A full strength legion was officially made up of 6,000 men, but typically all legions were organized at under strength and generally consisted of approximately 5,300 fighting men including officers. It is difficult to determine whether non-combatants like field surgeons and clerks were included in the 5,300 or helped bring the total number of men up to the official 6,000.

The basic structure of the army is as follows:

Contubernium: (tent group) consisted of 8 men.

Centuria: (century) was made up of 10 contubernium with a total of 80 men commanded by a centurion

Cohorts: (cohort) included 6 centuriae or a total of 480 fighting men, not including officers. In addition the first cohort was double strength but with only 5 centuriae instead of the normal 6.

Legio: (Legion) consisted of 10 cohorts.

Additionally each Legion had a 120 man Alae (cavalry unit) called the Eques Legionis permanently attached to it possibly to be used as scouts and messengers.

Therefore the total fighting strength of a Legion:

The First Cohort totaling 800 men (5 double-strength centuries with 160 men each) 9 Cohors (with 6 centuries at 80 men each) for a total 4,320, and an additional 120 man cavalry for a grand total of 5,240 men not including all the officers.

The basic designation of the 10 cohors was the same throughout all the Legions. They were arranged in battle so that the strongest and weakest units would be mixed throughout the formation maximizing moral and effectiveness

Cohort I: Was made up of the elite troops. Its direct commander was the Primus Pilus, the highest ranking and most respected of all the Centurions.

Cohort II: Consisted of some of the weaker or newest troops.

Cohort III: No special designation for this unit.

Cohort IV: Another of the four weak cohorts.

Cohort V: Again, no special designation.

Cohort VI: Made up of "The Finest of the Young Men".

Cohort VII: One of the four weak cohorts and a likely place to find trainees and raw recruits.

Cohort VIII: Contained "The Selected Troops".

Cohort IX: One of the four weak cohorts and a likely place to find trainees and raw recruits.

Cohort X: Made up of "The Good Troops".