Where is the most densely populated square kilometre in the UK?

After the recent release of England and Wales Census data for 2021 I decided to take a quick look at which areas have the highest population density. I've done this kind of thing before, for the whole of Europe and also in relation to thinking about measures of population density more generally - in short, my view is that using the arithmetic mean approach to density makes very little sense at the country level because it doesn't reflect how we actually experience density (unless we live somewhere like Monaco). That's why I came up with the very simple idea of using 'lived density' instead. But this piece is about my attempt to find the area of the UK with the highest population density in a single square km. And note that I say 'in the UK' in the title because the highest density area of the UK is within London - the highest densities in (e.g.) Glasgow, Edinburgh, Cardiff or Belfast are about half that of London's densest areas. Here's where I think the most densely populated square kilometre in the UK is, based on 2021 Census data.

This is the answer to my question - read on to see how I got here

Here's a map of the southeast of England (mostly) showing 1km density - brighter colours = higher density. There are a few very high density areas outside London, but not necessarily exactly where you might expect.

No big shock to find London has the highest densities

That London has the highest population density areas of the whole UK is not a surprise. Previously my calculations on the highest population densities in London led to figures in the range of 20,000 to 25,000 people in a single square km. But of course this refers to residential density, and my previous calculations arrived at a figure of about 120,000 people in a single square km for daytime population density (see below, where the top figure is daytime population and the bottom figure is residential).

Daytime density in parts of Central London is over 100,000 per sq km

The centres of cities such as Manchester and Birmingham in particular have some pretty high densities - over 10,000 per sq km - but London has by far the highest density and the largest number of areas above 15,000 per square km in relation to residential population density. If you're interested in daytime (i.e. workday) population density then I wrote about that in a previous post.

Okay, so where was I? Yes, trying to find the single square km in the UK with the highest population density. I can say UK with some confidence even though I'm only using data for England and Wales, like I've already said, because my previous analysis shows that the answer is definitely in London.

How do we estimate where the highest density is?

First of all, we attach the output area population data from the 2021 census to the 2021 output area boundaries and we get something pretty uninspiring, like the basic map below. There are over 180,000 output areas and they are quite small. This was all done in QGIS, but I've not really bothered trying to make it look pretty here because I just wanted to find the answer.

All output areas, with 2021 population data

Then we create a 1km grid that covers all of England and Wales. At this point I should point out that the answers you get when doing any kind of 'per sq km' calculation do of course depend upon where you draw your grid Shift it a little up or down, or left or right and you will obviously get slightly different results. But not radically different numbers (as we'll see later on). Also, if you're using output areas to do population density calculations - instead of a 1km grid - then your numbers won't make a lot of sense because the vast majority of them (90%) are under 1km in size. 

In fact, most output areas are tiny, as in the example from the Isle of Dogs in London (below), where we see one output area with a density figure of over 142,000 people per square km - but of course it's WAY smaller than a square km, which are illustrated with the big black lines and the big numbers. Using a population density figure like this would make no sense. This stuff is pretty obvious but sometimes people quote these kinds of figures without realising how small output areas actually are.

Dense? Yes. 140,000 per sq km? No.

Once I had the 1km grid overlaid on England and Wales I think used population-weighted centroids for the output areas and assigned them each to a single 1km grid square and then added up the total population in each. This is of course not perfect because output areas don't align perfectly with the grid but it's close enough in the areas of interest to take us closer to a plausible answer. Here's what that looks like in part of Manchester and Birmingham - messy maps but you get the point. The big numbers are the 1km cell populations and the small numbers are the output area populations.

Central Manchester wasn't like this 20 years ago

This rivals parts of London for density

Okay, so what next? Well, next was to filter for all those square km with more than 15,000 people. Bear in mind that at this stage we are still talking in rough figures because of the mismatch between output area boundaries and the grid - an example of this is shown below but you can see it's not a terrible fit - only those output areas with their population-weighted centroid within the 1km square are added to that square's total. So the overlapping ones with most population outside are assigned to a neighbouring square. Again, not perfect, but logical. The big numbers here are the population in each square - with only squares with more than 15,000 shown and labelled.

