The stone is a unit of measure, abbreviation st[1] which, when it ceased to be legal for trade in United Kingdom in 1985, was defined in British legislation as being a weight or mass [sic] equal to 14 avoirdupois pounds (about 6.35 kilograms).[2] It was also formerly used in several Commonwealth countries.
Eight stones make a hundredweight in the Imperial system. Given its imprecise definition, it is arguable whether one should use kilograms (a mass) or newtons (a weight/force) as the equivalent SI unit.
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The stone was originally used for weighing agricultural commodities. Historically the number of pounds in a stone varied by commodity, and was not the same in all times and places even for one commodity. Potatoes, for example, were traditionally sold in stone and half-stone (14-pound and 7-pound) quantities but the OED contains examples including:[3]
| Commodity | Number of Pounds |
|---|---|
| Wool | 14, 15, 24 |
| Wax | 12 |
| Sugar and spice | 8 |
| Beef and Mutton | 8 |
Another example is the definition of the "stone" in the 1772 edition of Encyclopaedia Britannica which reads, "STONE also denotes a certain quantity or weight of some commodities. A stone of beef, in London, is the quantity of eight pounds; in Hertfordshire, twelve pounds; in Scotland sixteen pounds".[4]
Although the United Kingdom's 1985 Weights and Measures Act[5] expressly prohibited the use of the stone as a unit of measure for purposes of trade (other than as a supplementary unit), the stone remains widely used within the United Kingdom and also in Ireland as a means of expressing human body weight. People in these countries normally describe themselves as weighing, for example, "11 stone 4" (11 stones and 4 pounds), rather than "72 kilograms" in most other countries, or "158 pounds" (the conventional way of expressing the same weight in the United States and Canada).[6]
Its widespread colloquial use may be compared to the persistence in the United Kingdom and in Ireland of other Imperial units like the foot, the inch, and the mile, despite these having been supplanted entirely or partly by metric units in official use and other contexts. Thus on a National Health Service Web site the user may select imperial units are widely used,[7] but the law requires that if this information is officially recorded, then such records shall be in metric units.[8]
When used as the unit of measurement, the plural form of stone is correctly stone (as in, "11 stone"), though stones is sometimes used, but not usually by British natives. When describing the units, the correct plural is stones (as in, "Please enter your weight in stones and pounds").
In many sports such as professional boxing, wrestling and horse racing, stone is still used as a means of measuring weight but is slowly becoming obsolete.