As India is expected to become the most populous country in the world, surpassing China with 1.4 billion people, according to the UN, demographers are unsure exactly when that will happen. But they know it will happen soon.

China has had the largest population in the world since at least 1950, when United Nations population figures were released. These two countries are home to over a third of the world’s 8 billion people.

“It’s really impossible to know exactly when India will overtake China,” said Bruno Schumacher, a demographer at the Catholic University of Louvain in Belgium. “There is some uncertainty not only about the population of India, but also the population of China,” he added.

Mathematical calculations from a number of polls, as well as birth and death records, predict that India will overtake China sometime in mid-April. But demographers warn they should be taken with a grain of salt as the numbers are vague and subject to revision.

“This is a rough approximation, a guess,” said Patrick Zerlan, head of population estimates and projections at the United Nations in New York.

How is the population calculated?

United Nations Population Division demographers make estimates based on projections from a variety of data sources.

“The last data update for both India and China was in July 2022,” said Sarah Hertog, UN Population Officer in New York.

“Demographers then use a statistical method to infer when India’s population will exceed China’s,” says Stuart Gittel-Bastein, a professor at the Khalifa University of Science and Technology in Abu Dhabi.

Where do the numbers come from?

The original database is based on censuses every ten years. China’s last census was conducted in 2020. Demographers have used birth and death records, as well as other administrative data, to estimate how much the population has grown since then.

The last census of India was conducted in 2011. The planned 2021 census has been postponed due to COVID-19. “Without a door-to-door count for more than a decade, sample surveys have filled in the gaps to help demographers and India itself,” said Alok Wajipegi of the New Delhi-based NGO, the Population Fund of India.

Among the most important is the Sample Registration System, a large-scale demographic survey of India that collects data on things like births, deaths, fertility, etc.

Why is India overtaking China?

China has an aging population with stagnant growth, even after the government abandoned its one-child policy seven years ago and just two years ago when it announced that families could have three children.

India has a much younger population, a higher birth rate and a decline in infant mortality over the past three decades.

“More babies are born in India every year than in any other country, and China has joined many European countries with more deaths than births each year,” said Dudley Poston Jr., professor emeritus of sociology at Texas A&M University.

According to the Associated Press