Historically, the urban limits of Rome were considered to be the area within the city walls. Originally, these were the Servian Wall which was built twelve years after Gauls' sack of the city in 390 BC. This contained most of the Esquiline and Caelian hills, as well as the whole of the other five. Rome grew out of the Servian Wall, but no more walls were constructed until almost 700 years later, when in 270 AD Emperor Aurelian began building the Aurelian Walls. These were almost 19 kilometres (12 mi) long, and were still the walls the troops of the Kingdom of Italy had to breach to enter the city in 1870. Modern Romans frequently consider the city's urban area to be delimited by its ring-road, the Grande Raccordo Anulare, which circles the city center at a distance of about 10km.

The Comune of Rome, however, covers considerably more territory and extends to the sea at Ostia, the largest town in Italy not to be a comune in its own right. The comune covers an area roughly three times the total area within the Raccordo and is comparable in area to the entire provinces of Milan and Naples, and to an area six times the size of the territory of these cities. The comune also includes considerable areas of abandoned marsh land which is neither suitable for agriculture nor for urban development.

Consequently the density of the comune is not that high, the communal territory being divided between highly-urbanized areas and areas designated as parks, nature reserves, and agricultural use. The Province of Rome is the largest by area in Italy. At 5.352 km², its dimensions are comparable to the region of Liguria, and more than three times the size of the greater metropolitan area of London.