Carat (mass)

Last updated
One carat brilliant.jpg
A one-carat brilliant.
General information
1 ct in ...... is equal to ...
    milligram    200
Conversions (imperial)
1 imp ct in ...... is equal to ...
    ounce    0.00705
Diamond-weighing kit, with weights labelled in grams and carats. Diamond Balance Scale 0.01 - 25 Carats Jewelers Measuring Tool.jpg
Diamond-weighing kit, with weights labelled in grams and carats.

The carat (ct) is a unit of mass equal to 200  mg (0.00705  oz ) or 0.00643 troy oz, and is used for measuring gemstones and pearls. [1] The current definition, sometimes known as the metric carat, was adopted in 1907 at the Fourth General Conference on Weights and Measures, [2] [3] and soon afterwards in many countries around the world. [lower-roman 1] The carat is divisible into 100 points of 2 mg. Other subdivisions, and slightly different mass values, have been used in the past in different locations.


In terms of diamonds, a paragon is a flawless stone of at least 100 carats (20 g). [4]

The ANSI X.12 EDI standard abbreviation for the carat is CD. [5]


First attested in English in the mid-15th century, the word carat comes from Italian carato, which comes from Arabic qīrāṭ قيراط , in turn borrowed from Greek kerátion κεράτιον 'carob seed', [6] [7] [8] a diminutive of keras 'horn'. [9] It was a unit of weight, equal to 1/1728 (1/123) of a pound (see Mina (unit)). [7] [10] [6]


Carob seeds have been used throughout history to measure jewelry, because it was believed that there was little variance in their mass distribution. [11] However, this was a factual inaccuracy, as their mass varies about as much as seeds of other species. [12]

In the past, each country had its own carat. It was often used for weighing gold. Beginning in the 1570s, it was used to measure weights of diamonds. [6]


An 'international carat' of 205 milligrams was proposed in 1871 by the Syndical Chamber of Jewellers, etc., in Paris, and accepted in 1877 by the Syndical Chamber of Diamond Merchants in Paris. A metric carat of 200 milligrams – exactly one-fifth of a gram – had often been suggested in various countries, [3] and was finally proposed by the International Committee of Weights and Measures, and unanimously accepted [3] at the fourth sexennial General Conference of the Metric Convention held in Paris in October 1907. It was soon made compulsory by law in France, but uptake of the new carat was slower in England, where its use was allowed by the Weights and Measures (Metric System) Act of 1897. [13]

Historical definitions

Carat before 1907 [14]
Cyprus 187
Brazil 192.2
Egypt 195
Ambonia 197
Florence 197.2
International carat
  Batavia, Borneo, Leipzig
South Africa (1)205.304
London-New York (1)205.303
Spain 205.393
London-New York (2)205.409
Berlin 205.44
Paris, East India 205.5
South Africa (2)205.649
Amsterdam 205.7
Lisbon 205.75
Frankfurt (on Main)205.77
Vienna 206.13
Venice 207
Madras 207.353
Bucharest 215
Livorno 215.99

UK Board of Trade

In the United Kingdom the original Board of Trade carat was exactly 3+16479691 grains (~3.170 grains = ~205 mg); [lower-roman 2] in 1888, the Board of Trade carat was changed to exactly 3+17101 grains (~3.168 grains = ~205 mg). [lower-roman 3] Despite its being a non-metric unit, a number of metric countries have used this unit for its limited range of application.

The Board of Trade carat was divisible into four diamond grains, [lower-roman 4] but measurements were typically made in multiples of +164 carat.

Refiners' carats

There were also two varieties of refiners' carats once used in the United Kingdom — the pound carat and the ounce carat. [lower-roman 5] The pound troy was divisible into 24 pound carats of 240 grains troy each; the pound carat was divisible into four pound grains of 60 grains troy each; and the pound grain was divisible into four pound quarters of 15 grains troy each. Likewise, the ounce troy was divisible into 24 ounce carats of 20 grains troy each; the ounce carat was divisible into four ounce grains of 5 grains troy each; and the ounce grain was divisible into four ounce quarters of 1+14 grains troy each. [15]


The solidus was also a Roman weight unit. There is literary evidence that the weight of 72 coins of the type called solidus was exactly 1 Roman pound, and that the weight of 1 solidus was 24  siliquae . The weight of a Roman pound is generally believed to have been 327.45 g or possibly up to 5 g less. Therefore, the metric equivalent of 1 siliqua was approximately 189 mg. The Greeks had a similar unit of the same value. [16]

Gold fineness in carats comes from carats and grains of gold in a solidus of coin. The conversion rates 1 solidus = 24 carats, 1 carat = 4 grains still stand. [17] Woolhouse's Measures, Weights and Moneys of all Nations [18] gives gold fineness in carats of 4 grains, and silver in troy pounds [18] of 12  troy ounces of 20  pennyweight each.[ clarification needed ]


