Imperial units
The imperial system of units, imperial system or imperial units is the system of units first defined in the British Weights and Measures Act 1824 and 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.
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
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. Before this date, the most precise measurement of the imperial Standard Yard was metres.Area
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, 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 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. The Weights and Measures Act 1985 defined a gallon to be exactly .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.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, now virtually unused since the metric system is used for all scientific purposes.
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 in vacuo 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
Since the Weights and Measures Act 1985, British law defines base imperial units in terms of their metric equivalent. The metric system is routinely used in business and technology within the United Kingdom, with imperial units remaining in widespread use amongst the public. All UK roads use the imperial system except for weight limits, and newer height or width restriction signs give metric alongside imperial.Traders in the UK may accept requests from customers specified in imperial units, and scales which display in both unit systems are commonplace in the retail trade. Metric price signs may be accompanied by imperial price signs provided that the imperial signs are no larger and no more prominent than the metric ones.
The United Kingdom completed its official partial transition to the metric system in 1995, with imperial units still legally mandated for certain applications such as draught beer and cider, and road-signs. Therefore, the speedometers on vehicles sold in the UK must be capable of displaying miles per hour. Even though the troy pound was outlawed in the UK in the Weights and Measures Act 1878, the troy ounce may still be used for the weights of precious stones and metals. The original railways are a big user of imperial units, with distances officially measured in miles and yards or miles and chains, and also feet and inches, and speeds are in miles per hour.
Some British people still use one or more imperial units in everyday life for distance and some types of volume measurement., many British people also still use imperial units in everyday life for body weight. Government documents aimed at the public may give body weight and height in imperial units as well as in metric. A survey in 2015 found that many people did not know their body weight or height in both systems. As of 2017, people under the age of 40 preferred the metric system but people aged 40 and over preferred the imperial system. As in other English-speaking countries, including Australia, Canada and the United States, the height of horses is usually measured in hands, standardised to. Fuel consumption for vehicles is commonly stated in miles per gallon, though official figures always include litres per equivalents and fuel is sold in litres. When sold draught in licensed premises, beer and cider must be sold in pints, half-pints or third-pints. Cow's milk is available in both litre- and pint-based containers in supermarkets and shops. Areas of land associated with farming, forestry and real estate are commonly advertised in acres and square feet but, for contracts and land registration purposes, the units are always hectares and square metres.
Office space and industrial units are usually advertised in square feet. Steel pipe sizes are sold in increments of inches, while copper pipe is sold in increments of millimetres. Road bicycles have their frames measured in centimetres, while off-road bicycles have their frames measured in inches. Display sizes for screens on television sets and computer monitors are always diagonally measured in inches. Food sold by length or width, e.g. pizzas or sandwiches, is generally sold in inches. Clothing is usually sized in inches, with the metric equivalent often shown as a small supplementary indicator. Gas is usually measured by the cubic foot or cubic metre, but is billed like electricity by the kilowatt hour.
Pre-packaged products can show both metric and imperial measures, and it is also common to see imperial pack sizes with metric only labels, e.g. a tin of Lyle's Golden Syrup is always labelled with no imperial indicator. Similarly most jars of jam and packs of sausages are labelled with no imperial indicator.
India
began converting to the metric system from the imperial system between 1955 and 1962. The metric system in weights and measures was adopted by the Indian Parliament in December 1956 with the Standards of Weights and Measures Act, which took effect beginning 1 October 1958. By 1962, metric units became "mandatory and exclusive."Today all official measurements are made in the metric system. In common usage some older Indians may still refer to imperial units. Some measurements, such as the heights of mountains, are still recorded in feet. Tyre rim diameters are still measured in inches, as used worldwide. Industries like the construction and the real estate industry still use both the metric and the imperial system though it is more common for sizes of homes to be given in square feet and land in acres.
In Standard Indian English, as in Australian, Canadian, New Zealand, Singaporean, and British English, metric units such as the litre, metre, and tonne utilise the traditional spellings brought over from French, which differ from those used in the United States and the Philippines. The imperial long ton is invariably spelt with one 'n'.