How a Roman legion was organised

From a small group to a legion

A common model for the imperial legion begins with eight messmates, a contubernium. Ten such groups made an eighty-man century. Six centuries normally formed a cohort, and a legion contained ten cohorts. A century’s name therefore does not mean it always contained one hundred men.

Why the first cohort is different

In an established early imperial arrangement, the first cohort had five double centuries rather than six ordinary centuries. Combining that arrangement with the remaining nine cohorts gives a theoretical infantry strength of 5,120, before adding the legion’s cavalry. This is a model for a particular organisational system, not a fixed total for every legion throughout Roman history.

Rank and reality

Centurions commanded centuries; a legionary legate held overall command, supported by other officers and specialists. Detachments could be sent elsewhere. Illness, vacancies and other duties affected how many men were available. Read a surviving strength report alongside the organisation chart to see why administrative structure and actual presence must be kept separate.