How to Calculate the Carat Weight of Gems

In gemology, the weight of precious stones are measured in units called carats. Carat may be subdivided into units called points, where a point is 1/100 of a carat. Jewelers and gemologists use the abbreviation ct for carats.

As a weight, the carat is a metric unit equal to 200 milligrams or equivalently 1/5 of a gram. A point is 2 milligrams. If you need to convert between ounces and carats, use the relations 1 oz = 141.74761562 ct, and 0.00705479239 oz = 1 ct.

NOTE:
The gemological definition of carat should not be confused with the metallurgical definition of carat. Jewelers rate the purity of gold, silver, and platinum with the carat (or karat) scale. The scale ranges from 1 to 24, with 24k being the purest and 1k being the least pure.

How to Weigh Gemstones to Find the Carat

You can use a finely calibrated digital scale to weigh a single precious stone or pearl, or many gems in bulk. Milligram scales can detect the weight of very light objects such as gemstones and will display the weight in milligrams. To convert this number to carats, divide it by 200.

For example, a large emerald taken out of its setting may weigh 125 mg. This is equivalent to 125/200 ct = 0.625 ct. In points, this is equal to 62.5 points.

You can use the calculator above to convert weights from milligrams, grams, or ounces to carats and points.

According to the Gemological Institute of America, the carat weight of a precious stone is one of the "Four C's" in gem appraisal. The other three factors that determine a gemstone's value are cut, clarity, and color. In diamonds, the cut and clarity of a gem are just as important as its size. Colored diamonds are much rarer than white diamonds; a yellow or pink diamond costs much more than a similarly sized white diamond.

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