After reading a post on the Only War forum I've done some very approximate number crunching.

A typical sector might encompass 7 million cubic light years, equivalent to a cube with sides almost 200 light years long

The Imperium proper could thus be thought of as a sphere whose center lies at Terra's Sol System, and whose radius is about 50,000 light years wide.

If this is true then the Imperium has a radius of approximately 250 sectors.

As a sphere, with a volume equal to sphere = (4/3) r cubed then there are 65,449,846 sectors. 65 million sectors!

However, as all the maps show a relatively flat galaxy, I think it's safe to assume that it isn't 100,000 light years deep. If we use the absolute worst case scenario, that each sector is the entire thickness of the galaxy, then we'll treat the Imperium as a circle and work out it's area: pi r 2.

That's 196,349 sectors. I'll treat that as a total estimate, and a bare minimum but 196 thousand sectors, 196 k battlefleets, 196k Imperial guard regiments (at least).

So pretty **** big then.

Wow….. That is BIG!!!! I like it!!! Great info Face Eater!

The galaxy is not a sphere, it shaped like a Compact Disc

1 sector thick, on average.

But yeah, still pretty big

On the subject of really big things:

The 3rd edition 40k rulebook contained a two-page spread detailing the different types of worlds found in the Imperium, their approximate range of populations, and the rough proportion of worlds of each type that make up the million-or-so worlds of the Imperium of Man.

Some years ago, a couple of posters on the Conclave forums (the once-official community for the Inquisitor narrative skirmish game) used those numbers to determine the approximate population of the Imperium of Man.

The value produced was quite staggering: 3.3x10^17. 330,000,000,000,000,000, to write it out long-form. Three hundred and thirty quadrillion people.

That is to say, the population of the Imperium is more than 47 million times the size of the population of Earth today.

It also means that the average planetary population (or any other value of "one in a million") is in the region of 330,000,000,000 - three hundred and thirty billion people. Clearly, this average is skewed upwards by some truly colossal hive worlds, but that still suggests that there may be worlds in the Imperium containing well over half a trillion human beings. It also means that there are just under a trillion humans for every single Space Marine.

A single life, on that sort of scale, really is meaningless…

Face Eater said:

I'm assuming the 196k IG regiments (at least) is due to the possibility that each sector has only one planet to raise a regiment from. When in all likely-hood, a sector would have about 50-300 planets, so maybe on average 100 planets to raise regiments each per sector. So, it's quite plausible it's 19,600,000 or greater IG regiments.

Going back to Face Eater's original point about the number of sectors in the Imperium, t here are in my view a couple of arguments that mitigate against against the idea of truly vast numbers of sectors along the lines you set out.

So why the bold larger than normal font message?

Nameless2all said:

Because I was trying to write on the forum while also looking after my baby son. As I only had one hand free, I dictated the message as a note on my iPhone5 & cut and pasted it across…and it seems to have kept the font.

Ohh well congrats with the baby. Wish you the best of luck. Got two little ones of my own, and let me tell you, they grow up fast.

This is why it's never a good idea to look too closely at any given sci-fi universe…

I agree. I guess it is also relevant to note that a given sector will have uninhabitable worlds (like gas giants) and worlds with very few human beings living on it. Maybe the 1 mil number is how many significant habitable worlds the Imperium has in it's "data records." This is just my opinion, but 5k sectors seams almost legit (but to small for me), and 196k might be a bit to big. But anyhoot, as long as you stay within that ball park when describing it to your players, you should be okay. I'm going to go with 40k sectors. 196 / 5 = 39.2, rounding up for error, = 40. Everything revolves around the 40k universe anyways.

If one dislikes the idea of 5,000 or so sectors (a perfectly reasonable opinion to hold, I hasten to add, I'm not wedded to that figure myself) and prefers to think in larger terms, I think there are a few options to increase the scale. One thing that occurred to me is that not every Imperial world need necessarily be within a Sector or subsector.

The assumption most of us work with is that all Imperial worlds are clustered together in political groups that form Subsectors, and these are clustered together into Sectors. The consensus (based on FFG material about Calixis-regional sectors, Dan Abnett's work on the Scarus sector, the Eye of Terror campaign/13th Black Crusade campaign, the Armageddon campaign and Battlefleet Gothic) seems to be that these form groups of 75-200 worlds. I suspect the truth is messier.

Have a look at the Badab War map . Note the Badab Sector in the centre. Note how it consists of eleven (by my count) irregularly sized subsectors. Note also how it's not the traditional "Sector cube" that the Ixaniad, Scarus and Calixis sectors are portrayed as being in the FFG sourcebooks.

However, note also the cluster of three smaller, subsector-sized political entities in the top left of the map. What is this? A sector? It's a bit small if it is. Note also what appears to be a free floating single subsector not associated with any surrounding sector-sized political groupings.

Now this is thin evidence, but I would posit the idea that it stands to reason that there are likely to be Imperial political groupings that are smaller than the "traditional" sector or subsector size.

Let's say you have a huge region of space - like a large nebulae or something - with only 6 habitable worlds in it. The nearest Imperial Sector is hundreds of light years or years' worth of warp travel away to all but the fastest ships. These 6 worlds are only a handful of light years apart, and warp travel between them is stable and safe. Let's say they're all Imperial worlds. Wouldn't it make sense for these worlds to form an Imperial political grouping? Or for the wider Imperium to impose one upon them?

I suspect that the Imperium is packed full of isolated smaller, regional administrative groupings like this, consisting of a handful of worlds. There are quite possibly tens of thousands of such "mini-sectors." All with different, colourful names ("The Segradan League, The Sargassid Worlds, The Red Nubulaen Autonomous Administrative Zone," etc etc) After all, the idea of a "Sector" is really just a political one.

Equally, there may be "super-sectors" in the Imperial heartland (The segmentum solar) consisting of thousands of densely packed worlds all within a few light years of each other.

This is all supposition of course, but I'm all for anything which adds layers of complexity and nuance to the 40k universe. It seems to me that such small regional groupings could be an interesting way of adding colour and dense layers of political intrigue to the settting. Oh, and emphasizing the vast scale of the Imperium, too!