Carats to milligrams:
multiply by 200

How many milligrams are in a carat?

One carat is exactly 200 milligrams. The metric carat, the unit used to weigh diamonds and other gemstones, is defined as 200 mg, which is the same as 0.2 grams. Five carats therefore make one gram.

The figure is fixed by definition, so jewellers anywhere can use 200 without rounding. A half-carat stone weighs 100 mg and a quarter-carat stone weighs 50 mg.

The carat to mg formula

Another quick check: a 0.75 carat stone is 150 mg. Three quarters of 200 is 150, so the answer agrees.

1. 1Write the weight in carats as a decimal.
2. 2Multiply by 200 to get milligrams.
3. 3Divide by 1,000 if you want grams instead.
4. 4Check by halving or doubling: 2 ct must be 400 mg, so 1.5 ct should sit between 200 and 400.

Chart: carats to milligrams

Because the factor is a clean 200, the whole table is easy to extend.

The 5 ct row shows 1000 mg, confirming that five carats make exactly one gram.

Where carats and milligrams meet

Gem dealers quote stone weights in carats, usually to two decimals, while lab scales and customs paperwork often read in grams or milligrams. A parcel of small melee diamonds weighing 2.35 g on the scale holds 2.35 ÷ 0.2 = 11.75 carats.

The carat is also divided into 100 points, so a 0.05 ct stone is called a five-pointer. In milligrams that stone weighs 10 mg, because each point is 2 mg.

A trick for checking any result is to remember that five carats equal one gram. If a stone weighs 12 carats, divide by five and you expect about 2.4 g, and the exact answer of 2,400 mg confirms it. This works well when you are scanning a long list of stone weights.

Pearls and other gems are sometimes sold by the gram rather than the carat, so dealers move between the two all the time. A strand weighing 18 g holds 90 carats of material. Writing both numbers on the invoice avoids arguments over price per unit.

When you compare two stones, convert both to the same unit before judging the difference. A 0.90 ct stone weighs 180 mg and a 1.00 ct stone weighs 200 mg, so the gap is only 20 mg, which is why prices often jump sharply at round carat weights rather than in line with the small difference in mass.

Common mistakes when converting carats to milligrams

• Confusing carat with karat. Carat (ct) measures the weight of gemstones, while karat (K) measures the purity of gold, so 18K gold has nothing to do with 200 mg.
• Dividing by 200 when converting carats to mg. Milligrams are smaller, so the number must grow.
• Mixing carats with ounces or grams without converting first, which gives a figure that is off by a large factor.
• Rounding the carat weight too early. A stone shown as 1.50 ct may be 1.496 ct on the scale, and the difference is about 0.8 mg.