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Human Geography · Foundations

Population

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On this page 9 sections
  1. In 30 seconds
  2. Why this matters
  3. The college version
  4. Eli explains
  5. Worked example
  6. Key takeaway
  7. Quick check
  8. Study tools
  9. Sources & references

In 30 seconds

Population geography studies how many people there are and how that number changes. Add up births minus deaths and you get ; the birth and death rates behind it are measured per 1,000 people each year. Total fertility rate tracks births per woman, with about 2.1 the level that keeps a population steady. The world passed 8 billion in November 2022, and growth is now slowing.

Why this matters

Almost every question in human geography starts with people: how many, how old, and growing or shrinking. Governments size schools, hospitals, pensions, and housing from these numbers, and businesses read them to find workers and customers. The composition matters as much as the total: a country with a young, fast-growing population faces very different pressures than one that is aging and stable. Knowing how birth rates, death rates, and fertility fit together lets you read a headline about "8 billion people" or a "falling birth rate" and understand what it actually means for the decades ahead.

The college version

Measuring how a population changes

Population geography begins with a simple accounting question: over a year, how does a population change? Three things move the total. People are born, people die, and people move in or out. The first two together give natural change; migration is the third and is handled separately. To compare places of very different sizes, demographers use rates rather than raw counts. The is the number of live births per 1,000 people in a year, and the is the number of deaths per 1,000 people in a year. They are called 'crude' because they divide by the whole population rather than adjusting for its age or sex makeup. Subtract the death rate from the birth rate and you get the , the growth a population would have from births and deaths alone, with migration set aside. If a country has a CBR of 18 and a CDR of 8, its RNI is 10 per 1,000, or 1.0 percent per year. A positive RNI means more births than deaths; a negative one means the reverse.

Fertility and the replacement level

The crude birth rate is easy to measure but blunt, because it depends partly on how many people happen to be of childbearing age. The is a sharper measure: it estimates the average number of children a woman would have over her lifetime if current age-specific birth rates stayed constant. A TFR of about 2.1 is called , the level that, ignoring migration, keeps a population's size roughly stable across generations. The figure is 2.1 rather than a flat 2 because a small margin allows for children who do not survive to have children of their own. When TFR sits above replacement, a population tends to grow from natural increase over the long run; when it sits below, it tends to shrink once its catches up. Globally the TFR fell from over 5 children per woman in the 1960s to about 2.3 in 2024, per the UN. That is why demographers now talk about growth slowing even though the world total is still rising.

How big, and growing how fast

Size and growth are different things, and it is easy to confuse them. The world's population reached 8 billion on 15 November 2022, according to the United Nations, and stood at about 8.2 billion in 2024. Those are size figures. Growth is the rate at which the total is changing. The annual growth rate peaked in the 1960s at over 2 percent per year (about 2.3 percent around 1963) and has more than halved, falling to under 1 percent, roughly 0.9 percent in 2023, per Our World in Data. So the population is still growing, but each year it adds a smaller share than it used to. Another way to see the slowdown: it took just 12 years for the world to go from 7 to 8 billion, but the UN projects it will take about 15 years, until 2037, to reach 9 billion. UN World Population Prospects 2024 projects the total to peak at roughly 10.3 billion around the mid-2080s and then edge down. Projections are not certainties; the UN revises them as new fertility data arrive.

Age structure and the population pyramid

A population is not just a total; it has a shape. Age structure describes how a population is divided among age groups and between the sexes, and it is usually drawn as a : back-to-back horizontal bars, males on one side and females on the other, youngest at the bottom and oldest at the top. The shape tells a story. An has a wide base that narrows quickly, meaning many children and high birth rates and, typically, a young and rapidly growing population. A stationary pyramid has bars of roughly even length up to the older ages, indicating low birth and death rates and little growth. A constrictive, or contractive, pyramid has a base narrower than its middle, the signature of low birth rates and an aging population that may eventually shrink. Age structure has real consequences: a young population needs schools and jobs, while an older one needs pensions and health care. As fertility falls and people live longer, the world's overall population is aging, and by 2024 about one in four people lived in a country whose population had already peaked.

Eli, the EliExplains learning guide

Eli explains

The same idea, in plain words

Explain it like I’m 10

Imagine a giant bathtub that stands for everyone alive. Births are water pouring in from the faucet; deaths are water draining out. If more pours in than drains out, the water level rises, and that rise is called natural increase. The birth rate and death rate just tell you how fast the faucet and drain are running for every 1,000 people. Fertility rate is a related idea: how many children a typical woman has in her life. About 2.1 is the magic number that keeps the level steady, because two children replace their two parents with a little to spare. Right now the world's tub still has more coming in than going out, so the level is rising past 8 billion, but the faucet has been turning down for decades, so the level is climbing more slowly than it used to.

Picture it like this

Population change is like a bathtub: births fill it, deaths drain it, and the water level is the total number of people.

