Convert grams to carats

Multiply the weight in grams by 5 and you have the weight in carats. A stone of 0.6 grams weighs 3 carats.

Formula: carats = grams × 5

The multiplier is a round number because the metric carat is 200 milligrams, exactly one fifth of a gram[1]. Dividing by 0.2 gives the same answer.

In your head the shortcut is to halve the figure and move the decimal point one place to the right. A weight of 0.25 grams is 1.25 carats. Half a gram gives 2.5 carats. One gram is exactly 5 carats, and ten grams come to 50 carats.

The same multiplier works across the whole range, from a single stone to a parcel of goods. A hundred grams of stones is 500 carats and a kilogram is 5,000 carats.

Going the other way you multiply by 0.2, and the carats to grams converter handles it.

The gram is a thousandth of a kilogram and the unit behind everyday scales: kitchen, postal, laboratory. The same unit describes scrap gold at a buyer’s counter and a parcel at the post office.

The metric carat is five times smaller than a gram and covers gemstones and pearls. Its symbol is ct and its value, 200 milligrams, is the one the Federal Trade Commission uses when it defines the weight of a diamond[1].

A ring is described in two units because of scale. The ring weighs several grams with its setting, while the stone set into it weighs hundredths or tenths of a gram. One figure describes the piece and the other the stone, which is why a description of a ring can give both.

The carat received its shared value in October 1907, when the General Conference on Weights and Measures recommended the metric carat of 200 mg to governments and adopted the wording unanimously[2]. The United States wrote it into law in 1913, and federal tables still name the metric carat as the unit for the sale of precious stones[3].

The gram belongs to the metric system and describes the mass of anything. The carat serves one group of goods, gemstones and pearls, and its value of 200 milligrams was recommended to governments in 1907. That is how one stone comes to carry two figures, with a multiplication by 5 between them.

Stone weights are quoted to the point, a hundredth of a carat. That amounts to 0.002 grams. To round a weight correctly it is measured more finely, to the thousandth of a carat[5], which is 0.0002 grams, five times smaller than the step of a scale reading to 0.001 grams. Federal rules add a tolerance in the same spirit: a weight given to one decimal place is read as accurate to the second, so “.5 carat” stands for anything between .495 and .504[1].

Pearls keep their own habits. The CIBJO standard allows pearl grains, liang, carats and methgals for natural pearls, and momme, kan, liang, carats and grams for cultured ones[4]. A seller uses the unit of the market, and the buyer converts it to the one they know.