  1. The United States adopted the metric carat definition on July 1, 1913, the United Kingdom on 1 April 1914.
  2. The pre-1888 Board of Trade carat, of which there were exactly 151+2764 per ounce troy, was approximately 205.4094 mg (3.169951 gr).
  3. The post-1887 Board of Trade carat, of which there were exactly 151+12; per ounce troy, was approximately 205.3035 mg (3.168317 gr).
  4. Unlike the modern carat, the Board of Trade carat was not used for measuring pearls; those were measured with pearl grains.
  5. The refiners’ carats were the offspring of the carat as a measure of fineness for gold.

Related Research Articles

The pound or pound-mass is a unit of mass used in British imperial and United States customary systems of measurement. Various definitions have been used; the most common today is the international avoirdupois pound, which is legally defined as exactly 0.45359237 kilograms, and which is divided into 16 avoirdupois ounces. The international standard symbol for the avoirdupois pound is lb; an alternative symbol is lbm, #, and or ″̶.

<span class="mw-page-title-main">United States customary units</span> System of units of measurement commonly used in the United States

United States customary units form a system of measurement units commonly used in the United States and U.S. territories since being standardized and adopted in 1832. The United States customary system developed from English units which were in use in the British Empire before the U.S. became an independent country. The United Kingdom's system of measures was overhauled in 1824 to create the imperial system, which was officially adopted in 1826, changing the definitions of some of its units. Consequently, while many U.S. units are essentially similar to their imperial counterparts, there are significant differences between the systems.

<span class="mw-page-title-main">Troy weight</span> System of units of mass

Troy weight is a system of units of mass that originated in 15th-century England, and is primarily used in the precious metals industry. The troy weight units are the grain, the pennyweight, the troy ounce, and the troy pound. The troy grain is equal to the grain unit of the avoirdupois system, but the troy ounce is heavier than the avoirdupois ounce, and the troy pound is lighter than the avoirdupois pound. One troy ounce equals exactly 31.1034768 grams.

<span class="mw-page-title-main">Grain (unit)</span> Unit of mass

A grain is a unit of measurement of mass, and in the troy weight, avoirdupois, and apothecaries' systems, equal to exactly 64.79891 milligrams. It is nominally based upon the mass of a single ideal seed of a cereal. From the Bronze Age into the Renaissance, the average masses of wheat and barley grains were part of the legal definitions of units of mass. Expressions such as "thirty-two grains of wheat, taken from the middle of the ear" appear to have been ritualistic formulas, essentially the premodern equivalent of legal boilerplate. Another source states that it was defined such that 252.458 units would balance 1 cubic inch (16 cm3) of distilled water at an ambient air-water pressure and temperature of 30 inches of mercury (100 kPa) and 62 °F (17 °C) respectively. Another book states that Captain Henry Kater, of the British Standards Commission, arrived at this value experimentally.

The ounce is any of several different units of mass, weight or volume and is derived almost unchanged from the uncia, an Ancient Roman unit of measurement.

<span class="mw-page-title-main">Avoirdupois system</span> System of weights based on a pound of 16 ounces

The avoirdupois system is a measurement system of weights that uses pounds and ounces as units. It was first commonly used in the 13th century AD and was updated in 1959.

<span class="mw-page-title-main">Pennyweight</span> Unit of mass

A pennyweight (dwt) is a unit of mass equal to 24 grains, 120 of a troy ounce, 1240 of a troy pound, approximately 0.054857 avoirdupois ounce and exactly 1.55517384 grams. It is abbreviated dwt, d standing for denarius –, and later used as the symbol of an old British penny.

<span class="mw-page-title-main">Solidus (coin)</span> Late Roman Empire gold coin

The solidus or nomisma was a highly pure gold coin issued in the Late Roman Empire and Byzantine Empire. Constantine introduced the coin, and its weight of about 4.5 grams remained relatively constant for seven centuries. In the Byzantine Empire, the solidus or nomisma remained a highly pure gold coin until the 11th century, when several Byzantine emperors began to strike the coin with less and less gold. The nomisma was finally abolished by Alexius I in 1092, who replaced it with the hyperpyron, which also came to be known as a "bezant". The Byzantine solidus also inspired the originally slightly less pure dinar issued by the Muslim Caliphate. In Western Europe, the solidus was the main gold coin of commerce from late Roman times to Pepin the Short's currency reform, which introduced the silver-based pound/shilling/penny system.

The dram is a unit of mass in the avoirdupois system, and both a unit of mass and a unit of volume in the apothecaries' system. It was originally both a coin and a weight in ancient Greece. The unit of volume is more correctly called a fluid dram, fluid drachm, fluidram or fluidrachm.