This is just of City Road in London

Surprised by the grid squares I found (below) that have more than 15,000 people? I did think Manchester would crack this list but not based on the 1km grid I used. Apart from that it's mostly what I'd expect. There's one square in each of Leeds, Birmingham, Leicester and Brighton and 63 in London. You might get some others in other places if you were to shuffle the grid around a bit. For example, I tried this in Portsmouth (a relatively very dense city) where the highest values on my 1km grid were approaching 14,000 per sq km. I managed to get a figure of 15,509 in a single square km by manually placing the square somewhere else (see below). But in these cases no matter what you do you're not going to reach London densities.

Portsmouth is a very dense city for the UK

The 67 1km squares with more than 15,000 people

Here are some zoomed-in versions showing the location of the highest density 1km squares across England (it's likely that these are also the highest in the whole UK, given that when I last did this Glasgow and Edinburgh had max densities of about 12,000 in any single square).

This is the Birmingham one

This is the Brighton one

This is the Leeds one

This is the Leicester one, to the east of the train station

The 63 London 1km squares, with population labels

Okay, get to the point mate

Run another filter and this time do it for over 20,000 in any single square km and what do you get? You get the map below, with three possible candidates for 'most densely populated square km in the UK'.

These areas all seem like plausible answers to my question

See below for the zoomed in versions showing more detail. The reason they are tilted slightly in this one and the maps above is because I changed the map projection so that the OpenStreetMap layer (© OpenStreetMap contributors) is nice and crisp because if you show it using the British National Grid projection it comes out a bit fuzzy, that's all. 

Notice London Marylebone in the square here

Bow Common Gas Works in the centre here

This area is around Upton Park

Okay, so which square should we choose as 'most densely populated'? Is it a case of just taking the one with the highest population figure? Maybe, but I wanted to investigate more first. In each case there are overlaps around the edges of the 1km squares due to the irregular shapes of output areas. But overall I believe it's the case that the numbers reflect pretty close to the true population so I think, based on my initial 1km grid, that the area in Bow (East London) is the most densely populated 1km square in the UK, with a total of approximately 21,000 people in the 1000 x 1000 metre square. 

Ah, but hold on now. What were we saying about moving the grid around and the impact it might have? Well, I also generated three extra grids and ran the calculations on those, so read on for my final answer!

The answer to the question is finally here

With my first alternative 1km grid I once again got figures of just over 21,000 in any given square km, but I got seven of these. With my next one I got three squares, two of which had more than 22,000 people in them. With my third I got three as well,, one of which had more than 24,000 people in it, and one of which had 23,000. See below for maps of these. The three top squares from my original grid are shown in red on these maps, for comparison.

Shuffle the grid, get more answers, but similar areas

Three alternative candidates

I think we have a winner

So, my initial grid seems to have not been very far off the mark in helping find the most densely populated square km in the UK. Yes, we could go on and on with grid placement, and do it programmatically but I think we're always going to end up in the same place, around Bow Common in East London. It's also encouraging that the 20,000 to 25,000 per sq km figures I previously calculated come out here too. By way of comparison, the most densely populated areas of Paris or London (or New York or Seoul) have more than 40,000 people in them, and some over 50,000.

The overlap here is the initial winning candidate from my search

So, based on all of the above, my answer to the question of where the UK's most densely populated square km is, is Bow in East London with a population of approximately 24,000 in a single square kilometre, as shown below. Yes, there is a bit of fuzziness in these numbers due to not being able to perfectly align grid squares to output areas but I'm fairly happy with the answer I arrived at here.

I think this is a plausible answer to the question I posed

Hey, hold on a sec! What, not more density numberwang? Yes indeed.

I'm happy with my answer, but just to prove beyond any reasonable doubt that in these kinds of things the answers you get depend upon your method I decided to cheat a little and see if I could use a random 1km grid approach to find any area with more than the population in the square above. This is not really in the spirit of the method because it's not applying a regular grid across England and Wales so I don't consider this the 'proper' answer. Instead, consider it an approach to find an even higher value, for research purposes. The location of the 'most densely populated square km' in the United Kingdom doesn't really change and that's the main thing. 

Here's what my 1km 'grid' looked line when I was on the hunt to find a higher figure than the square above. This is almost 28,000 separate 1km squares but they do not form a grid, so I consider this method a bit of a cheat so I don't use it as my final answer above, but I wanted to see what happened when you did it.

