How Life Expectancy Is Calculated: Life Tables Explained
When headlines say "life expectancy is 73 years," almost no one means that a newborn today will die at 73. Life expectancy is a summary statistic of a life table — and understanding the table explains nearly every confusion people have about these numbers, including why a calculator's estimate for you differs from your country's published figure.
The life table
A life table follows a hypothetical cohort of, say, 100,000 newborns through life, applying each age's observed death rate. Its core column is qx: the probability that someone alive at exact age x dies before age x+1. From qx the table derives how many survive to each age (lx), and from that, ex: the average remaining years for someone who has reached age x. "Life expectancy at birth" is just e0 — the average across the whole table, including everyone who dies young. That averaging is the source of most misunderstandings.
Period vs. cohort life expectancy
Almost all published figures are period life expectancy: they apply today's death rates at every age, as if you lived your whole life under current conditions. Cohort life expectancy instead follows real birth cohorts through time, letting rates improve as medicine advances. Period figures are a snapshot; cohort figures are a projection. Calculators like ours use period tables (they're what's documented) — which means they tend to understate lifespans slightly if mortality keeps improving.
Why your estimate differs from your country's average
This is the single most useful thing to understand. Life expectancy at birth averages in everyone who died young — but if you're 50 and reading this, you've already outlived them. Your relevant number is life expectancy at 50 (e50), which is always higher than e0. This "survivorship conditioning" is why our calculator's estimate for a 70-year-old man sits roughly a decade above his country's published life expectancy at birth. It isn't an error; it's the correct conditional statistic. See the methodology for the exact computation.
The Gompertz–Makeham model in plain English
Life tables are published for whole populations, but a calculator needs a smooth curve it can adjust per person. The Gompertz–Makeham model is the standard tool: it says your risk of death has a small constant background component plus a component that grows exponentially with age — roughly doubling every 8 years or so in adulthood. That exponential rise is one of the most robust regularities in demography, observed across centuries and continents. Our calculator fixes the curve's shape and solves for the level that reproduces your country's published e0, then shifts it with your lifestyle inputs. One curve, every output — so life expectancy, median, percentiles, and survival probabilities always agree with each other.
What the number can't tell you
Life tables describe populations. They don't know your genes, your medical history, your luck. Two people with identical calculator inputs can live 30 years apart. The estimate is the center of a wide distribution — useful for planning and perspective, useless as a personal prophecy. For the complete honest accounting, read the disclaimer, then run your own numbers.
Try it with your own numbers: the free death calculator builds your personal estimate — life expectancy, median age at death, survival percentiles, and a live death-clock countdown — in under a minute, entirely in your browser.