Determining the world's largest city shouldn't be too complicated, right? Do a census, count up the people and be done with it. The city with the largest number gets the title: "world's largest city."

But as BBC explains, coming to that conclusion can be messy because what defines a "city" isn't as clear-cut as you might imagine. With no international standard for measuring a city, determining it's size is a challenge:

Three scholars who in 2009 compared eight different lists of top cities by population, found there were 30 "top 20 cities" in total.

One of them, Rich Greene, associate professor of geography at Illinois University in the United States, says even the most authoritative list, from the UN, "compares apples with pears".

"We tend to think of 3 concepts of the city," he says.

"One being a municipal definition - the legal city if you will. A second we call the urbanised area, or the physical city - the built-up portion. A third we call the metropolitan area, which some people refer to as the influence of the city - how far out does the city go in terms of its influence, such as commuting patterns."

And even those definition can be hazy. For example, population statistics for Chinese cities can be especially misleading since what they call a city or municipality is more like what others would call a province, and it includes rural farmlands. A far cry from the huge cities with 30 million people that we like to imagine.

Nonetheless, we like to measure and compare. In our never-ending quest to know the biggest, fastest, strongest, which city can lay claim to the title, "world's largest city"? Even all-knowing Google isn't sure if it's Tokyo, Shanghai, or Seoul. But BBC says that generally Tokyo comes out on top:

Most experts will tell you that Tokyo is the world's largest metropolis, with a population of about 36 million people.

But the core of the city has only eight million people living in it.

The reason it gets into the record books is that the surrounding region - which includes the country's second city Yokohama, as well as 86 other towns and cities - has become so built up that it is now one huge continuous urbanised area.

Take that for what it's worth, because it all depends on how you slice it.

The world's biggest cities: How do you measure them [BBC]

Photo: tokyoform/Flickr

This post was originally published on Smartplanet.com