What is a population pyramid?
A population pyramid is a chart that shows the distribution of a population by age group and sex: males on the left, females on the right, with the youngest age groups at the bottom and the oldest at the top. Its shape reveals at a glance whether a population is young and growing, ageing and shrinking, or roughly stable.
How to read a population pyramid
Each horizontal bar counts the people in one five-year age group, called a cohort: 0–4 years at the base, 5–9 above it, and so on up to 100+ at the top. Bars for males extend to the left, bars for females to the right. The vertical axis is age — and, read from bottom to top, time: a cohort appears one notch higher with every five years that pass.
The irregularities are the interesting part. A bulge is an unusually large generation — the children of a baby boom — moving upward as it ages. A notch is a missing generation: births that never happened during a war, famine or economic crisis, or a cohort thinned by mass mortality or emigration. A pyramid is a country's history written in bars: China's 1961 pyramid records the Great Famine as a bite taken out of its base, and the two world wars stayed visible in Germany's bars for half a century.
The three classic pyramid shapes
Demographers group pyramids into three broad shapes, which correspond to stages of the demographic transition.
Expansive — young and growing
A wide base tapering quickly toward the top: each generation of children is larger than its parents' generation. This is the signature of high fertility and rapid growth — and growth will continue even if fertility falls, because ever-larger cohorts are still reaching childbearing age. Nigeria is a textbook example today.
Constrictive — ageing and shrinking
The base is narrower than the middle: fewer children are born than would replace their parents. The population ages, the working-age share shrinks, and total numbers eventually decline. Japan, with one of the world's highest median ages, is the classic case.
Stationary — roughly stable
The sides are nearly vertical through the working ages: fertility close to the replacement level of about 2.1 children per woman keeps cohort sizes even. The United States sits near this shape, held close to stable by near-replacement fertility and immigration.
What a population pyramid reveals
Dependency. Comparing the bars under 15 and over 64 with the working-age bars between them gives the dependency ratios: how many children and how many older people each 100 people of working age support. Every country page on this site computes both.
Transition stage. As countries develop, they move from high birth and death rates to low ones — the demographic transition. The pyramid's shape shows where a country stands along that path: expansive early, stationary in the middle, constrictive late.
Momentum. Today's children are tomorrow's parents. A wide base guarantees decades of further growth even after fertility falls, and a narrow base makes decline hard to reverse — which is why the world's 2050 projection is largely already written into the current pyramid.
Frequently asked questions
Why is it called a pyramid?
When the chart was popularized in the late 19th century, nearly every population had high birth rates and high mortality, so the chart really was pyramid-shaped. The name survives even though many countries today look more like barrels, urns or spinning tops.
Why do women outnumber men at older ages?
About 105 boys are born for every 100 girls, so the male bars start slightly longer. But male mortality is higher at every age, and the gap compounds over a lifetime: from roughly age 55 the female bars take over, and among people aged 90 and over, women often outnumber men two to one.
What data are these charts based on?
All pyramids on this site are drawn from the United Nations World Population Prospects, 2024 revision (medium variant): harmonized estimates for every country from 1950 onward, and projections to 2100. See our sources.