Where the picture stops working

The bathtub leaves out migration entirely, which is a real third pipe moving people between countries; it also hides age structure, since a tub only shows the water level, not whether the population is mostly young or mostly old, which matters enormously for what happens next.

Worked example

Suppose a country of 10 million people records 150,000 births and 90,000 deaths in one year. The crude birth rate is 150,000 divided by 10,000 (thousands of people), which is 15 per 1,000. The crude death rate is 90,000 divided by 10,000, or 9 per 1,000. The rate of natural increase is the birth rate minus the death rate: 15 minus 9 equals 6 per 1,000, which is 0.6 percent per year. In raw numbers that is 150,000 minus 90,000, or 60,000 more people from natural increase, before counting anyone who moved in or out. If the same country also had a total fertility rate below 2.1, you would expect this natural increase to shrink over time as its age structure ages, even though the population is still growing today.

Key takeaway

Population geography tracks how many people there are and how that changes: births minus deaths give natural increase, a total fertility rate near 2.1 holds a population steady, and although the world passed 8 billion in 2022, its growth is now slowing.

Quick check

3 questions here, of 5 in this lesson’s practice set. Answers stay hidden until you check.

Question 1 of 3foundational

How is the rate of natural increase for a country calculated?

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Question 2 of 3intermediate

A total fertility rate of about 2.1 is called 'replacement level' because, ignoring migration, it is roughly the rate that:

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Question 3 of 3foundational

According to the United Nations, the world's population reached 8 billion in:

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Practice all 5

Keep learning

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Practice this lesson
Study tools & related lessonsYou’ll learn to · Common mistakes · Easily confused · Key vocabulary · Related

You’ll learn to

  • Define crude birth rate, crude death rate, and the rate of natural increase, and calculate natural increase from birth and death rates.
  • Explain total fertility rate and why about 2.1 is called replacement level.
  • Distinguish population size from population growth, and explain what it means that global growth is slowing.
  • Interpret an age structure from a population pyramid, distinguishing expansive, stationary, and constrictive shapes.
  • Attribute key population figures to their source and year, including the world total.

Common mistakes

  • Confusing population size with growth rate, so that 'growth is slowing' is read as 'the population is falling.'

    A slowing growth rate still adds people; the world total is rising toward a projected peak around the mid-2080s even as the yearly growth rate drops below 1 percent.

  • Treating the total fertility rate and the crude birth rate as the same thing.

    The crude birth rate depends on the population's age mix; TFR estimates lifetime births per woman and is the better measure of underlying fertility.

  • Assuming replacement level is exactly 2.0.

    It is about 2.1, because a small margin above two accounts for children who die before reaching reproductive age.

  • Including migration when computing the rate of natural increase.

    Natural increase is births minus deaths only; migration is a separate component of total population change.

  • Reading a broad-based population pyramid as a sign of a large total population.

    A broad base signals a high birth rate and a young age structure, not the country's overall size; a small country can have an expansive pyramid.

Easily confused

Crude birth rate vs. Total fertility rate

CBR counts births per 1,000 of everyone and is skewed by age structure; TFR estimates lifetime births per woman and is the cleaner gauge of fertility.

Population size vs. Population growth rate

Size is how many people there are (about 8.2 billion in 2024); growth rate is how fast that number changes (under 1 percent per year and falling).

Expansive pyramid vs. Constrictive pyramid

An expansive pyramid has a wide base from high birth rates and a young population; a constrictive one has a pinched base from low birth rates and an aging population.

Key vocabulary

Crude birth rate (CBR)
The number of live births per 1,000 people in a population in a given year.
Crude death rate (CDR)
The number of deaths per 1,000 people in a population in a given year.
Natural increase
The change in a population from births and deaths only, with migration excluded.
Rate of natural increase (RNI)
The crude birth rate minus the crude death rate, usually stated per 1,000 people or as a percentage; positive when births exceed deaths.
Total fertility rate (TFR)
The average number of children a woman would have over her lifetime if current age-specific birth rates stayed constant.
Replacement-level fertility
A total fertility rate of about 2.1 that keeps a population's size stable over the long run, ignoring migration; the margin above 2 allows for children who do not survive to reproduce.
Age structure
The distribution of a population across age groups and between the sexes.
Population pyramid
A back-to-back bar chart of a population's age structure, males on one side and females on the other, youngest at the bottom.
Expansive pyramid
An age structure with a broad base, indicating high birth rates and a young, rapidly growing population.
Constrictive pyramid
An age structure with a base narrower than its middle, indicating low birth rates and an aging, potentially shrinking population.

Sources & references

  1. World Population Prospects 2024: Summary of Results — United Nations Department of Economic and Social Affairs, Population Division
  2. Global Issues: Population — United Nations
  3. Population Growth — Our World in Data
  4. Peak global population and other key findings from the 2024 UN World Population Prospects — Our World in Data
  5. Population Topics (Population Estimates and Demographic Concepts) — U.S. Census Bureau

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Researched 2026-08-19

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