1,106.4 sq. km, which is 427 sq. miles
It’s actually 17,345 per sq. mile, making it the forth densest country or dependent territory in the world behind Singapore, Monaco and Macau. The only other country to have a density of over 10000 per sq.mile is Gibraltar. While there are denser cities around the world, people can always choose to live outside them and commute, or have their businesses outside the city limits. If you live outside any of those five city states then you live in a different country, with all the problems of different legal systems, as well as dealing with crossing a national border (or internal border in Hong Kong’s and Macau’s case) every day.
