Residents of Kolkata are called Calcuttans. As of 2001, Kolkata city had a population of 4,580,544, while the urban agglomeration had a population of 13,216,546. The sex ratio is 828 females per 1000 males– which is lower than the national average, because many working males come from rural areas, where they leave behind their families. Kolkata's literacy rate of 80.86% exceeds the all-India average of 64.8%. Kolkata Municipal Corporation area has registered a growth rate of 4.1%, which is the lowest among the million-plus cities in India.

Bengalis comprise the majority of Kolkata's population, with Marwaris and Bihari communities forming a large portion of the minorities. Some of Kolkata's notable communities include Chinese, Tamils, Marwaris, Anglo-Indians, Armenians, Tibetans, Maharashtrians and Parsis. Major languages spoken in Kolkata are Bengali, Hindi, Urdu, English, Maithili, and Bhojpuri. According to the 2001 census, 77.68% of the population in Kolkata is Hindu, 20.27% Muslim and 0.88% Christian. Other minorities such as Sikhs, Buddhist, Jews and Zoroastrian constitute the rest of the city's population. 1.5 million people, who constitute about a third of the city's population, live in 2,011 registered and 3,500 unregistered (occupied by squatters) slums.

Kolkata reported 67.6% of total Special and Local Laws (SLL) crimes registered in 35 Indian mega cities. Kolkata police district registered 10,757 IPC cases in 2004, which was 10th highest in the country. The crime rate in the city was 81.4 per 100,000 against the national rate of 168.8 in 2004. Kolkata's Sonagachi area, with more than 10,000 sex workers, is one of India's largest red-light districts.