Diamonds are measured and valued by their carat weight. The word carat comes from the time when diamonds were weighed against carob seeds back in the 12th century. The exact weight for a carat is 0.20 grams. While carat weight has the greatest influence on the size of a diamond, the actual size for any given carat weight can vary greatly depending on the diamond cut. Furthermore, an increase in carat weight won't increase the diamond face size by the same about. For example, as shown in the carat-mm size table, the increase from 0.50 carat to 1.00 carat will be just 25%.

Likewise, the size of a 1.00 carat round diamond wont be the same as a princess cut ( usually smaller) or a marquise ( usually larger). When choosing a diamond it's important to check the measurements on the diamond certificate. In some cases, going lower on carat weight - and thus saving money - can in fact get you a similar size or even larger looking diamond.