Population growth is the change in the number of people or organisms living in a population over time. For human populations, it happens when births and people moving in outnumber deaths and people moving out. A population can still grow while its growth rate is falling, which is why population size and population growth rate should not be treated as the same measure.
For the world as a whole, migration does not change the total because movement between countries only redistributes people. Global population change therefore depends mainly on births and deaths, while the population of an individual country, city, or region can be strongly affected by migration.
- Births add people to a population, while deaths remove them.
- Net migration measures the difference between people entering and leaving an area.
- A falling percentage growth rate does not automatically mean the population is shrinking.
- Age structure matters: a young population may keep growing for decades even after fertility falls.
What Does Population Growth Mean?
Population growth means that a population contains more individuals at the end of a period than it contained at the beginning. Demographers measure this change using births, deaths, and migration, while ecologists use similar ideas when studying animal, plant, and microbial populations.
A simple way to picture population change is a water tank. Births and immigration are like water entering the tank; deaths and emigration are like water leaving it. The water level rises when more flows in than out, stays similar when the flows balance, and falls when more leaves than enters. The comparison is simple, but it captures the basic demographic accounting surprisingly well.
| Term | Meaning | Effect on Population Size |
|---|---|---|
| Births | New individuals born into the population | Increase |
| Deaths | Individuals who die during the period | Decrease |
| Immigration | People moving into an area | Increase |
| Emigration | People moving out of an area | Decrease |
| Natural Increase | Births minus deaths | Can be positive or negative |
| Net Migration | Immigration minus emigration | Can be positive or negative |
| Growth Rate | Population change expressed relative to population size | Shows the speed of change |
| Population Momentum | Growth or decline caused partly by the existing age structure | Can continue after fertility changes |
How Is Population Growth Calculated?
The basic demographic calculation starts with the population already living in an area, then adds births and people arriving while subtracting deaths and people leaving.
Population change = births − deaths + immigration − emigration
The migration part is often written as net migration, meaning immigration minus emigration. The same relationship can therefore be shortened to:
Population change = natural increase + net migration
Suppose a town starts with 100,000 residents. During one year it records 1,200 births, 800 deaths, 1,500 people moving in, and 1,000 moving out. Natural increase is 400 people, net migration is 500, and the town gains 900 residents. Its population becomes about 100,900.
A simple percentage calculation would put the annual increase near 0.9%. Official statistical agencies may calculate annual growth rates with methods based on midyear populations or exponential rates, so their published percentage can differ slightly from a basic start-to-end calculation.
Population Growth Rate
The growth rate shows how fast a population changes relative to its size. This makes it easier to compare places with very different numbers of residents.
A town that gains 10,000 people sounds as though it is growing rapidly. Yet if it already has 10 million residents, the gain represents only about 0.1%. A town of 200,000 gaining the same 10,000 residents grows by about 5%.
Births, Deaths, and Migration Shape Population Change
Every change in a human population can ultimately be traced to births, deaths, or migration. Their relative importance varies widely between places and over time.
Natural Increase
Natural increase is the number of births minus the number of deaths. When births exceed deaths, natural increase is positive. When deaths exceed births, the population experiences natural decrease unless migration is large enough to offset it.
Birth counts depend partly on the number of people at reproductive ages, not only on how many children the average woman has. Death counts are also shaped by age structure. An older population can record many deaths even when people live relatively long lives.
Net Migration
Net migration is the number of people entering an area minus the number leaving it. Positive net migration adds population. Negative net migration reduces it.
Migration can outweigh natural change. A region may have more deaths than births and still grow because enough people move there. Another area may have more births than deaths yet lose population because outward migration is larger.
The United Nations projects that immigration will be the main source of population growth through 2054 in 52 countries and areas. This is one reason national population trends cannot be understood from fertility figures alone.
Age Structure and Population Momentum
Age structure adds another layer. A country with a large generation of children and young adults can continue growing after fertility reaches roughly replacement level because a large number of people are entering the ages when births occur.
