1 carat (CT) is equivalent to 0.2 grams. In essence, divide a carat weight by 5 to get the weight in grams.
Naturally large diamonds are rare, so a single one-carat diamond will always be worth more than two half-carat diamonds of the same colour, clarity and cut.
Each carat contains 100 points. In that case, a 0.5 carat stone has 50 points, and a 0.33 carat (third of a carat) stone contains 33 points.
"Carat" is a measurement of "weight", not "size" thus the overall diameter (mm) of a diamond could vary slightly - larger or smaller - even though the carat weight is correct.