The three-story monumental granite building is 234 feet (71 m) long and 179 feet (55 m) deep.
It includes a basement, sub-basement and attic level, with 16-foot (4.9 m) ceilings at the basement levels and 10-foot (3.0 m) thick foundation walls, which are surrounded by a 25-foot (7.6 m) deep dry moat for light and ventilation.
The basement connects to a tunnel under 8th Street that was used for the delivery of mail to the post office.
The basement material is red Missouri granite, while the upper floors are gray granite from Hurricane Island, Maine, between 3 feet (0.91 m) and 4 feet (1.2 m) in thickness.
The building surrounds a skylit inner courtyard, 48 feet (15 m) by 55 feet (17 m).

High ceilings predominate in the main structure, with first floor ceilings at 26 feet (7.9 m) and second and third floors at 22 feet (6.7 m).
Interior structure is a mixture of wrought and cast iron, supporting arched brick floors in a system that was referred to at the time of construction as "fireproof."
The building's windows were provided with fireproof shutters.

The principal facade is the southern, along Olive Street, which features an iron mansard dome.
Each street elevation features a central pavilion which in turn bears a portico.
The Olive Street elevation's pediment is ornamented by the 1877 sculpture "America at War and America at Peace" by Daniel Chester French, his first major commission.
Double-hung windows are set in cast iron frames throughout the building.
 Cast iron trim and molding frames interior sides of windows and doors.
Interior detailing was extensive, with art glass panels, mosaic tile floors and bronze door knobs imprinted with the Seal of the United States.
Thirty offices on the second, third and fourth floors featured red Bologna marble fireplace mantels.
The three-story monumental granite building is 234 feet (71 m) long and 179 feet (55 m) deep. It includes a basement, sub-basement and attic level, with 16-foot (4.9 m) ceilings at the basement levels and 10-foot (3.0 m) thick foundation walls, which are surrounded by a 25-foot (7.6 m) deep dry moat for light and ventilation. The basement connects to a tunnel under 8th Street that was used for the delivery of mail to the post office. The basement material is red Missouri granite, while the upper floors are gray granite from Hurricane Island, Maine, between 3 feet (0.91 m) and 4 feet (1.2 m) in thickness. The building surrounds a skylit inner courtyard, 48 feet (15 m) by 55 feet (17 m).
High ceilings predominate in the main structure, with first floor ceilings at 26 feet (7.9 m) and second and third floors at 22 feet (6.7 m). Interior structure is a mixture of wrought and cast iron, supporting arched brick floors in a system that was referred to at the time of construction as "fireproof." The building's windows were provided with fireproof shutters.
The principal facade is the southern, along Olive Street, which features an iron mansard dome. Each street elevation features a central pavilion which in turn bears a portico. The Olive Street elevation's pediment is ornamented by the 1877 sculpture "America at War and America at Peace" by Daniel Chester French, his first major commission. Double-hung windows are set in cast iron frames throughout the building.  Cast iron trim and molding frames interior sides of windows and doors. Interior detailing was extensive, with art glass panels, mosaic tile floors and bronze door knobs imprinted with the Seal of the United States. Thirty offices on the second, third and fourth floors featured red Bologna marble fireplace mantels.
United States Customhouse and Post Office (St. Louis, Missouri)