This effect is called population momentum. The reverse can occur in an older population. Even if fertility rises, a relatively small number of adults at childbearing ages can make rapid population recovery difficult.
Why Population Growth Speeds Up and Slows Down
Human population growth changes as societies move between different combinations of birth and death rates. Demographers describe a widely observed sequence called the demographic transition.
High Birth and High Death Rates
For much of human history, both fertility and mortality were high. Families could have many children, yet infectious disease, food shortages, poor sanitation, and high childhood mortality kept long-term population growth relatively slow.
Death Rates Fall First
Population growth accelerates when survival improves while births remain high. Better food supplies, cleaner water, sanitation, vaccination, medical care, and reductions in child mortality can widen the gap between births and deaths.
Birth Rates Later Decline
Fertility commonly falls after mortality has already dropped. Changes associated with this stage can include lower child mortality, longer education, urban living, wider access to contraception, later family formation, and changing economic conditions.
Once births and deaths move closer together, population growth slows. If births eventually remain below deaths and migration does not offset the difference, population size can begin to decline.
How Fast Is the World Population Growing?
The human population is still growing globally, but the percentage growth rate is far below its twentieth-century peak. The latest United Nations World Population Prospects revision places the global population at about 8.2 billion in 2024 and expects growth to continue for several more decades.
- The world reached approximately 1 billion people in the early nineteenth century.
- It passed 8 billion in 2022.
- The global annual growth rate reached roughly 2.3% in the early 1960s and has since fallen to below 1%.
- Global fertility was estimated at about 2.25 births per woman in 2024, compared with 3.31 around 1990.
- More than half of countries and areas now have fertility below 2.1 births per woman.
- The UN’s 2024 projection expects the world population to reach about 10.3 billion in the mid-2080s before edging down to roughly 10.2 billion by 2100.
These numbers show why the statement that the human population is still growing does not mean it is growing faster every year. The total can continue rising while the percentage rate falls.
Population Size and Growth Rate Are Different
A population can add a large number of people even when its percentage growth rate is modest. This happens because the rate is applied to a much larger population base.
For example, a population of 1 billion growing by 1% gains about 10 million people in a year. A population of 100 million growing by the same percentage gains about 1 million. The rate is identical, but the numerical increase is ten times larger.
The reverse distinction is just as useful. A country whose growth rate falls from 2% to 1% has not necessarily begun shrinking. It is still growing as long as the rate remains positive; it is simply adding people more slowly relative to its size.
What Is Replacement Fertility?
Replacement fertility is the fertility level at which one generation is approximately replaced by the next over the long run, assuming no migration. A figure near 2.1 births per woman is commonly used for populations with low mortality.
The number is not an automatic population on-off switch. A country can have fertility below 2.1 and keep growing for years because of population momentum, longer life expectancy, or immigration. It can also have fertility near replacement level while experiencing little growth if its age structure is older.
The exact replacement level is not identical everywhere. It depends partly on mortality before the end of reproductive ages and on the sex ratio at birth. 2.1 is therefore a useful demographic benchmark rather than a universal physical constant.
Why Falling Fertility Does Not Immediately Stop Growth
One of the easiest population trends to misread is the delay between falling fertility and falling population size. The reason is the number of people already alive and their distribution across age groups.
Imagine that a country experienced high fertility for several decades and now has a very large young generation. Even after families begin having fewer children, millions of young adults may enter reproductive ages at roughly the same time. The number of births can remain above the number of deaths because many people are having fewer children each.
This is why demographic projections follow entire birth cohorts rather than treating fertility as the only variable. Modern projection systems also model mortality, age structure, and migration.
Exponential and Logistic Population Growth
Population ecology uses mathematical models to describe possible growth patterns. Two of the best known are exponential growth and logistic growth. They are useful models, but real populations rarely follow either curve perfectly for long periods.
Exponential Growth
Exponential growth occurs when a population grows at a constant proportional rate. As the population becomes larger, the number added during each period also becomes larger.