Lots of squares, but not a grid approach

Then we can filter for areas with above 24,000 people and see what we get. That's what's shown below, and the square from the answer above is highlighted in red here for comparison.

Ruh-Roh, spanner in the works? 

Is this now a showdown between West and East London for the title of 'most densely populated square km in the UK'? Well, maybe, but only if you're using this cheat method of single squares. So who wins on this approach? See below for the zoomed in maps with the numbers.

All over 24,000 here (remember this is approximate)

East London wins again (original winner in red)

So, as we can see, the answer to the 'where is the most densely populated square kilometre in the UK?' question doesn't seem to change, even if we cheat and allow ourselves to place our 1km squares randomly in order to maximise the single square total. The area in the map above has four new squares with 25,000 or more - though of course these numbers are an approximation of course, as I've explained above. This kind of cheating approach does however have the benefit of showing how areas of West London come close to being the highest density if we play a bit of gridshift.

What does this actually look like on the ground though?

Here's a Google Maps link to more or less the centre of the red square above, which is also pretty much in the middle of the blue squares. Here are some images from Street View - and recall that this is what about 25,000 people per sq km density looks like in London. This is half the density of the highest density places in Paris, Barcelona or New York, and about a quarter of the highest densities globally (found in cities like Manila, Cairo and Dhaka).

Burgess Street, E14

Broomfield Street, E14

Lindfield Street, E14 (still space for parks)

Are we done now?

Yes (for now).

Two decades of city centre population growth in England

Back in 2013 I published a paper in the journal Cities with the following title: 'English urban policy and the return to the city: A decade of growth, 2001–2011'. I actually wrote it in 2012 and it drew upon some of the urban policy things I'd written about in my PhD a few years earlier. If I had access to the paper now, or if I wanted to spend $27.95, I could even read it again but the gist of it was this: people came back to the city centre, in quite big numbers, between 2001 and 2011. 

You don't need an academic journal paper to tell you that because you couldn't see it just by walking around, obviously. But really it was about a bit more than population and it discussed New Labour urban policy, the Urban White Paper of 2000 and lots of other stuff. I've also just discovered that it has 61 citations on Google Scholar, which means as many as 61 people were at one point potentially aware of its existence and some might even have read it. Okay, too much preamble, time for a map - showing the population of the central Manchester and Salford wards that more than doubled in population between 2001 and 2020 (I don't have the census data for wards yet).

Source data: ONS ward-level population estimates, 2001-2020

Why was I thinking about this? 

It's because of the release this week of a big wodge of local Census 2021 data for England and Wales. I thought I'd go and look at the changes from 2001 to 2021 but then I couldn't find quite what I was looking for (ward population data) so I went and looked at the mid-year population data for wards instead, and how I find myself writing about it here because it's quite interesting.

If you haven't already spotted these things, three observations from the map above:

1. Salford Quays ward had less than 1,000 people living in it in 2001.
2. Central Manchester/Salford has the largest collection of contiguous wards (6) in England and Wales that more than doubled in population between 2001 and 2020 - the population now in the six wards above is about 85,000, compared to 24,000 in 2001 and 50,000 in 2011.
3. Deansgate and Piccadilly wards in Manchester look like a rabbit (Piccadilly) getting a piggyback on a goose (Deansgate - see below). This is the unintended consequence of me looking at map data for too long, but it's a clear fact. 

I'm not wrong, am I?

The numbers are of course not surprising if you've ever stepped foot in an English city centre over the past 20 years but it's quite interesting to be able to put a number on things. So now I'll share more maps of different cities, showing the 2001, 2011 and 2020 population data, like I have for central Manchester above (all areas shown more than doubled in population from 2001 to 2020).

Birmingham

Bristol

Leeds

Leicester

Liverpool

East London (1) - plus a bit non-East London

East London (2)

Newcastle

Nottingham

Sheffield - check out those numbers!

Southampton

Who are all these new people? Well, again it's not that hard to fathom if you have been paying any attention but a mix of students in new private sector accommodation, the so-called 'young professionals' estate agents seem to love and a mix of other folk, including some families and whatnot. Could also say stuff about brownfield sites here, as well as New Labour urban policy (the old 60% target) but that doesn't interest me much these days so I'll leave it at that.

