Cohort Size Calculator
Estimate how many people share your birth year, today and in the future.
Source: UN World Population Prospects 2024 (medium variant) · 2026 · last verified 2026-09-12
Source: WHO Global Health Observatory abridged life tables (lx) · 2021 — WHO's 19 age groups, interpolated to single years · last verified 2026-09-12
How this is calculated
Your birth year is converted to a current age, then matched against a modelled population snapshot to estimate how many people from your cohort are alive today. Dividing by the modelled survival ratio to your age backs out the estimated cohort size at birth, which is then projected to age 65 and 80. A cohort can only ever shrink from its birth-year size as people within it die at different ages, so each later-age estimate is necessarily smaller than the one before it.
For example, someone born in the world 30 years ago belongs to a global cohort of well over 100 million people at birth — by the time that cohort reaches age 65, mortality will have reduced it meaningfully, and by age 80 more substantially still, with the exact reduction depending heavily on country and sex-specific mortality curves. This is closely related to individual survival probability — the same mortality data applied to a whole population instead of one person — and to the National Age Percentile calculator, which shows where your age sits within your country's current population.
Frequently asked questions
- How is cohort size estimated?
- Today's modelled population at your birth year's implied age tells us how many people from that cohort are alive now. Dividing by the modelled survival ratio to that age backs out an estimate of the original cohort size at birth, which is then projected forward to age 65 and 80 using the same mortality curve.
- Why is confidence "low" on this one?
- It combines two independently modelled datasets (population snapshot and mortality curve) rather than a single direct source, so the margin for error compounds more than most calculators on this site.
- What is a birth cohort?
- A birth cohort is the group of people born in the same year (or year range) in a given country or worldwide. Demographers track cohorts over time because people born in the same year tend to share exposure to the same historical events, economic conditions, and mortality trends.
- Why does a cohort shrink over time?
- A cohort's size only ever decreases after birth — people within it die at varying ages, so the number of people from that birth year who are still alive falls steadily as the cohort ages, following the mortality curve for that country and sex.
- What data source does this use?
- This tool combines a modelled current population snapshot with mortality/survival curves derived from UN and WHO life table data, rather than querying a live registry — see the calculation explanation above for how the two are combined.
- How accurate are the projections?
- Treat the figures as reasonable estimates rather than precise counts — they combine two independently modelled datasets, so the margin of error compounds more than a calculator drawing on a single direct data source. They are most reliable for large populations and less reliable for small countries or narrow age bands.
- What is a cohort size useful for in planning?
- Cohort size estimates are used in workforce planning, pension and social security projections, school and healthcare capacity planning, and market sizing for products aimed at a specific age group — understanding how a cohort shrinks over time helps forecast future demand for age-linked services.
- How does this relate to survival probability?
- Cohort size at a future age is essentially survival probability applied to a whole population rather than one individual — the same underlying mortality curve that estimates one person's chance of reaching a given age is used here to project how many of their entire birth cohort will still be alive at that age.