The imperial system of units, imperial system or imperial units (also known as British Imperial or Exchequer Standards of 1826) is the system of units first defined in the British Weights and Measures Act 1824 and which continued to be developed through a series of Weights and Measures Acts and amendments.
The imperial system developed from earlier English units as did the related but differing system of customary units of the United States. The imperial units replaced the Winchester Standards, which were in effect from 1588 to 1825. The system came into official use across the British Empire in 1826.
By the late 20th century, most nations of the former empire had officially adopted the metric system as their main system of measurement, but imperial units are still used alongside metric units in the United Kingdom and in some other parts of the former empire, notably Canada.
The modern UK legislation defining the imperial system of units is given in the Weights and Measures Act 1985 (as amended).
Implementation The Weights and Measures Act 1824 was initially scheduled to go into effect on 1 May 1825. The Weights and Measures Act 1825 pushed back the date to 1 January 1826. The 1824 act allowed the continued use of pre-imperial units provided that they were customary, widely known, and clearly marked with imperial equivalents.
Apothecaries' units
Apothecaries' units are not mentioned in the acts of 1824 and 1825. At the time, apothecaries' weights and measures were regulated "in England, Wales, and Berwick-upon-Tweed" by the London College of Physicians, and in Ireland by the Dublin College of Physicians. In Scotland, apothecaries' units were unofficially regulated by the Edinburgh College of Physicians. The three colleges published, at infrequent intervals, pharmacopoeias, the London and Dublin editions having the force of law.
Imperial apothecaries' measures, based on the imperial pint of 20 fluid ounces, were introduced by the publication of the London Pharmacopoeia of 1836, the Edinburgh Pharmacopoeia of 1839, and the Dublin Pharmacopoeia of 1850. The Medical Act 1858 transferred to the Crown the right to publish the official pharmacopoeia and to regulate apothecaries' weights and measures.
Units
Length Metric equivalents in this article usually assume the latest official definition. Previously, the most precise measurement of the imperial Standard Yard was metres.Sears et al. 1928. Phil Trans A, 227:281. + Table of length equivalent units Unit Abbr. or symbols Relative to previous Feet Metres Notestwip typographic measurethouth1.44 twip Abbreviation of "thousandth of an inch". Also known as mil.Jerrard and McNeill, Dictionary of Scientific Units, second edition, Chapman and Hall; cites first appearance in print in Journal of the Institution of Electrical Engineers (G.B.) vol. 1, page 246 (1872).barleycornBc inchin (″) 1 metre ≡ 39 incheshandhh Used to measure the height of horsesfootft (′) h 1 12 inyardyd 3 Defined as exactly by the international yard and pound agreement of 1959rodrd yd The rod is also called pole or perchfurlongfur milemi or leaguelea Maritime unitsfathomftm The British Admiralty in practice used a fathom of . This was despite its being of a nautical mile (i.e. ) until the adoption of the international nautical mile.The exact figure was , but was in use in practice. The commonly accepted definition of a fathom was always 6 feet. The conflict was inconsequential, as Admiralty nautical charts designated depths shallower than in feet on older imperial charts. Today, all charts worldwide are metric, except for USA Hydrographic Office charts, which use feet for all depth ranges.cable One tenth of a nautical mile. Equal to under the strict definition.nautical milenmi Used for measuring distances at sea (and also in aviation) and approximately equal to one arc minute of a great circle. Until the adoption of the international definition of in 1970, the British nautical (Admiralty) mile was defined as .The nautical mile was not readily expressible in terms of any of the intermediate units, because it was derived from the circumference of the Earth (like the original metre). Gunter's survey units (17th century onwards)link fur Decimalization of furlong and acre to facilitate surveying calculationschainch 66 or of a furlong. The distance between the two wickets on a cricket pitch.
Area + Table of area units and equivalents UnitAbbr. or symbolRelative to previous Relation to units of length Square feet Square yards Acres Square metres Hectares perch* × rood × acre × square milesq mi × 640Note: *The square rod has been called a pole or perch or, more properly, square pole or square perch for centuries.
Volume
The Weights and Measures Act 1824 invalidated the various different gallons in use in the British Empire, declaring them to be replaced by the statute gallon (which became known as the imperial gallon), a unit close in volume to the ale gallon. The 1824 act defined as the volume of a gallon to be that of of distilled water weighed in air with brass weights with the barometer standing at at a temperature of .. (The date of coming into effect was 1 May 1825). The 1824 act went on to give this volume as . The Weights and Measures Act 1963 refined this definition to be the volume of 10 pounds of distilled water of density weighed in air of density against weights of density , which works out to .10 pounds = 4535.9237 grams. @ 0.998859 g/mL => 4546.092 mL The Weights and Measures Act 1985 defined a gallon to be exactly (approximately ).
+ Table of equivalences Unit Imperialounces Imperialpints Millilitres Cubic inches US ounces US pints fluid ounce (fl oz) 1 gill (gi) 5 pint (pt) 20 1 quart (qt) 40 2 gallon (gal) 160 8 Note: The millilitre equivalences are exact, but cubic-inch and US measures are correct to 5 significant figures.
