Population growth describes the change in the number of people in a defined group—commonly a country, region, or the world—over time. It can be expressed as an absolute change (number of people added) or as a rate (percentage change per year). Growth results from the combination of births, deaths and migration and shapes economic needs, public services and environmental pressures.

Measurement and basic concepts

Demographers measure growth with indicators such as the crude birth rate, crude death rate and the rate of natural increase (births minus deaths). The overall population growth rate incorporates net migration as well. Percent growth per year is a common way to report change: for example, an increase from 100 to 110 in one year is a 10% rise, an illustration often used to explain the concept. When growth compounds, it can be modelled as exponential; a simple rule of thumb used in planning is the "rule of 70," which estimates doubling time by dividing 70 by the annual growth rate (in percent). For further explanation see this example.

Causes and dynamics

Several interacting factors determine whether a population grows, stabilizes or declines. Key drivers include:

  • Fertility: average number of births per woman influences long-term change.
  • Mortality: improvements in healthcare and sanitation lower death rates.
  • Migration: movement between areas alters size independently of births and deaths.
  • Age structure: a young population can sustain growth even if fertility falls, a phenomenon known as population momentum.

Historical patterns and regional variation

Human population growth has varied over history. Pre-industrial societies typically combined high birth and death rates, producing relatively slow growth. The demographic transition model describes a shift to lower death rates followed by lower birth rates as societies develop, producing a period of rapid growth that later slows. Today, some regions experience rapid expansion, while others face low fertility and population aging. Urbanization commonly accompanies these shifts, concentrating people and changing economic and environmental demands.

Impacts, examples and policy responses

Population change influences housing, education, labor markets, food systems and ecosystems. Rapid growth can strain infrastructure and natural resources; decline or aging populations create challenges for pensions and workforce replacement. Governments and organizations respond with policies such as family planning, investments in education and health, urban planning, and migration management to shape outcomes. Accurate measurement—via censuses, surveys and registration systems—is essential for effective planning.

Distinctions and notable facts

It is important to distinguish natural increase (births minus deaths) from overall growth, which also includes net migration. Growth models range from exponential, which assumes continual proportional increase, to logistic formulations that incorporate limits or carrying capacity. Projections are inherently uncertain: small changes in fertility, mortality or migration sustained over decades can produce substantially different population futures. For introductory definitions and related terms consult general resources on population.