According to the United States Census Bureau, Fort Devens has a total area of 17.7 km² (6.8 mi²). 17.4 km² (6.7 mi²) of it is land and 0.2 km² (0.1 mi²) of it is water. The total area is 1.32% water.

There are 3 households out of which 33.3% have children under the age of 18 living with them, 33.3% are married couples living together, 0.0% have a female householder with no husband present, and 66.7% are non-families. 66.7% of all households are made up of individuals and 66.7% have someone living alone who is 65 years of age or older. The average household size is 2.00 and the average family size is 4.00.

In Fort Devens the population is spread out with 9.6% under the age of 18, 21.6% from 18 to 24, 51.1% from 25 to 44, 15.9% from 45 to 64, and 1.7% who are 65 years of age or older. The median age is 32 years. For every 100 females there are 850.5 males. For every 100 females age 18 and over, there are 1,251.5 males.

The median income for a household in Fort Devens is $0, and the median income for a family is $0. Males have a median income of $19,423 versus $0 for females. The per capita income for Fort Devens is $10,354. 78.6% of the population and 0.0% of families are below the poverty line. Out of the total people living in poverty, 0.0% are under the age of 18 and 0.0% are 65 or older.