Manhattan, New York (For Reference)
Area: 59.5 km2 (22.96 sq mi)
Population density: 26,939/km2 (69,771/sq mi)
Because Manhattan has so many office buildings, it swells to about 3.94 million during the day, giving a population density of about 171,000 per square mile / 66,000 per square km.
The Upper West Side is primarily residential, with 209,000 people / 1.9 square miles, meaning about 110,000 people per square mile / 42,500 per square kilometer.
The most densely populated city in the world.
Area: 38.55 km2 (14.88 sq mi)
Population density 42,858/km2 (111,000/sq mi).
Malé, Island (divisions Henveiru, Galolhu, Machchangolhi, Maafannu), Republic of Maldives
Area: 1.952 km2 (0.754 sq mi)
Population density: 47,416/km2
Fadiouth Island, Dakar
Area: 0.15 km2 (0.06 sq mi)
Population density: 60,000/km2 (155,400 / sq mi)
St. James Town Neighborhood, Toronto, Ontario, Canada
One of the most densely populated neighbourhoods in North America
Area: 0.23 km2 (0.09 sq mi)
Population density: 60,915 / km2 (155,905 sq mi)
Tin Shui Wai New Town, Yuen Long District, Hong Kong
Area: 4.3 km2 (1.7 sq mi)
Population density: 62,790/km2 (162,600/sq mi)
Migingo Island, Lake Victoria, claimed both by Kenya and Uganda
Area: 0.002 km2 (0.0008 sq mi)
Population density: 65,500/km2 (169,500/sq mi)
Ap Lei Chau, Island, Hong Kong
Area: 1.32 km2 (0.51 sq mi)
Population density: 66,755 people / km² (172,900 people / sq mi)
Tondo district, Manila, Philippines
Area: 9.10 km² (3.52 sq mi)
Population density: 69,297 per km² (179,470 per sq mi)
Mathare slums, Nairobi, Kenya
Area: about 3 km2 (1.2 sq mi)
Population density: hard to estimate, probably something between 60,000-100,000/km2 (155,000-259,000/sq mi).
Kibera, Nairobi, Kenya
Area: 2.3-2.5 km2 (0.8 – 1 sq mi)
Population density: hard to estimate, something between 74,000 – 114,000/km2 (192,000 – 295,000/ sq mi)
Rocinha favela, Rio de Janeiro, Brazil
Area: 0.86 km² (0.33 sq mi)
Population density: +80,000 /km2 (207,000/sq mi)
Dhaka Kotwali Thana, Dhaka, Bangladesh
Area: 2.07 km2 (0.80 sq mi)
Population density: 101,693/km2 (263,380/sq mi)
Santa Cruz del Islote, Island, Colombia
Area: 0.012 km2 (0.005 sq mi)
Population density: 103,917/km2 (269,000/sq mi)
Marine Lines, Mumbai, India
Area: 1.8km2 (0.7 sq mi)
Population density: 114,001/km2 (295,000/sq mi)
Mong Kok, Kowloon Peninsula, Hong Kong
According to the Guinness Book of Records Mong kok is the most densely populated place in the world (although using unconfirmed estimates, there are probably even more crowded places
Area: 1.1km2 (0.4 sq mi)
Population density: 130,000/km2
Dharavi slums, Mumbai, India
Area: 1.7-2,2 km2 (175-220 hectares)
Population density: around 300,000/ km2 (777,000 / sq mi).
Dharavi is probably the most densely populated spot in the world.