"The Pentagon is virtually a city in itself. Approximately 23,000 employees, both military and civilian, contribute to the planning and execution of the defense of our country. These people arrive daily from Washington, D.C. and its suburbs over approximately 30 miles of access highways, including express bus lanes and one of the newest subway systems in our country. They ride past 200 acres of lawn to park approximately 8,770 cars in 16 parking lots; climb 131 stairways or ride 19 escalators to reach offices that occupy 3,705,793 square feet. While in the building, they tell time by 4,200 clocks, drink from 691 water fountains, utilize 284 rest rooms, consume 4,500 cups of coffee, 1,700 pints of milk and 6,800 soft drinks prepared or served by a restaurant staff of 230 persons and dispensed in 1 dining room, 2 cafeterias, 6 snack bars, and an outdoor snack bar."
The fun thing about this is the shear detail of the description.
But for sure, it's a big building: "three times the floor space of the Empire State Building in New York" and when they built it the used 435,000 cubic yards of concrete (or 33,000 cubic metres which is 3 London double deckers piled in all directions)
More facts here
Another fascinating fact is that they built this monster of a building in 16 months!