The building measures 105 meters (344 feet) long, 43.5 meters wide (143 ft) and with a vault soaring 33.5 meters (109 feet) high. The cupola on top of the church reaches a height of about 58m (190ft). The church follows a ground plan similar to that of the Notre-Dame Cathedral with a nave of five bays and a choir aisle with 24 chapels.
The style of St. Eustache is Gothic, not unlike Notre Dame. However, during its long construction time the Gothic style had gone out of fashion and renaissance was the new style of choice. Hence the church does boast a great deal of Renaissance detail, especially the decorations and the arches.