Things You'll Need
Instructions
Obtain a jewelry or diamond weight scale. You will need a scale that is very sensitive, and can register milligrams.
Remove the diamond from the jewelry 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 equaling one carat.