The census figures for the ancient world are estimates at best. Thanks to the concept of the Roman Census, there are some figures specifically related to the Roman Empire, but these are often deemed unreliable as the people who were included in each periodic census could change. (ie for counting actual population vs. citizen males vs. provincial citizens for tax purposes etc.). Prior to the mid 4th century BC, all surviving figures are generally disregarded as completely fictitious, but after that, a pattern of reasonable population figures begins to emerge. However, it is still difficult to determine, especially as the Republic expanded to include various provinces, whether population figures include these areas, or just the city of Rome itself. Also clouding the science of the census is whether or not the count in various years was limited to male citizens, citizens and their families, women, freedmen, slaves and/or everybody else in between.
Understanding these difficulties, there is little choice but to determine the population of the Roman Empire using various consensus estimates. The population of the world circa AD 1 has been considered to be between 200 and 300 million people. In that same period, the population of the early empire under Augustus has been placed at about 45 million. Using 300 million as the world benchmark, the population of the Empire under Augustus would've made up about 15% of the world's population. Of this 45 million people, Augustus declared within in his own census information that:
* In 28 BC the citizen population was 4,063,000 (including both men and women)
* In 8 BC - 4,233,000
* In AD 14 - 4,937,000
By contrast, in the census of 70 BC, prior to the major civil wars of the late Republic (and considerably more conquests in Gaul and the East), some have estimated the population of the 'Empire' at a more considerable 55 to 60 million people. This falls more in line with estimates at the height of imperial power in the mid 2nd century AD, and might be inflated considering the lack of the previously mentioned expansion. The census of 70 BC showed 910,000 men held citizenship, which is far short of the Augustan citizen numbers (roughly 4 million), but more than the overall numbers (roughly 45 million) just a century later. The large discrepancy would seem to account for the fact that Augustus probably counted more than even citizen men and related family members (including women). He may have included non citizen freemen, freedmen and slaves as well, but this we can never be certain of. A Claudian census in 47 AD places citizen population at just under 7 million people. This, despite its near unbelievable rate of growth from just 50 years prior, can be partially attested by the great vilification of Claudius for including Gauls and other provincials in the Senate as well increasing the citizen roles. In fact, citizen growth was more a measure of Romanization than it was of birth rate. By this time, Roman citizenship was experiencing its first major shift from something of Italian origin, that would continue to evolve over the next few centuries.
At the height of Roman power in the mid 2nd century AD, conservative opinion is that the Empire was comprised of some 65 million people. Assuming that the world population was still roughly about 300 million people this would mean that the Roman population was approximately 21% of the world's total. However, less conservative estimates have added far more people living within the official borders of the Empire, perhaps as much as doubling the figure. With this in mind, the population of the Empire may have approached 130 million people or perhaps over 40% of the world's total! However, as these numbers for the ancient period are widely divergent and imprecise, it could be assumed that either number or any in between has the potential to be correct. Still the increase from 45 to 65 million in about a century is believable and can be credited to the conquests of Britannia and Dacia, and several annexations of client kingdoms dating from the time of Augustus. (mostly by Claudius)
Breaking down the 65 million population estimate, some additional assumptions can be made:
* 500,000 soldiers (legionaries totalling 150,000 and auxilia making up the rest)
* Approximately 600 Senators made up the elite of the elite.
* Perhaps up to 30,000 men filled the roles of Equestrians (knights), or the second tier of the aristocracy.
* 10 to 30% or 6 million to 19 million people lived in the cities, leaving the vast majority of some 46 to 59 million people to live in the country as independent and mostly tenant farmers.
* Rome itself was made up of over 1 million people and, though it would shrink remarkably after the fall of the west, no city would surpass that number until the great urban population booms of the industrial age, 1,500 years or more later.
* The slave population of Rome approached 500,000 on its own, probably half of which were owned by the 600 men of the Senate. Additional estimates have suggested that of the total 65 million people, 2 to 10 million may have been slaves.
After the plagues of the 160's to 170's AD, and the wars of Marcus Aurelius, the population of the empire fell from its previous high, likely down to about 40 million in total. By the beginning of the 4th century, and the reign of Constantine, civil wars and foreign incursions had taken their toll. The number had grown again, likely to somewhere around 55 million, but the rate of growth had obviously slowed considerably. By this time too, a major shift in imperial power was taking place from the west to the east. The population of Rome was in decline and Byzantium (or Constantinople) was on the rise. The west likely made up about 40% of the Empires total population with the remainder in the east. By the mid 6th century, wars, disease and emigration brought the population of Rome perhaps as low as 30 thousand to 100 thousand people; a far cry from its height just a few hundred years earlier. By contrast, in the same period, Constantinople may have numbered somewhere between 750,000 to 1 million people itself in the time of Justinian.