A decade ago, China and India were within a few million people of each other. Now their population curves point in almost opposite directions.

China is sliding down a slope that, if it holds, could roughly halve its numbers within a lifetime. India is still climbing, and won’t stop for another forty years or so. The two most populous nations on earth are heading somewhere genuinely different, and the gap between their paths is one of the more striking things in recent demographic data.

A quick note on what follows. We are not demographers or statisticians, and this is a piece of reading and reflection on published projections, not a forecast of our own. Everything here rests on long-range population modelling, which describes “if this holds” scenarios rather than fixed outcomes. Population-level trends are not predictions about any one place or person.

The two headline numbers

Start with China. Its population peaked in 2021 and started falling the next year. The exact peak was 1.4126 billion. And that 2021 total had grown by just 480,000 people over the year before, a rounding error at that scale. Then the plateau tipped into decline.

The far end of the projection is the number that stops people short. Under the UN’s middle-of-the-road scenario, China’s population is projected to fall to about 633 million by 2100, roughly where it sat in the late 1950s.

India is on a different clock. The UN expects it to keep growing until the early 2060s, then peak and ease down. One country nearly halving while the other adds hundreds of millions before turning: that is what makes the pairing worth examining.

Why China’s peak arrived when it did

Fertility is most of the story. A country’s fertility rate is the average number of children a woman has over her lifetime, and China’s has fallen a long way.

The Conversation’s analysis notes it slipped from 2.6 in the late 1980s to 1.3 in 2020 and 1.15 in 2021. A population holds steady over generations at a rate of about 2.1. China has been well below that for years, and by 2024 its rate was around 1.01, among the world’s lowest.

Another signal came earlier than the headline peak. China’s working-age population peaked in 2014, seven years before the overall total did. When fewer people are entering their childbearing years, and each of them is having fewer children, an overall peak becomes a matter of timing rather than possibility. The 2021 turn was the first year China’s population had shrunk since the great famine of 1959 to 1961.

How to read “633 million by 2100”

Carefully, is the short answer. A projection is not a prediction in the everyday sense. As the Population Reference Bureau puts it, “These assumptions, though based on research and expert opinions, are not certain.” A figure like 633 million is what you get if a particular set of assumptions about births, deaths, and migration holds true across seventy-five years. Change the fertility assumption and the number moves, sometimes by a lot.

How much it moves is worth seeing. Under earlier UN work, China’s projected 2100 population ranged from about 488 million to 1.153 billion, depending on whether you assumed low or high fertility, with the middle scenario in between. That is not a spread you can wave away. Our World in Data notes that “all population projections are sensitive to the underlying assumptions.”

What is far steadier is the direction. Pew Research Center found that “regardless of the precise timing of China’s population peak or the magnitude of its projected decline, there is near-universal consensus that the nation’s population is on a trajectory of decline.” So the exact 2100 figure is one scenario among several. The downward slope it sits on is close to a settled expectation.

What makes India’s path different

The UN’s World Population Prospects 2024 puts it this way: “The population of India, which is expected to remain the world’s largest throughout the century, will likely decline by 12 per cent after reaching its peak in the early 2060s at about 1.7 billion.” The “will likely” in the report’s own wording is not accidental. Even the UN hedges its central line.

The reason for the gap is again fertility, paired with age. India’s rate sits far closer to that steady-state level, around 1.96 in 2024 against China’s 1.01. And its people are younger. In 2025 India’s median age was about 29.8 years against China’s 41.1, an eleven-year gap. A younger population has more people still to enter their childbearing years, which is why India keeps growing even as its fertility slowly falls.

Clare Menozzi, a senior population affairs officer in the UN’s Population Division, framed the ranking as expectation rather than certainty. As she put it, “India is currently the largest country in the world in terms of population, and it is projected to stay so throughout the century.”

What the two curves tell us, and what they don’t

They tell us that the same broad force, falling fertility, plays out very differently depending on when a country’s decline sets in and how young it is when it arrives. China dropped below the steady-state fertility level long enough ago that its shrinking is now baked into the age of its people. India is decades behind on the same curve, with a young population that keeps growth going even as birth rates ease.

What the curves don’t tell us is exactly where either country lands. The China range of 488 million to over a billion for the same year, depending on fertility assumptions, is the honest picture of long-range uncertainty. The India peak of “about 1.7 billion” is a central estimate wrapped in its own range. What we take from the divergence is less about the precise endpoints and more about the shape: two neighbors who were near-twins in size a decade ago now sit on curves that will look nothing alike by the time a child born today reaches old age. The endpoints are scenarios that hinge on decisions and conditions no model can fix in advance. The parting of the two lines, though, has already begun.