When people talk about the size of diamonds, they speak in terms of carats. A diamond's size is measured by its carat weight. A carat equals 200 milligrams. This would mean a two-carat diamond weighs 400 milligrams. Two diamonds may be the same size, yet their diameters can differ. Identically sized diamonds simply mean they are of the same weight, yet don't necessarily share the same dimensions.
Obtain a jewellery or diamond weight scale. You will need a scale that is very sensitive, and can register milligrams.
Remove the diamond from the jewellery setting. You can't accurately measure a diamond while it is still in a setting.
Turn the scale on and set the diamond on the scale. Make note of the weight in milligrams.
Calculate the carat size of the diamond by its weight, with 200 milligrams equalling one carat.