A population pyramid is two back-to-back bar graphs, one showing the number of males and one showing females in a particular population in five-year age groups. Males are conventionally shown on the left and females on the right, and they may be measured by raw number or as a percentage of the total population.

A great deal of information about the population broken down by age and sex can be read from a population pyramid, and this can shed light on the extent of its development. There are two basic shapes of population pyramid.

Triangular population distribution

The triangular distribution may also be called a pyramid or exponential distribution. The wide base indicates a large number of children but the rapid narrowing shows that many people die between each age band. Therefore the pyramid indicates a population in which there is a high birth rate, a high death rate and a short life expectancy. This is the typical pattern for a less economically developed country where there is little access to or incentive to use birth control, poor hygiene (often due to a lack of clean water) and little access to health services.

Note that there tend to be more females than males in each age group. This is because females tend to have a longer life expectancy.

Rectangular population distribution

The narrow base of the rectangular population distribution indicates relatively few children and young people, and the lack of change in size between consecutive age groups that very few people die until they reach old age. This pyramid, then, is typical of a more economically developed country, which has low birth and death rates and a long life expectancy, and in which contraception is widely used and there is good public hygiene and health care.

The base of the pyramid may even be narrower than the middle. This indicates a falling birth rate.

Uses of population pyramids

Population pyramids can be used to find the number of economic dependents being supported in a particular population. Economic dependents are defined as those under 15 (children who are in full time education and therefore unable to work) and those over 65 (those who have retired). Of course, in some less economically developed countries children start work well before the age of 15, and in some more economically developed countries it is not usual to start work until 18 or 21, and people may work beyond the official retirement age of 65, but the definition provides an approximation. The government must plan the economy in such a way that the working population can support these dependents.

Population pyramids are also useful in planning for the future. They can be used in helping to predict changes in the age structure of the population over the next fifty or so years so that plans can be introduced to cope with the predicted changes. A current example of this are the British Government's plans to tackle the ageing population in the United Kingdom.

Social historians may find information of interest in population pyramids. For example, the "chip" in the UK pyramid for males born in the 1920s is due to the large number of that group lost in action during the Second World War. Correspongingly for females, death in childbirth or death in girl infants due to being exposed by their parents will show on the female side of the pyramid. In Germany there are far more males aged 20-35 than females. This is due to an influx of immigrants from Turkey and Yugoslavia who came to work in Germany in the 1990s, leaving their wives and families behind.