This is actually mostly a repost of figures I did on the Freebootaz forum a while back but I figured I might as well share them now that I have them more ironed out.
I've been trying to figure out exactly how big the empire is. Frankly Geography isn't getting me anywhere.
40k has a frustrating propensity for vagueness, giving exact figures forces the GW boys into that annoying continuity hole that they seem to have difficulty staying inside of. However there are some figures that can be taken directly from canon materials.
We know for a fact that there are 1 million planets within the GEoM's empire. This can be interpreted a number of ways, either that it is 1 million earth sized or bigger which is generous at best, or (and this is infinitely more likely) that there are one million planets of various sizes under imperial control. We can safely assume that most planets in the imperium would be viable planets for colonization (agriworlds, forgeworlds, essentialy the generic boring POS planet) which probably accounts for a majority of the empire. There are a bunch of relatively uninhabitable worlds in the empire (ala Catachan) as well as a number of worlds that are overpopulated to the extreme (ala Armageddon or any comparable hive world with populations in the hundreds of billions). However overall it seems reasonable to assume that the worlds of the empire would average out to have populations equal to the modern day earth. It's probably more than this, but I prefer to work with low end estimates rather than high end estimates.
In the Rulebook a Minea-Class Hive world with a population of 154 billions, the Imperial garrison strength (PDF) is 2,000,000 men...The annual planetary draft is 1,249,000 men (IG)...
If we assume that this figure is indicative of the relative percentages for the imperium then some 2% of any world's population goes to the military. 2% may seem low considering the whole "In the Grimdark of the future there is only war" thing but China only has 0.17% of the population in the military and we consider the chinese military to be disgustingly large. The USA has nearly triple the per capita soldier distribution, 0.47%, but even then that pales in comparison to a 2% militarization.Obviously there are exceptions to this, Cadia for example has a near 100% draft rate, for obvious reasons, but we cannot safely say if the majority of hive worlds would be over this figure.
So 1 million worlds in the empire
Estimated average Population per earth comprible planet 5,000,000,000
Estimated Armed Forces provided to the Empire 100,000,000
Total Population of Empire 5,000,000,000,000,000
Estimated Total Military Forces added to the IG per year to the various IG divisions 50,000,000,000,000 *(this is per annum recruitment not total size)
These are Low end estimates, there is plenty of evidence to suggest that the these figures should be double or even triple my estimates. Unfortunately GW is wary of giving exact figures on the subject.
We also know that roughly 1000 loyalist space marine chapters exist. We know that codex rules demand that chapters stay at no more than 1000 marines per chapter, not including initiates and serfs. The notable exceptions to this being the Space Wolves who have additional companies, the Ultramarines who maintain their own miniature empire in the systems local to their chapter world including Navy, IG armies, and Titans, and the Black Templars who are so fanatically devoted to the emperor that they are free of accusations of heresy and thus have grown to the size of a space marine legion. There are also dying chapters, chapters who no longer access to their gene seed because it was destroyed, chapters who are unable to get new recruits till they have accomplished some sort of penance, or the chapters who are so decimated by one campaign or another that it will be centuries till they are at full strength. I'm assuming that overall it will end up coming out about even.
But this means that in the 40k universe there are only 1,000,000 space marines at any moment in time. This number does not include any space marines sequestered by the Inquisition of the Ordos Xenos or the Grey Knights who are not tecnically Adeptus Astrates. This is potentially low or high depending on if GW means there are a thousand active
space marine chapters or a thousand total
As to the size of the imperial Navy there is really little that can be substantially determined in terms of fleet size. We can safely say that there are from 50-75 ships of various classes in a sector but there is really no canon on how many sectors there are in the Empire.
However if we assume that there are anywhere from 1-5 colonized worlds in a sector of space that means that there are anywhere from 250,000 to a million sectors in the imperium.
That means that the strength of the navy is somewhere between 12,500,000 and 75,000,000 ships of varying sizes and classes. (this assumes that the imperium doesn't bother having a naval presence in sectors it has no population in, something we know to be untrue)
We can also infer the size of the astrates navy as well.
Assuming that each chapter has 1-3 battle barges and around 7-10 strike cruisers that means that there are some roughly. 1,000-3,000 battlebarges in the universe and 7,000-1,000 strike cruisers at any time. This is of course the minimum but GW fluff is surprisingly minimalist where the space marines are numerically concerned.
The BFG ruleset is even more frustratingly vauge than the actual 40k ruleset, not really something that seems possible but is true. Starting off there seems to be a bit of debate over how large a starship in the GEoM's navy actually are. An emperor class could be anywhere between 3km to 15km long depending upon the source. I'm fond of using the http://www.merzo.net
sizes simply for lack of having a better starting off point. 40k has to be about the only scifi franchise to not have created some sort of sourcebook or another with sizes and cross-sections of the ships of it.
However we can make some rough estimates based off of the fluff. If we assume that the average escort ship is 1.5km (ish) long The only ship that I know of that is roughly that size is the star destroyer in star wars which has a crew of around 50,000.
The modern aircraft carrier.
Length: 1,092 feet (332.85 meters).
Beam: 134 feet (40.84 meters); Flight Deck Width: 252 feet (76.8 meters).
Beam: 250m, Width: 200m
Crew: 50,000+ (estimated)
However I would not be surprised if the crew of a cobra class was triple that considering exactly how many of basic functions of the ship are done by hand. So for the purposes of working with simple numbers lets say that the figure was somewhere in between 50k and 100,000.
So an escort is crewed by somewhere between 50-100k crewmen of various roles.
A capitol ship is some seven times the size of a cobra class, just in length. It would not be impossible to assume a crew in the high hundreds of thousands or low millions per ship.
I already estimated the navy to consist of 12,500,000 and 75,000,000 ships total. Unfortunately we do not know if this is all escorts, some escorts or no escorts at all. Not knowing the distribution of forces puts the size of the naval personnel somewhere between 6,250,000,000 and 75,000,000,000,000 people. I am aware of how vague this is, but GW is decidedly annoying when it comes to quantifying materials.