The city of Rome was the capital of an empire that stretched from the Pillars of Hercules in the West to the Euphrates River in the East. This dissertation seeks to quantify the Augustan (27 BCE- 14 CE) and Severan (193-211 CE) populations of this city. For the time of Augustus, the population shall reckon from a group of 250,000 male citizens in receipt of state-funded largesse, estimating the number of women, children, slaves, freedmen, and foreigners that such a number may support. The Severan population will be measured using a statistical model estimating the floor area of all residential housing and dividing this total by the average floor area per person. A range of interpretations will emerge from this evidence, so I shall offer both a possible and a probable range of population, so that others can see the upper and lower limits allowed by the data.