Roman Legion Manpower Varied from 4,200 to 5,200 Men Across Eras

The Roman legion's size fluctuated dramatically over a millennium. During the mid-Republic (mid-3rd to mid-2nd centuries BC), it was a citizen militia of about 4,200 infantrymen, as recorded by Greek historian Polybius. The legion was divided into velites (1,200 light skirmishers), hastati (1,200), principes (1,200), and triarii (600). In 107 BC, Gaius Marius allowed landless proletarii to enlist, standardizing equipment and training. The legion then comprised ten cohorts, each of six centuries of 80 men, yielding a theoretical strength of 4,800; the First Cohort was double-strength (800 men), plus officers and specialists, bringing the paper strength to 5,000–5,200. Velites and rank distinctions faded, and legionaries became heavy infantry armed with pilum and gladius. During the Crisis of the Third Century (c. 235 AD) through Diocletian and Constantine, legions shrank to 1,000–1,200 men for administrative flexibility. However, actual field strength was always lower: Vindolanda tablets show at least one-third absent on detached duties, so a legion on campaign operated at 60–80% capacity. At the Battle of Pharsalus (48 BCE), Julius Caesar's legions were heavily understrength, with some cohorts numbering as few as 200 men versus a nominal 480.