The Gemological Institute of America, GIA, one of the most respected of the diamond grading laboratories, lists the carat weight to the hundredth of a carat, as do many of the other common grading labs. This follows the regulations set by the FTC, Federal Trade Commission, in the area of precious gems. At the next most respected diamond-grading laboratory, AGS, the American Gemological Society, the carat weight is listed to the thousandth of a carat. They follow the rules for precious gem weights as laid out by the International Committee on Weights and Measures.

All laboratories that are respected, objective and independent will weigh only unset stones. A stone's weight can be calculated from its dimensions once it has been set, but this is a method that will not be used by a reputable laboratory as it is not accurate. A diamond can also be removed from its setting in order to be weighed, but this will add the potential of damage to the diamond during the removal and remounting procedure. It is best to make certain that a stone has a certificate prior to its being set, as the certificate will allow you to see at any time what the precise carat weight is, along with all of its other measurable dimensions.