Population growth is the change in the number of people living in a given area over time. It’s determined by a simple equation: the number of births minus the number of deaths, plus the net number of people migrating in or out. Globally, the world’s population stands at roughly 8.1 billion people in 2025, with a net gain of one person every 56 seconds.
How Population Growth Is Calculated
At its core, population growth comes down to three components: births, deaths, and migration. The growth rate for any country in a given period equals the crude birth rate minus the crude death rate, plus the net migration rate. If more people are born than die, and more people move in than leave, the population grows. If the reverse is true, it shrinks.
To put that in real terms, the U.S. Census Bureau estimates the current global pace as one birth every 9 seconds and one death every 10 seconds, with one net international migrant every 100 seconds. That slight surplus of births over deaths, plus migration flows, is what keeps the world population ticking upward.
Why Populations Grow (and Eventually Slow Down)
Population growth isn’t random. It follows a well-documented pattern called the demographic transition, which unfolds in stages as societies modernize.
In the earliest stage, both birth rates and death rates are high, so the population stays relatively stable. Many children are born, but many also die young, and few people survive to old age. This was the norm for most of human history.
Growth accelerates in the second stage, when health improvements cause death rates to fall while birth rates remain high. This is the period when populations balloon. Families still have many children, but far more of those children survive. Historically, this is the era of large extended families.
In the third stage, birth rates start to catch up with the already-low death rates. Parents adapt to the reality that most of their children will survive and choose to have fewer. Women gain more economic and social power, which also drives family sizes down. Growth continues but at a slower pace.
By the fourth stage, both birth and death rates are low, and rapid growth ends. The population stabilizes. Many wealthy nations are here now. A potential fifth stage exists where fertility drops below the level needed to maintain the population, leading to gradual decline.
The Role of Life Expectancy
One of the biggest drivers of population growth over the past century has been the dramatic rise in how long people live. Global life expectancy jumped more than 6 years between 2000 and 2019 alone, climbing from 66.8 years to 73.1 years, according to the World Health Organization. That gain came primarily from declining mortality rather than from people spending fewer years with illness or disability.
The COVID-19 pandemic reversed some of that progress. By 2021, global life expectancy had fallen back to 2012 levels, dropping to 71.4 years. But the long-term trend remains strongly upward: better sanitation, vaccines, antibiotics, and improved nutrition have all contributed to people living longer, which means more people alive at any given time.
Replacement Fertility: The Breakeven Point
For a population to hold steady over time without immigration, women need to have, on average, about 2.1 children each. This is called “replacement level fertility.” The number is slightly above 2 because not every child survives to adulthood. In countries with higher child mortality, the replacement number can be significantly higher.
When fertility drops below replacement level and stays there, the population eventually shrinks. When it stays above replacement, the population grows. Many of the world’s wealthiest countries are now well below 2.1, while many of the poorest remain well above it.
Where Growth Is Fastest and Slowest
Population growth is deeply uneven across the globe. The fastest-growing countries in 2025 are concentrated in sub-Saharan Africa: South Sudan leads at 4.52% annual growth, followed by Niger at 3.65%, Somalia and Angola both at 3.32%, and Benin at 3.26%. At those rates, South Sudan’s population would double in roughly 16 years.
On the other end of the spectrum, Europe’s population peaked around 750 million in 2020 and is projected to fall to about 590 million by 2100. China, India, Europe, and the United States are all on very different trajectories. China’s population has already begun declining, while India recently surpassed it to become the world’s most populous country. These diverging paths are shaped by differences in fertility, life expectancy, and migration patterns.
The Demographic Dividend
When a country’s birth rate drops, something interesting happens to its economy. The share of working-age adults grows relative to the number of dependent children and elderly people. This shift, called the “demographic dividend,” can accelerate economic growth because more people are producing and fewer are dependent on support.
But falling fertility alone doesn’t guarantee economic gains. It only creates the potential. Turning that potential into real prosperity requires investment in education and healthcare, good governance, functioning labor markets, and sound economic policy. Without those foundations, a large young population can become a source of instability rather than growth.
Population Growth and the Environment
More people means more demand for food, water, land, and energy. The expansion from 2 billion people in 1927 to 8 billion today has dramatically increased the need for energy, which still comes primarily from fossil fuels. As global population grows, carbon emissions tend to rise alongside it.
The relationship between population size and environmental impact, though, is far more complex than simple headcounts suggest. Environmental impact is typically understood as a product of three factors: population size, affluence (GDP per capita), and technology. A framework from environmental science expresses this as Impact = Population × Affluence × Technology.
The numbers reveal a striking inequality. The bottom 50% of the global income distribution, roughly 3.9 billion people, contributes only 11.5% of greenhouse gas emissions. The top 10%, about 771 million people, accounts for nearly 50%. Countries where population growth is highest, such as those in Africa, are responsible for only about 4% of global emissions, compared to high-income countries where fertility is low. This means that reducing population growth in the fastest-growing regions would not, by itself, significantly cut global carbon output.
That said, the link between population and emissions isn’t negligible everywhere. In Nigeria, with an annual growth rate of 2.4% and a population of 227 million, research shows that population growth exerts nearly as much influence on carbon emissions as rising income per person. And projections suggest that energy-related emissions in Africa will remain persistently high as populations grow and living standards improve. Meeting the energy needs of a growing global population after 2040 will require an estimated minimum of 156 exajoules per year, a massive amount without strong improvements in energy efficiency.
Where Global Population Is Headed
The world’s population is still growing, but the rate of growth has been slowing for decades. The peak growth rate occurred in the late 1960s at around 2% per year. Today it is closer to 0.9% and continuing to decline. Most projections suggest the global population will peak somewhere between 9 and 10 billion later this century before beginning a gradual decline.
The key uncertainty is fertility. If women in developing countries continue to have fewer children as education and economic opportunity expand, the peak will come sooner and lower. If fertility rates in wealthy nations tick back up at very high levels of development, as some demographers have suggested is possible, populations in those countries could stabilize rather than shrink. Either way, the era of explosive global population growth is winding down.