This entry describes small units of mass, today typically used for weighing precious stones. For the measure of the fineness of gold, e.g., 18-karat gold or 18-carat gold, see karat.

For the German and Scandinavian units of mass, see karat.


The metric carat, 1907 – present, a unit of mass used for weighing precious stones, = 200 milligrams = 3.086 troy grains1. Adopted by the 4th CGPM in 1907, for “diamonds, fine pearls, and precious stones.” Also called the international diamond carat.

For diamonds, the carat is subdivided into 4 carat grains (which are not troy grains), and sometimes into 100 points. A 2¼ carat diamond would be a diamond of 250 points. Another unit, used only within the trade, is grainer; 1 grainer = 50 milligrams.

In the United States, use of the metric carat began on July 1, 1913; prior to that the value 205.3 milligrams was used. According to the current national standard in the United States2, the carat is not to be used at all; milligrams should be used instead. This prohibition is a good example of a collision between the SI purists on the one hand and existing law and trade practices on the other.

In some areas the general introduction of the metric system affected the introduction of the metric carat in interesting ways. In Germany, a law of 17 May 1856 abolished the carat, and the carat was not mentioned in later weights and measures legislation. But German law did not outlaw the use of particular names for units, only requiring that any unit be referable to an approved standard. Since the new carat = 200 milligrams, and the kilogram was an approved standard, the metric carat was legal.

In the United Kingdom, the Weights and Measures Act of 1878 specified that precious stones could only be sold by troy ounces, that is, carats were not legal in trade.  The Weights and Measures (Metric System) Act of 1897, however, authorized metric units of mass, and by extension the metric carat. It was specifically authorized by an Order in Council of 14 Oct. 1913. The legality of the metric carat for trade in precious stones and pearls was reaffirmed by the Weights and Measures of Act of 1963.

Nation Adoption of metric carat Prior value, mostly 19th century,
Austro-Hungary Vienna diamond carat, 206.1
Belgium 10 March 1913
compulsory by royal decree of 31 Oct. 1913
17 Oct. 1890: the Assn. of Diamond Cutters of Antwerp made the carat 205.183 (1 kg = 4875 carats);
29 April 1895: Chamber of Commerce of Antwerp makes carat 205.3
Bulgaria 10 April 1910
Denmark 1 April 1910; effective immediately
France 22 June 1909;
effective 1 Jan. 1912
Germany Kölnische diamant-karat = 205.5
Holland 7 April 1911 205.1
Italy 7 Jan. 1910;
effective 1 Jan. 1912
Bologna carat, 188.6
Florence carat, 196.5
Turin carat, 213.5
Venice carat, 207.1
Japan 11 Nov. 1909
Norway 27 May 1910,
promulgated 17 June
Persia 209.5
Portugal decree of 19 April 1911
(the quilate metrico)
199.1? & Brazil?
Romania Royal Decree 8 March 1910
Russia 205.1
Spain Royal Order 11 March 1908 199.9
Sweden 10 June 1910, effective 24 June 1909
Switzerland 24 June 1909
United Kingdom Order in Council 14 Oct. 1913 according to Kelly (1835), Diamond carat, 205¼ (1 oz troy = 151½ carats);
prior to 1888, 205.409;
1888 to 1913, Board of Trade carat, 205.304
United States (for imports of stones)
ordered 17 June 1913;
effective 1 July 1913

1. G. F. Kunz.
The new international metric carat of 200 milligrams.
Transactions of the American Institute of Mining Engineers, (New York Meeting, February 1913).

Pages 1225 – 1245.

G. F. Kunz.
The New International Diamond Carat of 200 Milligrams.
Transactions of the American Institute of Mining Engineers, vol. 47, 1914. (Butte Meeting, August 1913).

Pages 748 – 769.

Circular No. 43, Bureau of Standards (Nov. 1, 1913).

2. IEEE/ASTM SI 10™-2002.
American National Standard for Use of the International System of Units (SI): The Modern Metric System.
New York: IEEE, 30 December 2002.

See Section 3.3.3.



“On and after July 1, 1913, the unit of weight for imported diamonds, pearls and other precious stones will be the metric carat of 200 milligrams.”

U. S. Treasury Dept. order, June 17, 1913.


Last year two countries, one probably the foremost in exporting diamonds and the other equally prominent in importing them put diamonds and all other gems under the international carat standard. This was a great step forward in the interest of the diamond trade. Until recently there were some 40 different standards in use in diamond centers all the world over, from a weight of 188.5 milligrams in Bologna, Italy, to a weight of 254.6 milligrams in Arabia. There were three different standards in use in the United States, although the generally accepted one was 205.3 milligrams. Besides, the carat was divided into halves, fourths, eighths, sixteenths, and so on to sixty-fourths.

This multiplicity of standards gave rise to much confusion, which finally became intolerable, and one country after another deoided to adopt for its use the new international carat weight of exactly 200 milligrams.

Spain had been the first country to take this step officially and was soon followed by Italy, Bulgaria, Denmark, Norway, Japan, Portugal, Roumania, Switzerland, Sweden, France, Germany, and the Netherlands. To these must now be added also Belgium, where the new carat weight of 200 milligrams went into effect on March 10, 1913, and the United States, where it is in use since July 1, 1913.

