Convert carats to grams

Multiply the number of carats by 0.2 and you have the weight in grams. A 2 ct stone weighs 0.4 grams.

Formula: grams = carats × 0.2

The multiplier is exact to the last digit, because the metric carat is fixed in metric terms: the Federal Trade Commission states that a carat is equivalent to 200 milligrams, one fifth of a gram[1].

Three weights show how the arithmetic works on stones you would see in a jeweller’s window. A quarter carat, 0.25 ct, comes to 0.05 grams, that is fifty milligrams. Half a carat gives 0.1 grams. A one-carat stone weighs 0.2 grams.

At those weights the gram figure starts with a zero and a point, which is why the trade describes stones in points instead. One hundred points make up a carat[1], so 0.25 ct is a 25-point stone and 0.7 ct is a 70-point stone.

Going the other way you divide by 0.2. The grams to carats converter does that for you.

The metric carat is a unit of mass for gemstones and pearls. One carat weighs 200 milligrams, a fifth of a gram, and its symbol is ct.

The gram is a thousandth of a kilogram and the unit everything else is weighed in: kitchen ingredients, a parcel, scrap gold at a buyer’s counter. A cut gemstone is light enough that grams describe it with zeros after the point, and that is where the separate unit comes from.

In the United States the metric carat became law in 1913. The National Institute of Standards and Technology records that the international metric carat was then defined as 200 milligrams, which ended the use of a number of unofficial and semi-official carats[2], and its tables still list the metric carat of 200 milligrams as the unit used for the sale of precious stones[2].

The value itself was agreed six years earlier, in October 1907. The minutes of the General Conference on Weights and Measures record that the carat then stood close to 200 milligrams but differed from country to country, and nowhere bore a simple relation to metric units. The delegates voted unanimously to recommend the metric carat of 200 mg to their governments[3].

On gold the word describes the alloy. One karat is one twenty-fourth of the metal by weight, so 24 karat gold is pure and 14K gold is fourteen parts gold in twenty-four. United States federal rules keep the two words apart, with karat for purity and carat for gemstone weight[1], so a search for carats in gold mixes two different questions.

The Federal Trade Commission treats the word Gold on its own as a claim of fine, 24 karat metal, and requires the karat designation to appear immediately before the word for any alloy[1]. That is the 14K or 18K stamp inside a ring.

British hallmarks work in thousandths instead. The Hallmarking Act 1973 lists six standards of fineness for gold — 375, 585, 750, 916.6, 990 and 999 — and the mark itself is a figure, 916 in the case of 916.6, not a carat number[7]. A 9 carat ring made in Birmingham therefore carries 375, and the same metal sold in the United States would be stamped 9K.

To turn fineness into karats, divide it by 1,000 and multiply by 24: 750 gives 18 karats. The weight of the piece is a separate measurement in grams, and if you want to compare it with the metal price, the grams to ounces converter handles troy ounces once you pick Troy (oz t) under Ounce type.