How big is a one-carat diamond?

The weight of a gemstone is measured by carats. The definition of a carat has changed over time, but the international standard for the weight of 1 carat is 200 milligrams, according to Zales Diamond Store. The diameter of a 1-carat diamond is about 6.5 millimeters.

Jewelers often refer to carats in one-quarter increments. Diamonds vary in size and can be measured as several carats or as low as a quarter of a carat, Zales states. When in reference to jewelry that weighs more than one carat, the carats may be measured in total carat weight, abbreviated as TW. Total carat weight is the total weight of all of the stones in a piece of jewelry combined.