Megacity

A megacity is an exceptionally large urban area. Most international organizations use a specific population threshold to define them. The United Nations Department of Economic and Social Affairs (UN DESA) defines a megacity as an urban agglomeration with over 10 million inhabitants.

Other researchers use different metrics for these massive centers. A report from the University of Bonn defines them as metropolitan areas with 10 million or more people. Some sources use a lower threshold of five to eight million people. These sources also require a high population density. They look for at least 2,000 people per square kilometre. You may also hear these areas called a conurbation, metropolis, or metroplex.

Defining the exact size of a megacity is often difficult. Experts struggle to determine where the outer limits of a city truly end. This makes it hard to estimate the total population accurately. To solve this, some have proposed an Urban Metric System. This system includes the measurement of Paropolis to help with calculations. This method has been used in limited ways, such as by Canada since 2018. Because of these different methods, the total number of megacities varies by source. The EU Global Human Settlement Layer (2024) lists 32 megacities. UN DESA (2025) counts 33. The OECD (2020) lists 39, and Demographia (2025) counts 42. In total, up to 53 unique places are identified as megacities across all sources.

Megacities are found across almost every continent. A large percentage of these urban agglomerations are located in China and India. Several other countries contain more than one megacity. These include Brazil, Indonesia, Japan, Pakistan, and the United States. In Africa, megacities exist in Angola, the Democratic Republic of the Congo, Egypt, Nigeria, and South Africa.

European megacities are found in France, Germany, Russia, the United Kingdom, and Turkey.

Latin American megacities are located in Brazil, Mexico, Colombia, Peru, and Argentina.

This global spread shows how urban growth is a worldwide phenomenon.

History shows that massive urban centers have always existed. In antiquity, Rome was the largest and most politically important city. It ruled over Europe, Western Asia, and Northern Africa. Scholars estimate its population was between 750,000 and 1,000,000 people by the end of the 1st century BC.

However, Rome's population declined sharply after 402 AD. This happened when Emperor Flavius Honorius moved the government to Ravenna. By the Early Middle Ages, Rome's population fell to only 20,000 people. Later, Baghdad was likely the world's largest city after its foundation in 762 AD.

Chinese capital cities like Chang'an and Kaifeng also saw huge population booms. In 742 AD, the Jingzhao Fu area near Chang'an had 1,960,188 people.

Modern megacities have grown at an unprecedented rate. In 1800, only 3% of the world's population lived in cities. By the end of the twentieth century, that number rose to 47%. In 1950, New York City was the only urban area with over 10 million people.

Since the 2000s, the Greater Tokyo Area has been the largest megacity. It includes areas like Yokohama and Kawasaki, with an estimated 37 to 38 million people.

However, as of 2025, the UN lists Jakarta as the world's largest megacity.

Another massive area is China's Pearl River Delta. It is anchored by Guangzhou and is a continuously built-up area.

Some satellite-based sources see Guangzhou and Shenzhen as one large megacity, but the UN sees them as disjointed.

Rapid urban growth creates significant social challenges. Much of this growth happens in developing countries. The UN forecasts that by 2030, nearly 5 billion people will live in cities. This means 60% of the global population will be urban.

This growth often leads to the expansion of slums or informal settlements. In 2018, the number of people living in slums passed 1 billion. This is nearly one-seventh of the world's population.

Many people move to cities due to massive internal and transnational migration. These settlements often lack quality housing, sanitation, drainage, and clean water. Residents may also lack officially recognized addresses, education, or healthcare.

These conditions create complex political and economic problems. In many overcrowded slums, high rates of disease occur due to unsanitary conditions and malnutrition. High population densities can also lead to higher crime rates. For example, many murders in Rio de Janeiro, Brazil, are gang-related and occur in favelas.

As the world moves toward higher urbanization levels, these issues become more central to global study. Understanding how megacities manage their resources and populations is a key part of modern geography and urban planning.