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What is longevity?
Longevity is a description of how long lives last — usually across a population, not a promise about one person.
Longevity is the general term for how long lives last. It is a broad idea that can be discussed for a single person as well as for a whole group. When SnapLongevity puts numbers on longevity, it uses population statistics: a country, a birth cohort, or a group defined by age and sex. That shift, from one life to a group of lives, is behind most misunderstandings about longevity figures.
Three words that are often mixed up
Swipe the table sideways to see all columns.
| Term | What it measures | What it does not mean |
|---|---|---|
| Lifespan | The length of one actual life. The longest life observed for the species is a separate idea, called maximum lifespan. | It says nothing about how long people typically live. |
| Life expectancy | An average of further years for a group, produced by a set of death rates. | It is not the age at which people die, and not a personal forecast. |
| Healthy life expectancy | The part of expected life estimated to be lived in full health, under a stated method. | It is not a measure of any individual's current health. |
The distinctions in this table are the ones used throughout SnapLongevity.
An average is not a typical person
A life expectancy of 82 does not mean that people die at 82. Deaths are spread across a wide range of ages, and the average sits somewhere inside that spread without describing anyone in particular. Two countries can report the same average while having very different distributions behind it — one with deaths tightly clustered in old age, another with more deaths in middle life balanced by more survivors reaching very old age.
Why population longevity changes
Population longevity changes when death rates change. Because deaths at young ages remove more potential years of life, changes in mortality early in life can have a large effect on life expectancy at birth. That is also why the headline figure can rise while the remaining life expectancy of people already in old age barely moves.
How to read any longevity figure
A longevity number is only interpretable with its context attached. Before comparing two figures, check that they agree on all five of the following. Most misleading longevity comparisons fail on at least one.
- Age: at birth, or remaining life from some later age?
- Sex: figures are published separately for women and men, and the two usually differ.
- Country or population: the group the death rates were observed in.
- Year: death rates are specific to a period, not permanent.
- Source and method: different agencies publish different estimates for the same country.
Every figure shown on this site carries that context with it, and where a value is not published, the site says so instead of estimating one. How the numbers are produced.