A population pyramid, also known as an age pyramid, is a diagram that displays the age and sex distribution of a population. It often resembles a pyramid, but can take a variety of shapes depending on the kind of population it depicts.

The population pyramid consists of horizontal bars that expand in both directions. The bars on the left side of the diagram show the male population, whereas the bars on the right display the female population. The size of the population is displayed horizontally (along the x axis), whereas its age groups are displayed vertically (along the y axis). The sex distribution can be displayed either in numbers or in percentages. Population pyramids help us to understand the age distributions of different populations by presenting their data in an easily understandable form.

Population pyramids can be divided into three distinct types depending on their shape: growing populations (common to undeveloped countries), balanced populations (developing countries) or shrinking populations (industrialized countries).

When studying population pyramids, you will probably notice that the amount of elderly women is often higher than that of elderly men. Although slightly more boys are born than girls in many societies, their mortality rate is also higher. This results in a higher life expectancy for women. When studying national population pyramids, the effects of famines, epidemics and recessions are easily distinguishable. These periods are often visible as a slight size decrease in one age group. Large age groups, such as people born during the post-World War II "baby boom", are also visible in population pyramids.

US Census Bureau