A total fertility rate of 2.1 children per woman is the standard threshold needed to sustain a population over time. This number, called “replacement level fertility,” means each generation produces exactly enough children to replace itself. It applies specifically to countries with low infant and child mortality. In nations where fewer children survive to adulthood, the rate needed can be significantly higher.
Why the Number Is 2.1, Not 2.0
Two parents need two children to replace themselves, so you might expect the magic number to be an even 2.0. The extra 0.1 accounts for two biological realities. First, slightly more boys are born than girls, at a ratio of roughly 105 males for every 100 females. Since only women bear children, a population needs slightly more than two births per woman to guarantee enough daughters reach reproductive age. Second, a small fraction of children will die before they grow old enough to have their own kids. In wealthy countries with advanced healthcare, that fraction is tiny, which is why 2.1 is sufficient. In countries with higher child mortality, the replacement rate can climb well above 2.1 because more births are needed to offset early deaths.
Where the World Stands Now
The global fertility rate in 2024 was 2.2 births per woman, down from about 5 in the 1960s and 3.3 in 1990. That decline has been remarkably steady. United Nations projections estimate global fertility will hit the 2.1 replacement level by 2050 and continue falling to 1.8 by 2100.
More than half of all countries already have fertility rates below 2.1. Nearly a fifth of countries, including China, Italy, South Korea, and Spain, have what demographers classify as “ultra-low” fertility: fewer than 1.4 births per woman. South Korea holds the record for the world’s lowest rate, at 0.80 births per woman as of 2025. At that pace, the country’s population of 51.8 million is projected to shrink by almost a third, to 36.2 million, by 2072.
These trends are why the UN now expects the world’s population to peak within this century. The projected size of the global population in 2100 is 700 million fewer people than what was anticipated just a decade ago.
How Migration Changes the Math
The 2.1 figure assumes zero net migration. In reality, immigration can dramatically lower the fertility rate a country needs to keep its population stable. Research measuring this effect found that countries with high immigration rates need far fewer births per woman. Singapore, with very high net migration, would need only 0.60 births per woman to maintain its population size long term. Norway would need 0.96, and Australia about 1.00. The relationship is straightforward: the more people moving in, the fewer births required to prevent population decline. As net migration approaches zero, the required fertility rate climbs back toward the traditional 2.1.
This is one reason immigration policy features so prominently in debates about demographic decline. For countries with fertility rates stuck well below replacement, immigration is the most immediate lever available to slow or reverse population shrinkage.
Why Population Keeps Growing After Fertility Drops
A country can have a fertility rate below 2.1 and still see its population grow for decades. This phenomenon, called population momentum, happens because of age structure. When a country has spent years with high birth rates, it ends up with a large share of young women in their childbearing years. Even if each of those women has fewer children than the previous generation, the sheer number of women having babies keeps total births high for a while. Demographer Nathan Keyfitz demonstrated that an immediate drop to replacement fertility in high-fertility populations could still produce decades of continued growth before the population leveled off.
The reverse is also true. Countries that have been below replacement for a long time develop a “top-heavy” age structure with fewer young people. Even if fertility ticked back up to 2.1 tomorrow, the small number of women of childbearing age would mean fewer total births than deaths for years to come.
Economic Effects of Staying Below Replacement
Persistent low fertility reshapes a country’s economy by changing the ratio of working-age people to retirees. As the population ages, fewer workers support more older adults through taxes, pensions, and healthcare systems. Japan has experienced this firsthand: its support ratio (the balance between what working-age people produce and what the full population consumes) fell at about 0.6% annually over the past decade, with government budgets taking an even harder hit at 0.9% per year.
The severity depends on just how far below replacement a country falls. Research on high-income countries found that fertility rates of 1.6 or above, while below replacement, don’t necessarily hurt overall living standards. Modest population decline can actually favor higher per-capita consumption because there are fewer dependents to support. The problems intensify at very low levels. A fertility rate around 1.1 (producing a population decline of roughly 2% per year) would reduce per-capita consumption by about 4% compared to a stable population. For high-income countries with rates below 1.6, the long-term support ratio is projected to settle at roughly 80% of its 2010 level, meaning the economy would need to generate the same output with a fundamentally different, older workforce.
South Korea, with its rate of 0.80, is heading into largely uncharted demographic territory. Countries at that level face compounding strain on public finances, shrinking labor forces, and the challenge of maintaining infrastructure built for a much larger population.
Replacement Level Is a Long-Term Target
One important nuance: reaching a fertility rate of exactly 2.1 doesn’t instantly stabilize a population. Replacement level fertility leads to zero population growth only if mortality rates stay constant and migration is neutral. In practice, populations are always in motion, shaped by shifts in life expectancy, immigration patterns, and the echo effects of past baby booms or busts. The 2.1 figure is best understood as the fertility rate at which a population would eventually stabilize if all other factors held steady over a long period. It’s a useful benchmark, not a switch that flips the moment a country hits it.