Of the four C's, carat weight is the simplest factor to determine the value of a diamond. As this is the measurement tool we use with all precious gemstones, the weight of a diamond or a gemstone is expressed in carats. Mathematically one carat equals one-fifth of a gram or 1/142 of an ounce. One carat is also described in a point system where 100 points totals one carat. A diamond weighing carat may be described as weighing 75 points or .75 carats.

Diamonds are precisely weighed before they are set into a ring or a piece of jewelry. As diamonds increase in size, their price per carat usually increases exponentially. As an example a two-carat diamond will be more than double the price of a one-carat diamond of the same quality.