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How life expectancy is calculated
Life expectancy is not a prediction of when anyone will die. A life table takes the age-specific death rates observed in one period and turns them into the average further years a group would live. SnapLongevity shows WHO's published values rather than recomputing them.
Life expectancy comes from a life table. Statisticians start with observed death rates for each age group in a population and a given year, then follow an imaginary cohort of 100,000 newborns through those rates. Nobody in that cohort exists. The table is a device for turning a year's mortality into a single interpretable number.
What it actually measures
The input is mortality, not health, wealth or lifestyle. If a population's recorded death rates at each age are known, the life expectancy follows arithmetically. That is why the figure changes when death rates change, and why it can move for reasons — an epidemic, a change in death registration — that have nothing to do with how long any individual will live.
In plain English
- Age-specific death rate
- The number of deaths in an age band during a year, relative to the population living in that band.Why it matters: Age-specific mortality rates are the core empirical input to a period life table; the remaining life-table quantities are derived through the table's methods and assumptions.
The columns
- nMx — the observed death rate in an age band.
- nqx — the probability of dying within that band, derived from the rate.
- lx — how many of the original 100,000 are still alive at the start of each age.
- ndx — how many die within the band.
- nLx — person-years lived within the band.
- Tx — person-years lived at that age and above.
- ex — life expectancy at that age: Tx divided by lx.
Observed death rates
Recorded for each age band in a population and year.
An imaginary cohort
100,000 newborns followed through those rates.
Survivors and person-years
How many remain alive, and how long they live within each band.
Life expectancy
Person-years at that age and above, divided by survivors.
These are the life-table columns listed above, in the order they are calculated.
Read in order, the table does something simple: it counts how much life the imaginary cohort gets to live from a given age onwards, then divides that total by the number of people still alive at that age. The answer is an average of remaining years — nothing more.
Where survival probabilities come from
In the life table, the conditional probability of surviving from one age to a later age is the ratio of the corresponding lx values. That is a direct ratio of published numbers, which is why SnapLongevity can show milestone probabilities without estimating anything.
At birth versus at a later age
Life expectancy at birth is the ex value on the first row: an average across everyone, including those who die young. Remaining life expectancy at a later age is the ex value on a later row, computed only from the survivors who reached it. They are different rows of the same table, answering different questions, and they cannot be converted into one another by subtraction.
Swipe the table sideways to see all columns.
| Figure | Group it describes | What it is for |
|---|---|---|
| Life expectancy at birth | The whole imaginary cohort, including those who die young. | Comparing populations in a single headline number. |
| Remaining life expectancy at age x | Only those still alive at that age. | Describing the expected further years of people who have reached it. |
Abridged tables, and where they stop
WHO publishes abridged life tables, grouped into five-year bands up to 85 and over. That is why the Explorer maps a chosen age to its official band, and why it cannot show survival to age 90 or 100 from this source: the table stops at the 85+ band. Rather than extrapolating beyond the published data, the site reports those milestones as unavailable.
The exact indicators, data year and validation steps behind those values are listed on the methodology page. How the numbers are produced.
Why it is not a predicted date of death
The life table describes a hypothetical cohort under one year's conditions. It contains no information about an individual — not their health, their circumstances, nor how mortality will change over their actual lifetime. Adding a life expectancy to a birth date produces a number, but not a meaningful one.