How to read a population pyramid, and a glossary

What a population pyramid shows

A population pyramid stacks the population of one year as horizontal bars by age, men on the left and women on the right, age 0 at the bottom and the oldest at the top. The length of each bar is the number of people of that age.

The shape records past births, the scars of wars and famines, and how fast the population will age. A notch marks a year with few births (Japan's 1966 'fire-horse' year), a bulge a baby boom. On this site you can drag the year and watch the shape move.

The three shapes

Expansive (wide base)

Each younger group is larger than the one above: many births, short lives, a fast-growing population. Typical of sub-Saharan Africa today and of most countries before 1950. Example: Nigeria

Bell-shaped (stationary)

Children and young adults are similar in number; births have levelled off and growth is slowing. The middle stage most countries pass through. Example: United States

Constrictive (narrow base)

Younger groups are smaller than older ones: decades of low fertility. The population ages and, without immigration, shrinks. Japan, Italy, South Korea. Example: Japan

The overview text on this site classifies a shape by the ratio of the 0–4 group to the largest five-year group under 65 (0.97 and above expansive, below 0.75 constrictive, bell-shaped between).

Glossary

Total fertility rate (TFR): The number of children a woman would have over her lifetime if the birth rates of a given year applied throughout. It is a snapshot of one year, not a prediction for any actual woman. Japan is around 1.2, Nigeria above 4. Ranking by fertility rate
Replacement level (about 2.1): The fertility rate at which each generation exactly replaces the previous one. It is slightly above 2 because some children do not reach adulthood and slightly more boys are born than girls. Below it, a population eventually shrinks unless people move in.
Median age: The age that splits the population in half: as many people are older as younger. It rises as births fall and people live longer. Niger is under 16, Japan around 50. Ranking by median age
Share aged 65+ (ageing rate): The share of the population aged 65 and over. Japan uses the labels 'ageing society' from 7%, 'aged society' from 14% and 'super-aged society' from 21%. Ranking by share aged 65+
Old-age dependency ratio: People aged 65 and over per 100 people of working age (15–64). At 50, two workers support each retiree; the inverse, 'workers per retiree', is often easier to grasp.
Demographic dividend and burden: When births fall, the share of working-age people rises for a few decades: fewer dependants per worker, which favours saving and growth (the dividend, or 'population bonus'). Later that large cohort retires and the ratio reverses (the burden, or 'population onus'). Countries in their demographic dividend
Natural change: Births minus deaths. When deaths exceed births the population shrinks 'naturally', and only net immigration can offset it. The 'natural change' card on country pages is per 1,000 people.
Net migration: People moving in minus people moving out, per year. Positive for destinations such as the US, Germany or Australia; negative for many countries with large diasporas.
Estimates and projections: Years up to 2023 are UN estimates reconstructed from censuses, registers and surveys. From 2024 the figures are projections: the 'medium variant' assumes a middle path for fertility, mortality and migration. Real outcomes diverge more the further you look ahead.
Life expectancy at birth: The average length of life a newborn would have if the death rates of that year stayed the same for its whole life. It summarises today's mortality rather than forecasting any individual's life. Ranking by life expectancy
Single years of age and five-year groups: UN data gives each single year of age (0, 1, 2 …100+), which is why country pyramids have 101 thin bars. Japan's regional projections use five-year groups (0–4, 5–9 …95+), so prefecture pyramids have 20 wider bars.

About the data

Country data come from the UN World Population Prospects 2024 (1950–2100), economic and social indicators from the World Bank, and Japan's prefectures and municipalities from the census and the IPSS regional projections (2020–2050). World Bank indicators refresh automatically every month. Figures may be reused with attribution.