Carat refers to the weight of the diamond and not its physical dimensions. The dimensions of a diamond are stated under Measurements on a diamond’s report. For round diamonds, there are two numbers which refer to the diamond’s longest and shortest diameters. If the difference between the two diameters is not more than 0.10 mm, the roundness of the diamond is considered feasible. If the difference only varies by not more than 0.06 mm, the round diamond is considered a perfect circle.
This significant diamond has a perfect round circle with the longest diameter being measured at 15.58 mm and the shortest diameter at 15.57 mm.
The diameters of two diamonds can be different though they bare the same weight. For example, a 2 carats round diamond that is ideally cut will measure up from 8.0 mm and above while a compromised 2 carats diamond may have a smaller diameter of 7.9 mm and below.