+ Unit measures defined by the Weights and Measures Act 1824, all measures determined by reference to the statute gallon of 277.274 cubic inches. UnitgallonsCapacity pint quart gallon1peck2bushel8quarter64Note: The 1824 Act removed the distinction between liquid and dry measure, specifying instead that the dry quantities shall be unheaped. The metric equivalences shown are approximate.
British apothecaries' volume measures These measurements were in use from 1826, when the new imperial gallon was defined. For pharmaceutical purposes, they were replaced by the metric system in the United Kingdom on 1 January 1971. In the US, though no longer recommended, the apothecaries' system is still used occasionally in medicine, especially in prescriptions for older medications. + Table of British apothecaries' volume units Unit Symbols andabbreviations Relative toprevious Exactmetric valueThe vinculum over numbers (e.g. ) represents a repeating decimal. minim ♏︎, , m, m., min ( pint) fluid scruple fl ℈, fl s 20 minims ( pint) fluid drachm(fluid dram, fluidram) ʒ, fl ʒ, fʒ, ƒ 3, fl dr 3 fluid scruples ( pint) fluid ounce ℥, fl ℥, f℥, ƒ ℥, fl oz 8 fluid drachms pint O, pt 20 fluid ounces gallon C, gal 8 pints Note:
Mass and weight In the 19th and 20th centuries, the UK used three different systems for mass and weight.
troy weight, used for precious metals; avoirdupois weight, used for most other purposes; and apothecaries' weight, used by physicians and apothecaries for medical prescriptions
The distinction between mass and weight is not always clearly drawn. Strictly a pound is a unit of mass, but it is commonly referred to as a weight. When a distinction is necessary, the term pound-force may refer to a unit of force rather than mass. The troy pound () was made the primary unit of mass by the Weights and Measures Act 1824 and its use was abolished in the UK on 1 January 1879, with only the troy ounce () and its decimal subdivisions retained. The Weights and Measures Act 1855 made the avoirdupois pound the primary unit of mass. In all the systems, the fundamental unit is the pound, and all other units are defined as fractions or multiples of it.
+ Table of mass units Unit Pounds In SI units Notes grain (gr) Exactly milligrams. drachm (dr) A dram is of an ounce ounce (oz) An ounce is of a pound pound (lb) 1 Defined by the Units of Measurement Regulations 1994 (SI 1994/2867) stone (st) 14 The plural stone is often used when providing a weight (e.g. "this sack weighs 8 stone"). A person's weight is usually quoted in stone and pounds in English-speaking countries that use the avoirdupois system, with the exception of the United States and Canada, where it is usually quoted in pounds. quarter (qr or qtr) 28 One quarter (literally a quarter of a hundredweight) is equal to two stone or 28 pounds. The term quarter is also used in retail contexts, where it refers to four ounces, i.e. a quarter of a pound. (The 1824 act defined a quarter as a unit of volume, as above: thus a 'quarter of wheat', 64 gallons, would weigh about 494 lb. The density of wheat is 0.770, and 291*0.770=.). hundredweight (cwt) 112 One imperial hundredweight is equal to eight stone. This is the long hundredweight, 112 pounds, as opposed to the short hundredweight of 100 pounds used in the United States and Canada.Weights and Measures Act ton (ton) 2240 Twenty hundredweight equals a ton (as in the US and Canadian systems). The imperial hundredweight is 12% greater than the US and Canadian one. The imperial ton (or long ton) is pounds, which is much closer to a tonne (about pounds), compared to the 10.7% smaller North American short ton of . The symbol "t" is used to denote a tonne. Gravitational units slug (slug) The slug, a unit associated with imperial and US customary systems, is a mass that accelerates by 1 ft/s2 when a force of one pound (lbf) is exerted on it. {| F = ma (Newton's second law) 1 lbf = 1 slug × 1 ft/s2 (as defined above) 1 lbf = 1 lb × g/gc (by definition of the pound force) g ≈ ft/s2 gc ≈ lbm⋅ft/lbf⋅s2 1 slug ≈ pounds
Natural equivalents The 1824 Act of Parliament defined the yard, pound and gallon by reference to the prototype standards, naming the prototypes the Imperial Standard Yard, the Imperial Standard Troy Pound and the Imperial Standard Gallon, and it also defined the values of certain physical constants, to make provision for re-creation of the standards if they were to be damaged. For the yard, the length of a pendulum beating seconds at the latitude of Greenwich at mean sea level was defined as inches. For the pound, the mass of a cubic inch of distilled water at an atmospheric pressure of 30 inches of mercury and a temperature of 62° Fahrenheit was defined as 252.458 grains, with there being 7,000 grains per pound.
Following the destruction of the original prototypes in the 1834 Houses of Parliament fire, it proved impossible to recreate the standards from these definitions, and a new Weights and Measures Act 1855 was passed which permitted the recreation of the prototypes from recognized secondary standards.
Current use
United Kingdom
, imperial units remain in everyday use in the United Kingdom. They are used for road distances and speeds, draught beer and cider, personal height and body weight, some sports and property advertising, and many pre-packaged foods are still produced in imperial sizes.