Oh, go on then, a few more.

Cardiff - yes, 100% NOT in England! But interesting

Bournemouth

I can't unsee it

Poundbury

This one was missing a label in the version above

How big is Tokyo?

How big is Tokyo? What's the population of Tokyo? What even is Tokyo? All excellent questions, so in this long read I'll try to explain my answers to these questions by way of data and maps, including some new analysis I've done. But first, here's a photo of the urban area, plus a map of Tokyo prefecture - I'll say more about it later on - including a bit about the remote Tokyo island shaped like a shark's tooth.

Yodalica, CC BY-SA 4.0, via Wikimedia Commons

The convex hull for Tokyo Prefecture

The short-ish version

If you look at a satellite image of the wider metropolitan region that includes Tokyo, Yokohama and many other places and then calculate a population for that continous urban area, you'll get one big figure. If you calculate the population of Tokyo based on the commuter zone you'll get another big number. If you calculate it at the prefecture level you get another, lower, figure and if you use the '23 Special Wards' definition of Tokyo you get another figure. 

But if you're reading this then you probably already know all that, so let's look at the latest numbers instead. For this, I derived four different 'Tokyo' populations using the most recent (2020) WorldPop data for Japan. There are actually four different versions of this data that you could use and I tried them all but the numbers differed very little between datasets so the figures here use the 2020 1km aggregated WorldPop data for Japan. So, based on this, here are some 'Tokyo' populations for 2020.

• 9.7 million people in the 23 Special Wards (this represents the inner-urban core of the Tokyo Metropolitan Area, and is part of Tokyo prefecture - i.e. the Tokyo Metropolitan Government (TMG) area). 
• 14.3 million people in Tokyo prefecture (i.e. the 'city proper' - this is the official Tokyo Metropolitan Government area but a big chunk of it goes well beyond the urban fabric). The official Tokyo Statistical Yearbook 2020 figure for this area says there are 14.05 million people in Tokyo prefecture, so my calculation isn't far off. Given that my calculations are based on 1km squares, it's not surprising there are some differences, given the unmodifiable chunkiness of 1km chunks of city vs the very detailed prefecture boundary.
• 34.6 million people in the wider Tokyo urban area, as defined by the European Commission's Global Human Settlement Layer project - take a look at their web map. This area is highly urbanised and in fact if you look closely (see map below) it appears that there are many continuously urban areas adjacent to it which are not included - so it may not be picking up all of the Kanto continous urban area. In fact, it definitely isn't. That's why I give another figure, below.

The GHSL project's definition of 'Tokyo' urban area

• 40.5 million people in a fairly continuously urbanised area within the much larger Kanto region. I drew this area very roughly myself based on the underlying satellite imagery but as you can see (below) it does appear to be a reasonable approximation for the wider urban area. Yes, there is a good bit of green in here but it's still very much an urban area.

The wider 'Tokyo' metropolitan area

So is the population of Tokyo 40.5 million, or what? Well, we'll talk about Tokyo definitions a bit more below but here's the answer I'd give.

"The population of the wider metropolitan area in which Tokyo is situated is approximately 40 million people. The continuously urbanised Tokyo commuter zone has about 35 million people. The official Tokyo local government area (i.e. the prefecture, or TMG) has 14 million people and the '23 Special Wards' of Tokyo contain just under 10 million people."

But of course this also raises more questions, such as: how big are these areas in terms of square miles or square km? And, how does this compare to other cities or urban areas? And what about population density?

But for now, I think we've reached the conclusion that Tokyo is a big city. I think we can all agree on that much. Or can we? But what actually is 'Tokyo' anyway?

How big is Tokyo? The many Tokyos

Geography boffins can talk all day about formal regions, functional regions, ontologies, epistemologies, boundedness, agglomeration, conurbations and so on. We can even go on Reddit or elsewhere on the web to read about how big Tokyo is or isn't, including that famous image of the wider Kanto area plonked over the UK. But for most sensible urban watchers, the 'city proper' isn't really a great way of assessing 'cities' because it's often a fairly arbitrary boundary that goes well beyond the urbanised area itself (as in the case of Tokyo) or indeed doesn't actually include all of the urbanised area at all (also true in the case of Tokyo). 

