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A cohort was a major subdivision of a Roman legion made up of several centuries. In the standard organization of the Principate, a normal cohort contained six centuries and about 480 soldiers, while the first cohort could be larger and more prestigious.
In brief:
| Unit | Strength | Command |
|---|---|---|
|
Century |
about 80 |
Centurion |
|
Cohort |
about 480 |
Senior centurion |
|
Legion |
roughly 5,000 |
Legate |
The figures are a comparative establishment, not a permanent headcount. Ancient units rarely remained at full strength throughout a campaign.
The cohort gradually became the legion’s principal tactical subdivision in the late Republic. The change should not be reduced to a single decree or a precisely dated “Marian reform”. In the earlier manipular legion, two centuries made a maniple. Cohort organisation retained many traditional centurionate titles even after the earlier distinctions between hastati, principes and triarii had lost their original significance.
For a normal Principate cohort, a common calculation is six centuries of roughly eighty men: an establishment of about 480 personnel. The first cohort held higher status and in some periods had an enlarged organisation. A model of five double-strength centuries, approximately 800 men, is useful in particular imperial contexts but cannot cover all Roman history. Establishment, total strength on the rolls and personnel actually present are different figures.
A cohort was large enough to receive a distinct mission yet compact enough to manoeuvre within the legion. A commander could deploy cohorts in several lines, keep some in reserve, reinforce a flank or assign them to an assault. The arrangement depended on terrain and plan rather than on one unchanging pattern of five cohorts in front and five behind.
Within the cohort, centurions controlled their centuries and the senior officer coordinated their action. Cohort organisation allowed a legion to divide into detachments while retaining command and to operate on broken ground more flexibly than a single undivided mass.
The term cohors was not confined to legionary subdivisions. The auxilia included infantry cohorts and mixed cohortes equitatae with a mounted component. Their independent command and organisation differed from those of a cohort within a legion. The labels quingenaria and milliaria do not guarantee the presence of exactly 500 or 1,000 men.
A concrete example is the strength report of the First Cohort of Tungrians from Vindolanda, dated around 92–97 CE. It lists 752 personnel, of whom 456 were absent on various assignments; 296 remained locally, including men unfit for duty. This was an auxiliary, not a legionary, cohort, but the document clearly demonstrates the difference between total strength and available personnel.
The Praetorian Guard, urban cohortes urbanae and the firefighting and night-watch service of the cohortes vigilum also used the term. A shared name does not imply identical duties, arms or recruitment.
How many centuries were in a cohort? A normal Principate cohort had six. The first cohort had a special organisation that changed over time.
How many soldiers were in a Roman cohort? About 480 is a common establishment for a normal legionary cohort. Actual strength could be substantially lower.
Did cohorts exist only in legions? No. Cohorts also served in the auxilia, Praetorian Guard and urban services, but their organisation differed.
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