Contubernium (8 men) > Centuria (80 men) > Cohort (480 men) > Legio (5,000 men)
Until the middle of the first century, 10 cohorts (about 5,000 men) made up a Roman Legion. This was later changed to nine cohorts of standard size (with 6 centuries at 80 men each) and one cohort, the first cohort, of double strength (5 doublestrength centuries with 160 men each).
Each legion had 59 centurions, one to command each centuria of the 10 cohorts.
The commanding officer of a Roman legion was called a legate, or Legatus Legionis.
Contubernium (8 men) > Centuria (80 men) > Cohort (480 men) > Legio (5,000 men)

Each Legion also had a 120 man Alae (cavalry unit) called the Eques Legionis permanently attached to it possibly to be used as scouts and messengers. 
Contubernium (8 men) > Centuria (80 men) > Cohort (480 men) > Legio (5,000 men)
The Roman legion was divided into 10 cohorts:
The First Cohort had more soldiers than the other cohorts. The first cohort was made up of 800 men (5 centuries with 160 men each). The other 9 Cohorts had a total of 4,320 soldiers (6 centuries of 80 men each).
Cohort 1
Cohort 2
Cohort 3
Cohort 4
Cohort 5
Cohort 6
Cohort 7
Cohort 8
Cohort 9
Cohort 10 
800 men
480 men
480 men
480 men
480 men
480 men
480 men
480 men
480 men
480 men 
Cavarly 
120 
A grand total of 5,240 men not including all the officers.
Interesting Fact 
During the first three centuries of the empire the army contained between 25 and 34 legions.
Can you work out how many soldiers there were? 
Each century, cohort and legion had its own standard (flags and symbols on a long pole).
Find out more here
