Carats to grams:
multiply by 0.2

How many grams are in a carat?

One carat is exactly 0.2 grams, which is 200 milligrams. The metric carat is the unit used to weigh diamonds and other gemstones, and its value is fixed, so there is no rounding in the factor.

It also means that five carats make exactly one gram. That makes the conversion simple enough to do in your head: carats divided by five gives grams.

Because the carat is defined as a fixed 200 milligrams, a stone's weight tells you its mass but not its price. Colour, clarity and cut all matter just as much. For everyday work the conversion is useful in two places: when a seller quotes a piece in grams and a buyer thinks in carats, and when you compare the weight of a stone with the metal that holds it. A ring with a 1 ct diamond carries 0.2 g of stone, a small share of the total weight of the ring.

The carat to gram formula

A larger stone works the same way. A 3.2 ct stone weighs 3.2 × 0.2 = 0.64 g, and a 12.5 g piece of rough is 12.5 × 5 = 62.5 ct.

1. 1Write the stone's weight in carats.
2. 2Multiply by 0.2, or divide by 5.
3. 3For milligrams, multiply the carats by 200 instead.
4. 4Check that the answer in grams is one fifth of the carat number.

Chart: carats to grams

Jewellers also split a carat into 100 points, so a 25 point stone is 0.25 ct, which is 0.05 g.

Where carat to gram conversion is used

• Diamond certificates, which state the weight to two decimals of a carat.
• Pearl and gemstone dealers who sell by the gram but price by the carat.
• Insurance and valuation papers, which may list a weight in grams for the whole piece.
• Estimating how much a ring or pendant adds to a bag or parcel.

These weigh almost nothing. A 1 carat diamond is only 0.2 g, which is lighter than a paper clip, so jewellery scales are accurate to hundredths of a carat.

Quick sanity checks for carats to grams

Small stones are easy to check with the five-to-one rule. A 0.5 ct stone is 0.1 g, a 2 ct stone is 0.4 g and a 100 ct parcel is 20 g.

Because the carat is so small, gem weights are quoted to two decimals. When two sellers quote a weight, ask for it in carats to two decimals so you compare like with like.

The factor is a definition, so it is the same for diamonds, rubies and pearls.

Common mistakes when converting carats to grams

• Mixing up carat and karat. Carat (ct) measures gem weight, while karat (K) measures the gold purity out of 24, so 18K gold has nothing to do with 18 ct of weight.
• Multiplying by 5 instead of 0.2, which makes the answer 25 times too big.
• Confusing carat weight with size. A heavier stone is not always larger across, because shape and cut change how a stone looks.
• Reading a point as a carat. A 50 point diamond is 0.50 ct, not 50 ct.