If 1,000 organisms grow by 10%, the increase is 100. If the population later reaches 10,000 and still grows by 10%, the increase becomes 1,000. On a graph, sustained exponential growth produces a J-shaped curve.
Logistic Growth
Logistic growth begins rapidly but slows as population density rises and limiting conditions become stronger. Ecologists represent the upper constraint with K, the carrying capacity.
A common form of the model is dN/dt = rN(1 − N/K), where N is population size, r represents the population’s potential rate of increase, and K represents carrying capacity. The resulting curve is usually described as S-shaped.
Carrying capacity is especially useful in ecology, where food, habitat, water, nesting sites, disease, predation, and competition can limit population size. Applying one fixed carrying-capacity number to the entire human population is much harder because technology, trade, consumption, infrastructure, and resource use change over time.
What Does Doubling Time Mean?
Doubling time estimates how long a population would take to become twice as large if its growth rate stayed constant. A commonly used approximation is the Rule of 70: divide 70 by the annual percentage growth rate.
- At 1% annual growth, doubling takes roughly 70 years.
- At 2% annual growth, doubling takes roughly 35 years.
- At 0.5% annual growth, doubling takes roughly 140 years.
This is an illustration, not a forecast. Fertility, mortality, and migration change over time, so a population rarely keeps exactly the same rate for the entire doubling period.
Population Growth in Everyday Situations
The same demographic rules can produce very different results depending on the population being measured.
- A growing suburb: New housing attracts thousands of residents from nearby cities. Births contribute to growth, but migration explains much of the short-term increase.
- An aging rural district: Deaths exceed births and younger adults move elsewhere for work. Natural decrease and outward migration both reduce population.
- A country with falling fertility: Families are having fewer children, yet a large young generation is reaching adulthood. Population momentum keeps the total rising for a time.
- A low-fertility country receiving migrants: Deaths may outnumber births, but positive net migration keeps the national population stable or growing.
- A recovering wildlife population: Protection increases survival and reproduction. Numbers rise quickly at first, then food, habitat, disease, and competition can slow the increase.
- A new school district: The total city population may grow slowly while the number of school-age children falls if the age structure shifts toward older adults. Population size alone does not reveal the demand for every service.
Why Population Statistics Can Be Misread
Several population measures sound similar but answer different questions. Reading them separately avoids some common errors.
- A falling growth rate is not the same as population decline. A population keeps getting larger while its growth rate remains above zero.
- Low fertility does not guarantee immediate decline. Age structure and migration may keep population numbers rising.
- More births do not always mean a higher fertility rate. A large number of women at reproductive ages can produce many births even when births per woman are falling.
- A large numerical increase does not always mean a fast rate. Large populations can add millions of people at fairly low percentage growth.
- Migration changes local and national populations but not the global total. A person crossing a border is subtracted from one national population and added to another.
How Population Growth Changes Age Structure
Population growth is not only about the total number of residents. It changes the balance between children, working-age adults, and older people. Two countries with the same population size can therefore have very different demographic needs.
A youthful, fast-growing population may require more schools, housing, maternal health services, transport, water systems, and future employment. A slowly growing or declining older population may place more demand on health care, long-term care, retirement systems, and efforts to maintain the labor force.
This is why demographers examine population pyramids and age-specific rates rather than relying only on a country’s total head count.
How Population Growth Affects Cities and Services
Rapid local growth can change demand faster than infrastructure can be expanded. Housing construction, classrooms, roads, public transport, water supply, electricity networks, waste systems, and health services may all need adjustment.
The effect depends on where growth occurs and who is being added. Ten thousand additional university-age adults create different needs from ten thousand young children or ten thousand retirees. Population planning therefore combines total growth with age, household size, location, and migration patterns.
How Population Change Unfolds
From Births and Deaths to Long-Term Population Change
1. Start With the Existing Population
Age structure determines how many people are approaching reproductive ages and how many are reaching ages where mortality is higher.
2. Add Births
Birth numbers depend on fertility rates and on how many people are in reproductive age groups.
