Carat weight is the weight of the diamond measured in carats. A diamond weight is simplistic of its characteristics to measure, and from the earliest times it was used to calculate a diamond's value.
Five carats are equal to one gram, each carat is divided into 100 points. A diamond that weighs 25 points is ¼ of a carat. When you talk about carat weight (ct) you're talking about diamond size.