The economics of the Roman army concerns the costs of maintaining the Imperial Roman army and the infrastructure to support it, as well as the economic development to which the presence of long-term military bases contributed. Supply contracts with the military generated trade with producers near the base, throughout the province, and across provincial borders.
Military burden
The size and growth of the Roman army of the Principate may be summarised as follows:
Note: Figures are based on official (not actual) unit strengths and exclude Roman navy effectives and foederati.
The Roman navy contained probably 30,000–40,000 marines, sailors, and oarsmen, of which 15,000–20,000 in the Mediterranean fleets at Misenum and Ravenna (contrary to popular belief, Roman warships of this period were not rowed by the forced labor of convicts or slaves, but by volunteer professional oarsmen) and perhaps the same again in the classis Britannica (English Channel fleet) and the fluvial flotillas on the Rhine and Danube.
Substantial numbers of irregular tribal troops (foederati) were in the paid service of the empire during the whole period: the number is unknown, but there were at least 5,500 in Britain alone around 175 AD (surrendered Sarmatian cavalry posted there by Marcus Aurelius ). If this figure is multiplied by 10 to represent other frontier provinces, it is possible that there may have been 50,000–60,000 such irregulars at any given time.
At its peak under emperor Septimius Severus () therefore, the standing Roman military establishment may have comprised over half a million effectives.
This was a very significant burden on the Roman economy, which was pre-industrial: at least 80% of its inhabitants worked in agriculture. A very high percentage of all the revenue from taxes and rents raised by the imperial government were spent on the military: about 80% of the imperial budget in c. 150. Other sources suggest less, but not less than 60%. Military spending constituted, by one estimate, about 2.5% of the empire's GDP, which seems a tolerable burden if compared to the US, today's global superpower, which spent 3.8% of its GDP on defence in 2006 (18% of the federal budget). But the comparison is misleading. Due to modern technology, a modern economy is far more productive per capita than the Roman economy: on one estimate, the average American in 1998 was at least 73 times more economically productive, in comparable terms (i.e. in international dollars), than a Roman in the 1st century AD. Another factor to consider is that half the population was 25 or less (compare modern societies 35–40) meaning that larger portion of the population were too young and not taxpayers. Another factor is that many items of production were not taxed such as those for domestic use, clothing, buildings. Therefore, taxes (and compulsory services) to support the Roman military would have taken a much greater share of surplus per capita production i.e. surplus to the subsistence needs of producers, almost all agricultural. For the average peasant, the taxes and services he was obliged to provide to the military would have represented a significant share of his disposable surplus if situated within reasonable distance of the units. Compulsory services performed by the richer members of society helped to supply military needs (and maintain the infrastructures of cities and the empire, in effect a tax that was paid for in money or labor).