A prime example of this is shown in the map below, of the Tokyo Metropolitan Government area (the TMG, shown in white - this is the prefecture boundary for Tokyo) - I haven't shown the far-flung islands that are actually part of the TMG prefecture but you can see that the area around most of it is very dense. The area within the dashed white line is home to 14 million people, as of 2020. This is one of Japan's 47 prefectures and it is the most populous by far - Kanagawa (containing Yokohama) immediately south of Tokyo has about 9.2 million people. 

Overbounded in the west, underbounded everywhere else

If you Google 'Tokyo prefecture' this (above) is the boundary you should see on Google Maps. It is equivalent, in administrative terms, to the local government area covering Greater London or New York City. But of course it is an administrative, technical, political boundary and not one that makes much sense if we think about human activities. It's a formal boundary rather than a functional one.

Nonetheless, this version of 'Tokyo' does exist and it is a real thing and within this space your local government authority is the Tokyo Metropolitan Government and the Governor is (at the time of writing) is Yuriko Koike. But really this section is more about the size of 'Tokyo' in areal units, so let's get to that. Note in the map below how the only 'Tokyo' that includes a big chunk of countryside is the official Tokyo Metropolitan Government area - i.e. the actual Tokyo prefecture itself).

• The 23 Special Wards cover an area of approximately 620 sq km / 240 sq mi.
• The Tokyo Metropolitan Government area (TMG, also known as Tokyo prefecture) covers an area of approximately 1,780 sq km / 687 sq mi- note that this refers to the area shown in the map below and does not include the islands that are part of the prefecture.
• The Tokyo commuter zone (i.e. the European Commission's GHSL boundary for 'Tokyo') covers 5,318 sq km / 2,053 sq mi.
• The area I defined - very roughly - as being the continous urban fabric surrounding Tokyo covers an area of 12,350 sq km / 4,768 sq mi. Note that my drawn area is not the same as the Kanto region. The Kanto region covers an area of 32,423 sq km / 12,518 sq mi and has a population of 42 million - so if we used the Kanto region as a proxy for 'Greater Tokyo' or 'the wider Tokyo Metropolitan area' then it would be adding in loads of almost empty countryside and that wouldn't make much sense.

Look at the urban fabric vs the boundaries here

But does any of this even help us? Do you know how big or how small 620 sq km or 240 square miles even is? Well done if you do, but I don't, so I need to have some way to compare it to something I know if I'm going to understand it. More on that below in relation to London but here are a few comparisons.

• New York City (i.e. the five Boroughs) covers 300 square miles (about 780 sq km). So, the 23 Special Wards (population just under 10 million) covers an area about 80% of the size of NYC but has more than a million extra people.
• Greater London (i.e. the 32 Boroughs plus the City of London) covers an area of about 1,570 sq km (606 sq mi) - that's about 90% of the size of the Tokyo Metropolitan Government prefecture area - and recall that the Tokyo prefecture area contains 14 million compared to about 9 million in Greater London. Add Scotland's population (about 5.4 million) to Greater London and you get roughly the same population as Tokyo prefecture.
• Delaware is about the same size is the the European Commission's GHSL boundary for 'Tokyo'. Delaware covers 1,982 sq mi or 5,133 sq km and has a population of just over 1 million as of 2021. By comparison, the GHSL 'Tokyo' area has almost 35 million people. Take Delaware, add in the population of Texas (almost 30 million as of 2020) and you're still not at metro Tokyo density levels. To get to that you'd also have to add Arkansas' 3 million residents. This would of course be a fascinating sociological experiment but in practice it may not work very well. 
• Île-de-France (the the Région Parisienne / the Paris Region) covers an area of 12,012 sq km (4,368 sq mi) and has a population of just under 13 million people - compared to the 40 million in my self-drawn 'Tokyo continous urban area' shape. This is also roughly the same area as Los Angeles County (including the water bits), which is home to just under 10 million people.

So you're saying that Delaware has the same area (roughly) as the urbanised Tokyo commuter zone but 34 million fewer people? Yes, that's about right. and the continous urban fabric of Tokyo's extended metropolitan area has more people than California in an area not much bigger than Los Angeles County? Also yes.

