tells you what would happen if a hypothetical group of newborns experienced today’s death rates at every age throughout their entire lives. That’s a crucial distinction because mortality rates change over time, usually improving. So the number is best understood as a summary score for how well a population is surviving right now, not a literal prediction of the future.
This is why life expectancy can shift noticeably from year to year. The U.S. figure dropped sharply during the COVID-19 pandemic, then rebounded. It rose from 78.4 in 2023 to 79.0 in 2024. Those swings reflect changing death rates across the population in real time, not changes in how long any particular person will end up living.
Period vs. Cohort Life Expectancy
There are actually two versions of this statistic, and they measure different things. The one you see in news headlines is called period life expectancy. It takes the death rates from a single year (or a short span of years) and applies them to every age group. It’s a useful benchmark, but it assumes mortality conditions never improve or worsen from that point forward.
Cohort life expectancy follows an actual birth group through time, using real death data from past years combined with projections about future mortality improvements. Because medical advances and public health gains tend to reduce death rates over time, cohort life expectancy is almost always higher than period life expectancy. In England and Wales, cohort figures typically run seven to eight years higher than period figures for the same birth year. The UK’s Office for National Statistics considers cohort life expectancy the more appropriate measure of how long someone born today will actually live on average, precisely because it accounts for likely future progress.
How the Number Is Built
Behind the single headline number sits a structure called a life table. Statisticians start with real data: how many people of each age lived in a population during a given year, and how many of them died. From those raw counts, they calculate the probability of dying at each age. Then they construct a hypothetical population, typically starting with 100,000 newborns, and run it through those probabilities year by year, tracking how many survive to each successive age.
At every age, the table records how many person-years the group collectively lives. Adding up all those person-years from a given age onward and dividing by the number of people still alive at that age gives you the average remaining years of life. At birth, that’s the life expectancy at birth. At age 65, it’s life expectancy at age 65, a separate and very useful figure.
Why Life Expectancy at Birth Can Be Misleading
Because life expectancy is an average, it’s sensitive to deaths at very young ages. A society with high infant mortality will have a dramatically lower life expectancy at birth even if adults who survive childhood live to old age. This is exactly why historical life expectancy figures can be so confusing. A life expectancy of 40 in a past century didn’t mean most people dropped dead at 40. It meant enough children died in infancy and early childhood to pull the average way down.
Research from Stanford University has shown that declines in infant mortality are the single largest driver of reduced lifespan inequality, the gap between those who die young and those who live long. Reducing child deaths doesn’t just raise the average; it narrows the spread of outcomes across the population. Deaths from infections and injuries in young people have a similar, though smaller, effect. By contrast, progress against diseases that mainly kill older adults, like cancer, raises the average age at death but does less to shrink the variation.
Life Expectancy at Different Ages
Life expectancy is not fixed at birth. It recalculates at every age because once you’ve survived past the risks of earlier years, your expected remaining lifespan changes. In the U.S. in 2024, life expectancy at birth was 79.0 years. But a person who had already reached age 65 could expect another 19.7 years on average, meaning they’d live to about 84.7. That’s nearly six years longer than the at-birth figure would suggest.
This gap exists because the 65-year-old has already survived the causes of death that affect children, teenagers, and younger adults (accidents, congenital conditions, violence, overdoses). Each year you live effectively removes a set of risks from your personal equation, and the remaining-life calculation adjusts upward accordingly. Women consistently outlive men in these statistics: in 2024, female life expectancy at birth was 81.4 years compared to 76.5 for males, a gap of nearly five years.
What Drives Differences in Life Expectancy
The enormous gains in life expectancy over the past century came overwhelmingly from controlling infectious disease. Clean water and sanitation alone accounted for nearly half of the total mortality reduction in major U.S. cities between 1900 and 1936. Antibiotics made bacterial infections survivable and enabled safer surgeries. Vaccines eliminated or controlled smallpox, measles, and polio. These interventions saved lives primarily among the young, which had an outsized effect on the population average.
Today, the factors that create gaps in life expectancy between groups are more behavioral and socioeconomic. A large international study covering 2010 to 2016 found that smoking was the single biggest contributor to life expectancy inequality, accounting for roughly 20% of the gap among men and 19% among women. Low income explained about 10% of the gap for men and 13% for women. High body weight contributed 8% for men and 12% for women. Other significant factors included low physical activity, high alcohol consumption, limited social contacts, and low fruit and vegetable intake. Growing up in a household where a parent had a manual occupation also correlated with shorter life expectancy, reflecting the long reach of early-life conditions.
Healthy Life Expectancy
Standard life expectancy counts all years lived, regardless of health. A person bedridden for their final decade counts the same as someone healthy until the end. That limitation led to the development of healthy life expectancy, sometimes called HALE. The World Health Organization defines it as the average number of years a person can expect to live in full health, adjusting for time spent living with illness or disability.
The calculation starts with a standard life table, then subtracts the equivalent years lost to disability at each age. These lost years are estimated from data on disease burden, adjusted for the fact that many people live with multiple health conditions simultaneously. The result is always lower than total life expectancy, and the gap between the two reveals how many years people in a given country typically spend in poor health. In many high-income countries, that gap is roughly eight to ten years, meaning a substantial portion of the years gained through modern medicine are lived with significant health limitations.