The system of imperial units or the imperial system (also known as British Imperial^[3] or Exchequer Standards of 1825) is the system of units first defined in the British Weights and Measures Act of 1824, which was later refined and reduced. The imperial units replaced the Winchester Standards, which were in effect from 1588 to 1825.^[4] The system came into official use across the British Empire. By the late 20th century, most nations of the former empire had officially adopted the metric system as their main system of measurement, although some imperial units are still used in the United Kingdom, Canada and other countries formerly part of the British Empire. The imperial system developed from what were first known as English units, as did the related system of United States customary units.

The Weights and Measures Act of 1824 was initially scheduled to go into effect on 1 May 1825.^[5] However, the Weights and Measures Act of 1825 pushed back the date to 1 January 1826.^[6] The 1824 Act allowed the continued use of pre-imperial units provided that they were customary, widely known, and clearly marked with imperial equivalents.^[5]

Apothecaries' units are mentioned neither in the act of 1824 nor 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, pharmacopoeiae, the London and Dublin editions having the force of law.^[7][8]

Imperial apothecaries' measures, based on the imperial pint of 20 fluid ounces, were introduced by the publication of the London Pharmacopoeia of 1836,^[9][10] the Edinburgh Pharmacopoeia of 1839,^[11] and the Dublin Pharmacopoeia of 1850.^[12] The Medical Act of 1858 transferred to The Crown the right to publish the official pharmacopoeia and to regulate apothecaries' weights and measures.^[13]

Metric equivalents in this article usually assume the latest official definition. Before this date, the most precise measurement of the imperial Standard Yard was 0.914398415 metres.^[14]

In 1824, the various different gallons in use in the British Empire were replaced by the imperial gallon, a unit close in volume to the ale gallon. It was originally defined as the volume of 10 pounds (4.54 kg) of distilled water weighed in air with brass weights with the barometer standing at 30 inches of mercury (102 kPa) at a temperature of 62 °F (17 °C). In 1963, the gallon was redefined as the volume of 10 pounds of distilled water of density 0.998859 g/mL weighed in air of density 0.001217 g/mL against weights of density 8.136 g/mL, which works out to 4.546092 L. The Weights and Measures Act of 1985 switched to a gallon of exactly 4.54609 L (approximately 277.4194 cu in).^[19]

These measurements were in use from 1826, when the new imperial gallon was defined, but were officially abolished in the United Kingdom on 1 January 1971.^[21][22] In the US, though no longer recommended, the apothecaries' system is still used occasionally in medicine, especially in prescriptions for older medications.^[23][24]

In the 19th and 20th centuries, the UK used three different systems for mass and weight.

The distinction between mass and weight is not always clearly drawn. Strictly a pound is a unit of mass, although it is commonly referred to as a weight. When a distinction is necessary, the term pound-force may be used to refer to a unit of force rather than mass. The troy pound (373.2417216 g) was made the primary unit of mass by the 1824 Act; however, its use was abolished in the UK on 1 January 1879,^[31] with only the troy ounce (31.1034768 g) and its decimal subdivisions retained.^[32] The Weights and Measures Act 1855 (18 & 19 Victoria C72) made the avoirdupois pound the primary unit of mass.^[33] In all the systems, the fundamental unit is the pound, and all other units are defined as fractions or multiples of it.

Although the 1824 act defined the yard and pound by reference to the prototype standards, 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 39.01393 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.^[5] However, 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 (18 & 19 Victoria. Cap. 72) was passed in 1855 which permitted the recreation of the prototypes from recognized secondary standards.^[33]

The imperial system is one of many systems of English units. Although most of the units are defined in more than one system, some subsidiary units were used to a much greater extent, or for different purposes, in one area rather than the other. The distinctions between these systems are often not drawn precisely.

One such distinction is that between these systems and older British/English units/systems or newer additions. The term imperial should not be applied to English units that were outlawed in the Weights and Measures Act 1824 or earlier, or which had fallen out of use by that time, nor to post-imperial inventions, such as the slug or poundal.

The US customary system is historically derived from the English units that were in use at the time of settlement. Because the United States was already independent at the time, these units were unaffected by the introduction of the imperial system.

