The weight of a diamond is measured in carats, not to be confused with karat which describes purity of gold.
Carats are a metric form of measurement and are similar to other measurements broken down into one hundred “points” for every one-carat.
Although carat weight does define various size ranges, the same carat weight diamonds can appear smaller or larger depending on the cut of the diamond, and whether or not the diamonds dimensions are cut proportionally to its weight.
Values of diamonds are assigned to certain weight ranges and the slightest of differences in weight can make a considerable difference. Precision instruments are used to measure diamond weight and weight is generally described to the hundredth of a carat in decimals. For example a 1.53-carat diamond would be described as “one point five three” or “one fifty three”
(above illustration is not to scale)