Thus practically every important country has fallen into line and adopted the carat as a definite weight, except England. As there still remained the unscientific, cumbersome, and antiquated fractional division of the carat into 64 parts, Albert I, King of Belgium, by royal decree on October 31, 1913, made it compulsory in Belgium for all calculations of values in the diamond trade after July 1, 1914, to be made in decimals. [Circular No. 43 of the Bureau of Standards, United States Department of Commerce, contains tables of the relations between the carat weight in former use in the United States and the new international metric carat, together with other information of interest.]

Dept. of Commerce, Bureau of Foreign and Domestic Commerce.
"Antwerp Diamond Trade" in
Daily Consular and Trade Reports.
Report 73, March 28, 1914. Page 1171.
Washington: U.S.G.P.O., 1914.


The definite legalization of the metric carat in England was brought about by an Order in Council of Oct. 14, 1913, in part as follows: “Now, therefore, His Majesty, by virtue of the power vested in him by the said Acts, by and with the advice of the Privy Council is pleased to approve of the new denominations of standard weights specified in the schedule hereto, and doth dfrect that the same, on and after the 18t day of April, 1914, shall be Board of Trade standards in like manner as they were mentioned in the Second Schedule of the Weights and Measures Act, 1878.”

The U. S. Bureau of Standards issued on Nov. 1, 1913, a special circular (No. 43) devoted to the metric carat. The circular gives a number of tables showing the equivalents of fractions of the old carat (2.053 mg.) in decimals of the metric carat of 200 mg., and vice versa. A section is devoted to the details of weighing and weights, offering some valuable suggestions in relation to the treatment of balance and scales and the proper conformation and handling of the new carat weights.

G. F. Kunz.
The new international metric carat of 200 milligrams.
Transactions of the American Institute of Mining Engineers, vol 47.
New York: the Institute 1914.
Page 769.


In 1877, principal merchants in the gem trade in London, Paris, and Amsterdam met and agreed on a standard value of 204.9624 milligrams.

U. S. National Bureau of Standards.
Jeweler's and Silversmith's Weights and Measures (2nd ed.) Circular 43.
Washington, DC: U.S.G.P.O., 1921.

United States National Bureau of Standards.
Miscellaneous Publication 233.
Washington, DC: U.S.G.P.O, 1960.

Page 6, footnote 10.


In Indonesia, ? – 20th1 century, a unit of mass used for diamonds, approximately 205 milligrams.

1. United Nations, 1966.


In Iran A map showing the location of Iran., ? – 20th1 century, a unit of mass, approximately 193.3 milligrams.

1. United Nations, 1966.

Ancient and medieval values of the carat


The Greek (κεράτιονkeration) and Roman (siliqua) carat, a unit of mass, about 189 milligrams, originally the weight of a seed of the carob tree (Ceratonia siliqua, also known as St. John's Bread), from whose pods the familiar chocolate substitute is made. In classical times 1 siliqua = 3 barley corns = 4 wheat grains. (Twelve hundred years later, Johnson's Dictionary (1755) defined a carat as four grains.) In modern times, conventionally taken as about 189 milligrams, based on the weight of the solidus, though later scholars place it at 187 milligrams. 

photo of a solidus

Solidus of Constantine II, circa 326. Courtesy CNG Coins (

From the early 4th through the 10th century, a gold coin, the solidus, played a preeminent role in Roman and Byzantine coinage. It weighed 24 siliqua, and from this the carat became identified with the fraction 1/24.

E. Babelon.
Traité des monnaies grecgues et romaines. Ière Partie. Théorie et doctrine.
Paris: Ernest Leroux, 1901.

Cols. 574-575. Includes references to earlier works.


Lindsay A. Turnbull, Luis Santamaria, Toni Martorell, John Rall and Andy Hector.
Seed size variability: from carob to carats.
Biology Letters vol. 2, pages 397-400 (2006) doi:101098/rsbl.2006.0476

This study of seed weights differs from earlier ones in applying modern statistical methods. Their most original contribution may be that they studied human subjects’ ability to reject outlier seeds by visual observation: “Our study shows that people are remarkably good at selecting seeds by eye, and can discriminate differences in weight of around 5%. In this instance, human selection seems, unusually, to have been stabilizing rather than directional leading to a distribution of carat weights around the mean weight of carob seeds rather than at the higher end of the distribution.”


The Egyptian carat, about 196 milligrams.

G. C. Miles.
Early Arabic glass weights and stamps.
Numismatic Notes and Monographs, no. 111.
New York: American Numismatic Society, 1948.


Syrian and Arabian carats, about 212 milligrams (On the authority of Phillip Grierson, we treat the Syrian and Arabian carats as essentially the same. The following discussion is mainly due to Grierson.)

This unit originated in the reform of the coinage by the Caliph 'Abd al-Malik (reigned 685 – 705), who chose to make the weight of the new dinar the weight of the mitkal, a well-known, pre-Islamic Arabic weight probably based on the weight of the Greek drachme, as represented by coins. As part of this reform, the carat was defined as 1/20th of a mitkal.

The increased mass meant that the unit no longer reflected the mass of ceratonia seeds, and required a redefinition of its relation to wheat and barley seeds: 1 carat = 4 barley corns = 5 wheat grains.

Philip Grierson.
The Monetary Reforms of 'Abd al-Malik: Their Metrological Basis and Their Financial Repercussions.
Journal of the Economic and Social History of the Orient. Vol. 3, no. 3 (Oct. 1960.) pages 241-264.

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