If you have a diamond-studded family heirloom or are looking for the perfect engagement ring, knowing the carat size will help you determine the value of the diamond. Verify the size of the stone independently to avoid an overzealous jewelry dealer taking advantage of you. Find a jeweler who has a policy that allows you to purchase the diamond, get the size verified on your own, and return the diamond if the carat size given is incorrect. You can measure it yourself, but it is best to go to an appraiser.
Use a tape measure. This is the least accurate way to determine measurement, but it will give you a general idea of your diamond carat weight. Find a tape measure that measures in millimeters. Set the diamond face up on a clean flat surface. You are measuring the diameter of the diamond at the widest point. This will be different depending upon the shape of your diamond, but generally it is the girdle area of the diamond. The girdle area is the widest point around the diamond, and typically separates the crown, or face, of the diamond that is visual in a jewelry setting from the part of the diamond that is not visible in a jewelry setting. Lay the beginning of the tape measure next to the diamond on one edge and stretch the tape measure tightly past the other end of the diamond. Measure to the very last edge of of the diamond. When you have a number, you can convert the millimeter measurement into carats using a conversion chart or online conversion tool. For example, a 4.1 mm diamond is .25 carats.
Print out a carat size comparison chart. This is less accurate than a tape measure, but will give you a general idea your diamond's carat size. You will find a size comparison charts on many diamond jeweler's websites. Look for one that has chart sizes for the different shapes, such as the round brilliant cut and the square princess cut. Once you have printed out the chart, place a loose diamond directly on top of the visuals until you find the closest match. It will tell you the size of your diamond by millimeters and carat weight. If your diamond is in ring form this will be harder to do, so you will have to eyeball it more or less.
Take your diamond to a diamond appraiser. This is the best way to know the exact size of your diamond. Diamond appraisers have been trained to measure diamonds and have special industry tools to give you the absolute, most accurate carat size. It is best to go to an independent appraiser who has no incentive for exaggerating about the size of your diamond, unlike a jewelry store. Go to an appraiser who has been verified by the Better Business Bureau and has a graduate gemologist diploma from a reputable gemology education institution, such as the Gemologist Institute of America.
Check the Gemological Institute of America's website for a list of reputable jewelry appraisers.