Observations on the area and population of NYC, Hong Kong, Shanghai and Beijing

Did some rudimentary calculation based on my limited experience, and got some interesting results. Numbers are rounded up. Will redo this exercise to include London, Paris, Tokyo, Seoul and other major cities in the future.

1. The core area of the cities has about 20,000 people per square kilometer.

Manhattan (NYC): 1.6 million people on 60 sq. km., density around 26,000;
Hong Kong Island (HK): 1.3 million people on 80 sq. km., density around 16,000*;
Puxi, Nine Districts (Shanghai): 6.5 million people on 280 sq. km., density around 23,000;
Dong Xi Wen Wu, The Inner Four Districts (Beijing): 2 million people on 93 sq. km., density around 22,000.

*It is much more densely populated than the data may suggest, because Hong Kong Island consists mostly of mountains, whereas the other three places are mostly plains.

2. The extended area of the cities has about 10,000 per square kilometer.

New York City (all five boroughs): 8.3 million people on 800 sq. km., density around 10,000;
Hong Kong (Hong Kong Island, Kowloon and the New Territories): 7 million people on 1000 sq. km., density around 7,000;
Puxi & Pudong (Shanghai, not including other 7 suburban districts and Chongming Island): 9.5 million people on 800 sq. km., density around 12,000;
Dong Xi Wen Wu & Haidian & Chaoyang (not including other 10 suburban districts and 2 counties): 8 million people on 1000 sq. km., density around 8,000.

3. For the metropolitan area, results are not very clear-cut:

NYC metro area (definition in controversy): 18 million people on 18,000 sq. km., density around 1,000.
Hong Kong & Shenzhen: 16 million people on 3,000 sq. km., density over 5,000.
Shanghai: 18 million people on 6,000 sq. km., density near 3,000.
Beijing: 17 million people on 17,000 sq. km., density around 1,000.