3. Subtract Deaths
Mortality rates and the age distribution determine how many people are removed from the population.
4. Add Net Migration
For a country or local area, people moving in minus people moving out can amplify, offset, or reverse natural population change.
5. Measure the New Total
The difference between the starting and ending populations gives numerical change; relating that change to population size gives a growth rate.
6. Carry the Age Structure Forward
Today’s children become tomorrow’s adults. This creates population momentum and explains why demographic change can continue long after fertility first rises or falls.
7. Update the Projection
Future estimates change when new evidence alters assumptions about births, survival, migration, or the size of particular age groups.
Why Population Projections Change
Population projections are conditional estimates, not fixed predictions. Demographers begin with the current population and make assumptions about future fertility, mortality, migration, and age-specific survival.
Small differences can accumulate over decades. Fertility is especially influential in long-range projections because a change in births today also changes the number of potential parents in a later generation. Migration can shift more quickly and is difficult to project far into the future.
The United Nations has revised its long-range population estimates as new fertility data have become available. Its 2024 revision projects a global peak of about 10.3 billion in the mid-2080s, around 700 million fewer people in 2100 than the UN expected in projections published a decade earlier.
What Population Data Cannot Tell Us With Certainty
Historical population estimates become less precise as records become older or incomplete. Even modern censuses, registration systems, and surveys can miss people, record migration with delays, or use slightly different definitions of residency.
Future totals carry another source of uncertainty: human behavior changes. Fertility preferences, migration patterns, health conditions, economic conditions, and life expectancy can move differently from the assumptions used in a projection. The farther a projection extends into the future, the more attention should be paid to its assumptions and alternative scenarios.
Population growth is therefore best understood as a moving balance. Births, deaths, migration, and age structure interact over time, and a change in one measure can take years or decades to appear fully in the total population.
Questions About Population Growth
What are the main causes of population growth?
Human population growth occurs when births plus immigration exceed deaths plus emigration. For the world as a whole, migration cancels out, so global growth depends on births exceeding deaths.
What is natural population growth?
Natural population growth, also called natural increase, is the difference between births and deaths. It does not include migration.
Can a population grow when fertility is below 2.1?
Yes. Population momentum, longer survival, and immigration can keep a population growing even when fertility is below the approximate replacement level.
Is the world’s population still growing?
Yes. The global population is still rising, although its percentage growth rate has fallen greatly from the levels recorded in the 1960s. The United Nations expects the global total to peak around the mid-2080s under its 2024 medium projection.
What is the difference between population growth and population growth rate?
Population growth is the numerical change in population size. The population growth rate expresses that change relative to the size of the population, commonly as a percentage per year.
Does a lower growth rate mean fewer people?
Not necessarily. A population can keep adding people while its growth rate falls. Population decline begins when total population change becomes negative.
Sources
- United Nations DESA – World Population Prospects 2024: Summary of Results. This official UN Population Division report provides global population, fertility, peak-population, and long-range projection figures used in the article.
- United Nations – Population. This official overview summarizes current UN figures for fertility, longevity, migration, and projected demographic change.
- U.S. Census Bureau – The Components of Demographic Change. The Census Bureau explains how births, deaths, and migration form the demographic balancing equation used to measure population change.
- World Bank – Population Growth (Annual %) Metadata. The World Bank’s indicator documentation explains how annual population growth rates are defined and measured in international datasets.
- OpenStax – Population Growth and Regulation. This peer-reviewed educational resource from OpenStax explains exponential growth, logistic growth, carrying capacity, and limits on biological populations.
- Nature Education – How Populations Grow: The Exponential and Logistic Equations. Nature’s educational reference explains the mathematics and biological meaning behind exponential and logistic population models.
- Our World in Data – Population Growth. This research-based data reference combines historical population series with UN estimates and projections, making long-term changes in population size and growth rates easier to compare.
- Our World in Data – Demographic Transition. This research summary documents the observed sequence in which mortality falls before fertility, producing a temporary period of faster population growth.