The interior is a perfect circle whose diameter and height are exactly the same, 43m.
The interior design of the hemispherical dome was quoted by Michelangelo in the new Sacristy of San Lorenzo (Firenze, Italy).
The second level consists of a row of blind windows alternating with square designs, which is the re-design in 1747.
The original design of the second level is seen in Pantheon[No.1]