The diameter of classical columns slightly decreases with height. Vitruvius calls it “entasis” and cites aesthetic reasons for its use already in Greek architecture. However, the exact geometrical shape of the entasis remained unexplained. It was subject to wide speculation even in later times. The large columns at the Porticus of the Pantheon in Rome are made with an entasis. This article describes the exact measurements of their proportion and a method to determine the geometry of the entasis. The Bern Digital Pantheon Model is used as the data source.