Okay, but what's this in Londons? How many Londons is this? Can we make sense of this by using Londons? Perhaps, so do read on.

Note how the white boundary goes beyond the urban area

Tokyos as Londons

Because I live in the UK, about 175 miles from London, and because London is the only big city I know fairly well, I'm going to use Londons to make more sense of the Tokyos above. What I did was take the shape of Greater London and then resize it to match the four different definitions of 'Tokyo' above. To be clear, though, when I say 'London' I am using it as shorthand for Greater London - that is, the 32 Boroughs plus the City of London that have a combined population of 9 million people. 

Once I had created four new Londons, re-sized to match the four different Tokyos above, I then calculated the population of these new Tokyo-based Londons using the same WorldPop 2020 data as I used for Tokyo. This gave me the following results.

• The area of London re-sized to match the area covered by Tokyo's 23 Special wards has 5.7 million people (vs 9.7 million in Tokyo's 23 Special Wards).
• London re-sized to match the Tokyo Metropolitan Government area (i.e. the prefecture) has 9.5 million people in it. It's only just a bit bigger than Greater London itself. The real Tokyo has 14.3 million people in the same area.
• London re-sized to match Tokyo's GHSL-based wider urban area has 12.5 million people in it, compared to almost 35 million in the real Tokyo urban area.
• London re-sized to match my wider 'urban fabric' definition of Greater Tokyo has 16.2 million people in it, compared to 40.5 million in the actual 'urban fabric' of the wider Tokyo area.

You can see all this on the maps below. The first one is just the re-sized Londons, the next one has labels too and the one after that shows the urban fabric to you can get a sense of the underlying settlement pattern in the south east of England.

Greater London re-sized to match different Tokyos

With labels, for a bit of context (a bit messy, sorry)

Same as the first one, but with building footprints added

I realise this can all get a bit confusing at times but just remember that the coloured 'London' shapes above have been re-sized to match the areas covered by the four different definitions of 'Tokyo' used above. Now, although I'm not thinking about population density here, the last map in particular makes me think of it, not least because I've seen people say that London is more densely populated than Tokyo. Well, as you've seen above, that really depends upon what you mean by Tokyo, so let's talk a little bit about density.

Population density

We've seen above that the Tokyo prefecture boundary goes way into the non-urban part of the administrative area and that the population in this particular Tokyo is packed tightly together. This is just another example of where using an administrative boundary to derive population density data makes no sense, or at least not much sense to me. I have written on this in the past, in relation to what I call 'lived density'. The basic idea in that piece was that population density metrics often make no sense at all if we are thinking about it in relation to what people actually experience in their day-to-day lives.

If we go to the Tokyo prefecture page on Wikipedia, we will find the following information.

• Area: 2,194 sq km (847 sq mi) 
• Population: 14.0 million
• Density: 6,363 people per sq km / 16,480 per sq mi

But of course the population distribution in Tokyo prefecture is very much not a good reflection of this. 

The arithmetic mean here is a bad measure because: 

a) it is not a good representation of the reality on the ground - i.e. the mean as a model is a bad fit here; and 

b) the area figure includes loads of rural areas and offshore islands that are also very much not part of metropolitan Tokyo. 

Yes, the maths are correct but the number is not very useful in my opinion. The density in the 23 Special Wards would be a much closer representation of lived experience for most Tokyo residents.

• 23 Special Wards area: 620 sq km / 240 sq mi
• 23 Special Wards population: 9.7 million
• 23 Special Wards population density: about over 15,000 per sq km / 39,000 per sq mi

Greater London's population density is about 5,666 per sq km or 14,670 per sq mi - so not as dense at all as the 23 Special Wards, or even as dense as the over-bounded Tokyo prefecture. I'm not sure of the original source of the claim that London has twice the population density of Tokyo but I saw it in this twitter thread and it stood out to me. I suppose it's all a matter of getting your Londons and your Tokyos defined in different ways though.

Hmm, how where was I? Oh yes, the actual size of the official 'Tokyo' and by that I mean the prefecture, i.e. the Tokyo Metropolitan Government area. 

