Diamonds are sold by Carat weight.

Diamonds, like other gemstones are sold by the carat (ct.) Carats are a unit of weight.
One carat = 200 mg. or 1/5 of a gram.
One point (pt.) = 1/100 of a carat
(example 50 pt. = 1/5 of a carat) (ct.) Carats are a unit of weight.
One carat = 200 mg. or 1/5 of a gram.
One point (pt.) = 1/100 of a carat
(example 50 pt. = 1/5 of a carat)

Actual size in millimeters. Estimated carat weight compared to mm diameter of radiant cut diamond.

CARAT WEIGHT=HEIGHT X DIAMETER X 0.0061