Honk Kong's walled city (click to see video) - image source: ArchNet Digital Library

The New York Times published a fascinating article a few days ago which asked the question: How many people can Manhattan hold? The answer is it depends on the density. If it were packed as densely as the fabled Kowloon Walled City (see first exhibit – click for video), its current 1.6 million resident population could be expanded fortyfold to 65 million!

Following that theme, Matt Yglesias estimated California’s current population of 37 million could be increased to 188 million if it were housed at the relatively modest density of New Jersey. A couple of years ago I calculated that the entire population of Australia could be accommodated within 25 km radius of Melbourne’s CBD, provided it were settled at the current density of Brooklyn.

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I decided to follow the NYT’s lead and calculate the theoretical holding capacity of some Australian capital cities (see second exhibit) by comparing them with some of the world’s denser places. To provide a consistent basis for comparison, I compared urbanised areas rather than administrative areas, drawing on the 2011 edition of the Demographia World Urban Areas database (urbanised areas – think of them as the extent of street lighting viewed from a satellite – are much smaller, that’s why the population of Australian cities in the second exhibit is smaller than you’re used to hearing).

Sydney is easily Australia’s densest city with 2,103 persons per sq km, while Brisbane is the sparsest (978 per sq km). However even Sydney is a long way behind Dhaka in Bangladesh, the world’s densest city with 35,000 persons per sq km. Dhaka has 11.4 million residents – it’s by no means the largest city in the world, but they’re packed into just 324 sq km.

Sydney ranks 92nd in the world by size of population but around 425th by density. As the second exhibit shows, the urbanised area of Sydney could hold 63 million people if it were populated at the same intensity as Dhaka.

Population each Australian city could accommodate if it were developed at the density of other selected cities

The city with the biggest population by far is the Tokyo-Yokohama conurbation. It has 36 million residents but they occupy 9,065 sq km, giving it a density around one ninth of Dhaka’s. That’s still much higher than Sydney – if Sydney were settled at Tokyo-Yokohama’s density, it could accommodate 7.2 million people, almost double its current population.

Most European cities are denser than Sydney but there are exceptions. Toulouse, Antwerp, Nice, Marseille, Lyon, Milan and Budapest are considerably lower density cities than Sydney (and all but the latter two are also lower density than Melbourne). The “new world”, though, is a more relevant comparison and might surprise.

It probably seems counter-intuitive, but Sydney is not as dense as Los Angeles. Indeed, Sydney could house an additional 530,000 residents if it were as dense as Los Angeles. Perhaps even more astonishing, Sydney is denser than New York. Yes, denser. It would have 540,000 fewer residents if it were populated at the same density as the Big Apple.

These are not anomalies – the explanation is that we customarily use average density. The very high densities in areas like Manhattan and Brooklyn are offset by the huge swathe of low density development in New York’s suburbs, where densities come down to 10 persons per hectare or less (the cut-off for an urbanised area in most cases is 4 person/ha). Los Angeles has few residents living at really high density, but it has few residing at really low density either (as discussed here, the best way to compare cities is to use weighted density).

Los Angeles is the only US city that is denser than Sydney, with San Francisco about the same. In fact there are only six that are denser than Melbourne – those two, plus New Orleans, New York, Las Vegas and Miami. Even Chicago, notwithstanding a population of 9 million, is more sprawling than Sydney or Melbourne. Atlanta, Boston, Houston, Philadelphia, Dallas, Washington and Phoenix all have larger populations than either Sydney or Melbourne, but lower densities.

Brisbane is Australia’s shocker (or perhaps nirvana, depending on one’s point of view). It’s not quite as dense as sun-belt sprawl icons like Dallas, Houston and Austin, all of which have 1100 residents per sq km. With 978 residents per sq km, Brisbane’s only a little denser than cities like Memphis, Indianapolis and Kansas City, but is no match for spacious Atlanta (700 per sq km).

Here’s another way of looking at the numbers. If the population of Dhaka were housed at Melbourne’s density, it would need to expand its geographical footprint from 324 sq km to 7,042 sq km. On the same basis, Tokyo-Yokohama would require 22,495 sq km. To put that in context, the area of the ACT is 2,400 sq km.

Or imagine if London’s 8.6 million residents were accommodated at Brisbane’s density – instead of the 1,623 sq km they currently occupy, they’d need 8,778 sq km.  That’s more than Tokyo-Yokohama uses to house 36.7 million people!

This might be the most practical way to look at it: Brisbane would have a population of 3,840,000 if it were developed at Sydney’s density, or about double its current population. Similarly, Adelaide and Perth would hold around half as many residents again, and Melbourne around a third more, if they had the same density as Sydney’s urbanised area.

That’s instructive for the planning of the other capitals because, whatever it’s infrastructure problems, Sydney is still a quintessentially “Australian” city. It might be arguable, but I’d contend most of Sydney’s infrastructure and environmental negatives (and let’s not forget Sydney still has many positives!) are a failure of politics, not an inevitable consequence of higher density.

Note: Kowloon’s walled city is a reminder that successful city life isn’t always pleasant parks and babbling brooks. The first exhibit (image source here) clicks through to a remarkable German film of Kowloon’s famous Walled City, which was demolished in 1994. Originally a fort, it was one of the densest and most anarchic urban settlements in the world. It covered approx 3 hectares with an estimated population density in 1987 of 1,255,000 persons per sq km.