The official Tokyo is HUGE!

The version of 'Tokyo' that gets us to 35 million people includes the following places: Tokyo; Yokohama; Kawasaki; Saitama; Chiba; Setagaya; Nerima; Ota; Sagamihara; Edogawa; Adachi; Funabashi; Itabashi; Kawaguchi; Hachiōji; Suginami; Koto; Ichikawa; Katsushika; Machida; Fujisawa; Kashiwa; Shinagawa; Kita; Koshigaya; Tokorozawa; Kawagoe; and Nak. That's a big place, but of course it's not a single place, even if it is basically one huge urbanised area with no major gaps in between.

But there is another Tokyo that is MUCH bigger than that Tokyo (kind of), though on the face of it you'd think it was much smaller. You can read about this on the Tokyo prefecture Wikipedia page in the 'Geography and government' section but the basic facts are as follows.

• It contains 62 different municipalities: 23 Special Wards, 26 cities, 5 towns, and 8 villages. 
• The most populous ward is Setagaya Ward (one of the 23), with a population of about 910,000. 
• It contains three villages with populations under 500
• It contains islands hundreds of miles away from the Japanese mainland.

Here's a map showing the convex hull of the Tokyo prefecture area, like the one at the beginning of this piece. The dotted line encloses all mainland and island areas that is officially, from an administrative point of view, 'Tokyo' - i.e. the prefecture version of Tokyo that has just over 14 million people. The sea is of course not part of Tokyo. 

Includes the Izu Islands and the Ogasawara Islands

Rotate Japan and put it inside Tokyo's convex hull

What's the island at the far south east corner of the map above? Why, that would be Minamitorishima, the island shaped like a shark's tooth (see below).

Minami-Tori-shima

Here's a photo of Minamitorishima taken in 1987 by a US Air Force Sergeant. Also known as Marcus Island, it is 1,848 km / 1,148 miles southeast of Tokyo and covers an area of about 1 square mile - about the same size as the City of London - and the closest island to it is more than 1,000 km away. It is, as you might expect, the most easterly point of 'Tokyo' and nobody lives there. This makes it easier to calculate the population density. Unsurprisingly, there is no Google Street View imagery here, although the satellite image is quite good. It doesn't look like the best place to be when the weather is bad though.

Taken by Chief Master Sergeant Don Sutherland, U.S. Air Force 

Conclusions

1. Tokyo is a big city, however we define it. But to say that Tokyo has 10 million or 14 million people would be a big under-representation of the true size and scale of the continous urban area of which Tokyo is the biggest settlement.

2. Not all Tokyos are the same but only one Tokyo is the 'official' Tokyo - i.e. the prefecture.

3. Some Tokyos are much more than just Tokyo. That is, any sensible definition of the functional scale and size of Tokyo is much bigger than the Tokyo Metropolitan Government area defined by the prefecture boundary.

4. One Tokyo (the 23 Special Wards) is much smaller than the other Tokyos.

5. The urban area of 'Tokyo' or 'Greater Tokyo' or 'Tokyo Megalopolis' is very often considered to be the largest urban area on earth - and as we have seen here the population can be calculated at between 35 and 40 million people - that's about the same population as Canada or California. Note that the Global Human Settlement Layer (which my 34.6m Tokyo population comes from) has Guangzhou and Jakarta urban areas ahead of Tokyo in terms of population, at 40 and 36 million people respectively.

6. Tokyo includes loads of islands far away from what we think of as Tokyo. If we draw a shape around it (i.e. a convex hull) then that area is far bigger than Japan itself. Yes, Japan fits into Tokyo's convex hull (if we chop it up a bit). Tell your friends.

7. The population of Tokyo is 9.7 million, 14 million, 35 million or 40 million depending upon what you mean by 'Tokyo'. But if we're talking about the population of the large, continuous urban area then I'd say that 'Tokyo' has between 35 million people and 40 million people.

8. Has anyone done a list of countries where you can see the nation's highest peak from the capital city? You can see Mount Fuji from Tokyo, about 100km / 62 miles away. This must be my next challenge.

Here's a parting gif for you - the GHSL area of Tokyo compared to the GHSL London area - they are defined the same way so we are comparing like-with-like here.