<span class="mw-page-title-main">Tola (unit)</span> Traditional Asian unit of mass

The tola also transliterated as tolah or tole, is a traditional Ancient Indian and South Asian unit of mass, now standardised as 180 grains or exactly 3/8 troy ounce. It was the base unit of mass in the British Indian system of weights and measures introduced in 1833, although it had been in use for much longer. It was also used in Aden and Zanzibar: in the latter, one tola was equivalent to 175.90 troy grains.

<span class="mw-page-title-main">Apothecaries' system</span> Historical system of mass and volume units used by physicians and apothecaries

The apothecaries' system, or apothecaries' weights and measures, is a historical system of mass and volume units that were used by physicians and apothecaries for medical prescriptions and also sometimes by scientists. The English version of the system is closely related to the English troy system of weights, the pound and grain being exactly the same in both. It divides a pound into 12 ounces, an ounce into 8 drachms, and a drachm into 3 scruples of 20 grains each. This exact form of the system was used in the United Kingdom; in some of its former colonies, it survived well into the 20th century. The apothecaries' system of measures is a similar system of volume units based on the fluid ounce. For a long time, medical recipes were written in Latin, often using special symbols to denote weights and measures.

A system of measurement is a collection of units of measurement and rules relating them to each other. Systems of measurement have historically been important, regulated and defined for the purposes of science and commerce. Systems of measurement in use include the International System of Units or SI, the British imperial system, and the United States customary system.

The fineness of a precious metal object represents the weight of fine metal therein, in proportion to the total weight which includes alloying base metals and any impurities. Alloy metals are added to increase hardness and durability of coins and jewelry, alter colors, decrease the cost per weight, or avoid the cost of high-purity refinement. For example, copper is added to the precious metal silver to make a more durable alloy for use in coins, housewares and jewelry. Coin silver, which was used for making silver coins in the past, contains 90% silver and 10% copper, by mass. Sterling silver contains 92.5% silver and 7.5% of other metals, usually copper, by mass.

English units are the units of measurement used in England up to 1826, which evolved as a combination of the Anglo-Saxon and Roman systems of units. Various standards have applied to English units at different times, in different places, and for different applications.

<span class="mw-page-title-main">History of measurement</span> Aspect of history

The earliest recorded systems of weights and measures originate in the 3rd or 4th millennium BC. Even the very earliest civilizations needed measurement for purposes of agriculture, construction and trade. Early standard units might only have applied to a single community or small region, with every area developing its own standards for lengths, areas, volumes and masses. Often such systems were closely tied to one field of use, so that volume measures used, for example, for dry grains were unrelated to those for liquids, with neither bearing any particular relationship to units of length used for measuring cloth or land. With development of manufacturing technologies, and the growing importance of trade between communities and ultimately across the Earth, standardized weights and measures became critical. Starting in the 18th century, modernized, simplified and uniform systems of weights and measures were developed, with the fundamental units defined by ever more precise methods in the science of metrology. The discovery and application of electricity was one factor motivating the development of standardized internationally applicable units.

The Cologne Mark was a unit of weight equivalent to 233.856 grams. The Cologne mark was in use from the 11th century onward. It came to be used as the base unit for a number of currency standards, including the Lübeck monetary system, which was important in northern Europe in the late Middle Ages, and the coinage systems of the Holy Roman Empire, most significantly the 1754 conventionsthaler, which was defined as 110 of a Cologne Mark and replaced the reichsthaler which had been 19 of a Cologne mark.

<span class="mw-page-title-main">Comparison of the imperial and US customary measurement systems</span>

Both the British Imperial and United States customary systems of measurement derive from earlier English systems used in the Middle Ages, that were the result of a combination of the local Anglo-Saxon units inherited from Germanic tribes and Roman units brought by William the Conqueror after the Norman Conquest of England in 1066.

<span class="mw-page-title-main">Imperial and US customary measurement systems</span> English (pre 1824), Imperial (post 1824) and US Customary (post 1776) units of measure

The imperial and US customary measurement systems are both derived from an earlier English system of measurement which in turn can be traced back to Ancient Roman units of measurement, and Carolingian and Saxon units of measure.

A number of different units of measurement were historically used in Cyprus to measure quantities like length, mass, area and capacity. Before the Metric system, the Imperial system was used. In between 1986-1988, metric system was adopted in Cyprus.

The Exchequer Standards may refer to the set of official English standards for weights and measures created by Queen Elizabeth I, and in effect from 1588 to 1825, when the Imperial Units system took effect, or to the whole range of English unit standards maintained by the Court of the Exchequer from the 1200s, or to the physical reference standards physically kept at the Exchequer and used as the legal reference until the such responsibility was transferred in the 1860s, after the Imperial system had been established.


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