Interpreting Population Pyramids and Age-Sex Structure

A population pyramid is a bar chart that shows the age and sex structure of a population, with age groups stacked from youngest at the bottom to oldest at the top, males on one side and females on the other. In brief: a wide base means many children and high birth rates (expanding population), straight sides mean a stable population, and a narrow base means low birth rates (declining population). The dependency ratio, (people aged 0–14 plus 65 and over) ÷ (people aged 15–64) × 100, measures how many dependents each 100 working-age people support.

Pyramids reveal the past and the future of a population at a glance, which helps governments plan schools, jobs, health care and pensions. This guide explains how to read a pyramid, the main shapes, the dependency ratio and sex ratio, compares the shapes in a table, works through calculations, outlines the history of the method and corrects common mistakes.

How do you read a population pyramid?

The vertical axis shows age groups, usually in five-year bands (0–4, 5–9, 10–14 and so on). The horizontal axis shows the number or percentage of people in each group, with males on the left and females on the right by convention. The width of a bar tells you how large that group is.

Look at four things. The base shows the recent birth rate. The sides show how quickly people die at each age: steep sides mean lower deaths at young ages, and a rapidly narrowing shape means high mortality. The top shows life expectancy and the number of elderly people. Bulges or notches show past events such as baby booms, wars, epidemics or large migration. The pyramid’s shape can also be seen as three broad groups: the young (0–14), working age (15–64) and elderly (65 and over).

What shapes do population pyramids have?

Expanding (triangle) pyramid

A wide base and rapidly narrowing sides indicate high birth rates and high death rates, and a young population that is growing quickly. This pattern is typical of many low-income countries, such as Niger or Nigeria, and requires heavy investment in education and jobs.

Stationary or slowly growing (column) pyramid

The sides are nearly straight, and the base is about as wide as the middle. Birth and death rates are both low, and the population is close to stable. Many countries in the later stages of development show this shape.

Declining (constricting) pyramid

A narrow base, a wider middle and a large top show low birth rates and an aging population, which will shrink without migration. Japan, Italy and Germany show this pattern, and it increases the cost of pensions and health care while reducing the workforce.

Special cases

Cities with many young workers, such as Gulf states with migrant labor, show a large bulge in working ages and a surplus of men. Countries after wars or epidemics may show a “notch” in the cohort that was born or died in those years.

What is the dependency ratio?

People under 15 and over 64 are generally considered dependents because they are mostly not in paid work and must be supported by those of working age (15–64). The total dependency ratio is (young + old) ÷ working-age × 100. It can be split into the youth dependency ratio (young ÷ working age × 100) and the old-age dependency ratio (old ÷ working age × 100). The ratio is a rough guide, since many people over 64 work and some people aged 15–64 are students or unemployed.

What does the sex ratio show?

The sex ratio is the number of males per 100 females. At birth it is slightly above 100, usually about 105 boys per 100 girls. Because men have higher mortality at most ages, women outnumber men among older people, so the ratio falls below 100 in the oldest groups. An unusual ratio can point to sex-selective practices, migration of workers or past wars.

Pyramid shapes compared

Worked example: dependency and sex ratio

1. Data: a country has 30 million people aged 0–14, 60 million aged 15–64 and 10 million aged 65 and over, so the total is 100 million.
2. Total dependency ratio: (30 + 10) ÷ 60 × 100 = 40 ÷ 60 × 100 = 66.7. There are about 67 dependents for every 100 working-age people.
3. Youth dependency: 30 ÷ 60 × 100 = 50.
4. Old-age dependency: 10 ÷ 60 × 100 = 16.7. The sum of 50 and 16.7 is 66.7, which matches.
5. Percentage in each group: children 30%, working age 60%, elderly 10%.
6. Sex ratio: if there are 52,000 boys and 50,000 girls in the 0–4 group, the ratio is 52,000 ÷ 50,000 × 100 = 104 males per 100 females.
7. An aging example: a second country has 15 million aged 0–14, 55 million aged 15–64 and 30 million aged 65 and over. The ratio is (15 + 30) ÷ 55 × 100 = 81.8, and most of it comes from the old-age part, 30 ÷ 55 × 100 = 54.5.

How did population pyramids develop?

Graphs of the population by age and sex came into use in the 19th century as national censuses became regular and statisticians looked for ways to show their results. The form became a standard tool of demography in the 20th century. The United Nations and national statistical offices now publish pyramids and projections for almost every country, and they are used to plan education, health care and pension systems. Projections of future pyramids are always uncertain because they depend on assumptions about fertility, mortality and migration.

Which mistakes are common?

Mistake 1: Mixing up the number and the percentage

Some pyramids show counts and others percentages. A pyramid of percentages cannot be used to compare absolute population sizes.

Mistake 2: Assuming that a wide base always means fast long-term growth

A wide base shows many children now. Whether the population keeps growing depends on later fertility and mortality.

Mistake 3: Treating everyone aged 65 and over as dependent

The dependency ratio is a simplification. Many older people work, and many young adults do not.

Mistake 4: Ignoring migration

Large inflows or outflows can distort the pyramid so that the shape does not reflect births and deaths alone.