Carats to Milligrams (ct to mg)

1 carat equals 200 milligrams exactly.

1 carat = 200 milligrams

How to convert carats to milligrams

To convert carats to milligrams, multiply the number of carats by 200. This factor is exact. To go the other way, multiply milligrams by 0.005.

milligrams = carats × 200

Example: 10 carats = 2,000 milligrams.

About these units

Carat (ct)

Metric (SI)Current unit

Exactly 200 milligrams. The unit for weighing gemstones such as diamonds. Not to be confused with the karat, a measure of gold purity.

Jewellers once weighed stones against seeds. The word carat traces back to the Greek keration, meaning a carob seed, and reached English through Italian and Arabic. Carob seeds were used as balance weights for gems and gold, though their mass varies about as much as any seed does. Before 1907 each country had its own carat, ranging from roughly 187 to 216 milligrams. It was used for gold early on and for diamonds from the 1570s. An international carat of 205 mg was proposed in Paris in 1871, but the 200 mg metric carat won out.

The metric carat was adopted at the Fourth General Conference on Weights and Measures in 1907. France made it mandatory, the United States followed in 1913 and the United Kingdom in 1914. Today it is exactly 200 mg, so five carats make a gram, and it is divided into 100 points of 2 mg each; a diamond described as 25 points is a quarter carat. Carat is used for gemstones and pearls, and it should not be mixed up with karat, which measures the purity of gold and is a separate idea.

1 carat = 0.0002 kilograms (exactly).

Sources: Wikipedia: Carat (mass); NIST Guide to the SI, Appendix B.8 (mass conversions)

Milligram (mg)

Metric (SI)Current unit

One thousandth of a gram. Used for medicine doses and vitamins.

No single inventor can be credited with the milligram. It arrived with the metric system that France introduced in the 1790s, where the gram was the basic unit of mass and thousandths of it were named with a prefix. When the International System of Units was later built around the kilogram, the rule against stacked prefixes meant that a millionth of a kilogram is written 1 mg, never a microkilogram, so the milligram remained a gram-based unit even though the gram itself is not the SI base.

A milligram is small enough that doses of medicine and vitamins are measured in it routinely. A US penny weighs about 2.5 g, so a milligram is roughly one 2,500th of a coin. The unit is also at the center of a well-known safety problem: milligram and microgram differ by a factor of 1,000, and the abbreviation mcg for micrograms is used on prescriptions precisely so that a handwritten micro sign cannot be misread as milli. Confusing the two has caused serious medication errors.

1 milligram = 0.000001 kilograms (exactly).

Sources: Wikipedia: Milligram; Wikipedia: Microgram; Wikipedia: Gram

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