According to a census conducted in 2011, Kathmandu metropolis alone has 2.5 million inhabitants, and the agglomerate has a population of more than 3 million inhabitants. The metropolitan city area is 50.67 square kilometres (19.56 sq mi) and has a population density of 3000per km² and 17000 per km square in city.

The city stands at an elevation of approximately 1,400 metres (4,600 ft) in the bowl-shaped Kathmandu Valley of central Nepal. It is surrounded by four major mountains: Shivapuri, Phulchoki, Nagarjun, and Chandragiri. Kathmandu Valley is part of three districts (Kathmandu, Lalitpur, and Bhaktapur), has the highest population density in the country, and is home to about a twelfth of Nepal's population.

Historically, the Kathmandu Valley and adjoining areas were known as Nepal Mandala. Until the 15th century, Bhaktapur was its capital when two other capitals, Kathmandu and Lalitpur, were established. During the Rana and Shah eras, British historians called the valley itself "Nepal Proper". Today, Kathmandu is not only the capital of the Federal Democratic Republic of Nepal, but also the headquarters of the Central Development Region of Nepal. The Central Region comprises three zones: Bagmati, Narayani, and Janakpur. Kathmandu is located in the Bagmati Zone.

Kathmandu is the gateway to tourism in Nepal. It is also the nerve center of the country's economy. It has the most advanced infrastructure of any urban area in Nepal, and its economy is focused on tourism, which accounted for 3.8% of Nepal's GDP in 1995-96. Tourism in Kathmandu declined thereafter during a period of political unrest, but since then has improved. In 2013, Kathmandu was ranked third among the top 10 travel destinations on the rise in the world by TripAdvisor,and ranked first in Asia.

The city has a rich history, spanning nearly 2000 years, as inferred from inscriptions found in the valley. Religious and cultural festivities form a major part of the lives of people residing in Kathmandu. Most of Kathmandu's people follow Hinduism and many others follow Buddhism. There are people of other religious beliefs as well, giving Kathmandu a cosmopolitan culture. Nepali is the most commonly spoken language in the city. English is understood by Kathmandu's educated residents.

Kathmandu's sister cities (Lalitpur Patan) and Bhaktapur are integral to Kathmandu's cultural heritage, tourism industry, and economy; therefore UNESCO's World Heritage Site lists all three cities' monuments and attractions together under one heading, "Kathmandu Valley-UNESCO World Heritage Site".