Carat weight is simply the gross physical weight of a gemstone, with one carat equaling 200 milligrams, or approximately 1/142 of an ounce. Gemstones vary in density, however, so whereas carat weight of a diamond is the same as with other gemstones, the corresponding size of the stone is not. For example, a one-carat stone made out of denser material, such as a ruby, will be smaller in size than a one-carat diamond. However, measuring carat weight for all stones is the same.
Get an accurate scale. As most diamonds are less than one-carat, and with one carat only measuring 200 milligrams, you will be dealing with very tiny weights when you measure most diamonds. Considering that diamond value increases dramatically with the size of the stone, every milligram counts, and you want to be sure that you capture the exact weight of your stone.
Weigh your stone. Make sure the scale is clean and dry, as any extra material or weight could dramatically affect your estimation of the size of your stone. Place your diamond on the scale and read the results. Many jewelers have sophisticated scales that do the work for you and read out the exact carat weight of the weighed stones but many scales will simply give you the weight in milligrams or ounces. Record this amount so that you can convert it to a carat weight.
Do the math. As one carat equals 200 milligrams, the weight of your diamond in milligrams divided by 200 equals your carat weight. If your scale measures in ounces, you will need a different, and slightly more complicated, calculation. There are 141.7476 carats in one ounce, so multiply your weight in ounces by 141.74762 to determine the number of carats.
If your stone is particularly valuable or if you just want to double-check your work, you can always have a diamond weighed, graded and certified by the Gemological Institute of America (GIA).