British law now defines each imperial unit in terms of the metric equivalent. The metric system is in official use within the United Kingdom for most official applications with Imperial units remaining in widespread use amongst the public.^[38] All UK roads use the imperial system except for weight limits, and newer height or width restriction signs give metric alongside imperial.^[39]

Units of measurement regulations require all measuring devices used in trade or retail to display measurements in metric quantities. Almost all traders in the UK will 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 some imperial units still legally mandated for certain applications such as draught beer and cider,^[40] road-signs,^[41] and 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 of 1878, the troy ounce may still be used for the weights of precious stones and metals. The original railways (many built in the Victorian era) 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, although more recent systems are metric, and London Underground uses metric.^[42]

Most British people still use imperial units in everyday life for distance (miles, yards, feet and inches) and volume in some cases (especially milk and beer in pints) but rarely for canned or bottled soft drinks or petrol.^[38][43] Though use of kilograms is increasing, most British people also still use imperial units in everyday life for body weight (stones and pounds for adults, pounds and ounces for babies). Some government documents aimed at the public give body weight and height not only in metric units (kilograms centimetres) but also in imperial units (stones and pounds, feet and inches).^[44] A survey in 2015 found that many people did not know their body weight or height in one system or the other.^[45] People under the age of 40 preferred the metric system but people aged 40 and over preferred the imperial system.^[46] The height of horses in some English-speaking countries, including Australia,[1] Canada, the United Kingdom and the United States is usually measured in hands, standardized to 4 inches (101.6 mm). Fuel consumption for vehicles is commonly stated in miles per gallon (mpg), though official figures always include litres per 100 km equivalents. When sold draught in licensed premises, beer and cider must be sold in pints and half-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 official government 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. The size (diagonal) of television and computer monitor screens is always denominated in inches. Food sold by length or width e.g. pizzas or sandwiches, is generally sold in inches. Clothing is always 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.^[47]

Some pre-packaged products show both metric and imperial measures and it is also common to see imperial pack sizes with metric only labels e.g. a 1 lb (454 g) tin of Lyle's Golden Syrup is always labelled 454 g with no imperial indicator. Similarly most jars of jam and packs of sausages are labelled 454 g with no imperial indicator.

India's conversion to the metric system from the imperial system occurred in stages 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. For the next five years, both the previous and new system were legal. In April 1962, all other systems were banned.^[48]

Today all official measurements are made in the metric system. However, 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.^[48][49]

In Standard Indian English, as in Australian, Singaporean, and British English, metric units such as the litre, metre, and metric 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'.^[49]

Hong Kong has three main systems of units of measurement in current use:

In 1976 the Hong Kong Government started the conversion to the metric system, and as of 2012 measurements for government purposes, such as road signs, are almost always in metric units. However, all three systems are officially permitted for trade,^[50] and in the wider society a mixture of all three systems prevails.

The Chinese system's most commonly used units for length are 里 (lei5), 丈 (zoeng6), 尺 (cek3), 寸 (cyun3), 分 (fan1) in descending scale order. These units are now rarely used in daily life, the imperial and metric systems being preferred. The imperial equivalents are written with the same basic Chinese characters as the Chinese system. In order to distinguish between the units of the two systems, the units can be prefixed with "Ying" (英, jing1) for the Imperial system and "Wa" (華, waa4) for the Chinese system. In writing, derived characters are often used, with an additional 口 (mouth) radical to the left of the original Chinese character, for writing imperial units. The most commonly used units are the mile or "li" (哩, li1), the yard or "ma" (碼, maa5), the foot or "chek" (呎, cek3), and the inch or "tsun" (吋, cyun3).

The traditional measure of flat area is the square foot (方呎, 平方呎, fong1 cek3, ping4 fong1 cek3) of the imperial system, which is still in common use for real estate purposes. The measurement of agricultural plots and fields, however, is traditionally conducted in 畝 (mau5) of the Chinese system.

For the measurement of volume, Hong Kong officially uses the metric system, though the gallon (加侖, gaa1 leon4-2) is also occasionally used.

During the 1970s, the metric system and SI units were introduced in Canada to replace the imperial system. Within the government, efforts to implement the metric system were extensive; almost any agency, institution, or function provided by the government uses SI units exclusively. Imperial units were eliminated from all road signs, although both systems of measurement will still be found on privately owned signs, such as the height warnings at the entrance of a parkade. In the 1980s, momentum to fully convert to the metric system stalled when the government of Brian Mulroney was elected. There was heavy opposition to metrication and as a compromise the government maintains legal definitions for and allows use of imperial units as long as metric units are shown as well.^[51][52][53] The law requires that measured products (such as fuel and meat) be priced in metric units, although an imperial price can be shown if a metric price is present.^[54][55] However, there tends to be leniency in regards to fruits and vegetables being priced in imperial units only. Environment Canada still offers an imperial unit option beside metric units, even though weather is typically measured and reported in metric units in the Canadian media. However, some radio stations near the United States border (such as CIMX and CIDR) primarily use imperial units to report the weather. Railways in Canada also continue to use Imperial units.

Imperial units are still used in ordinary conversation. Today, Canadians typically use a mix of metric and imperial measurements in their daily lives. However, the use of the metric and imperial systems varies by age. The older generation mostly uses the imperial system, while the younger generation more often uses the metric system. Newborns are measured in SI at hospitals, but the birth weight and length is also announced to family and friends in imperial units. Drivers' licences use SI units. In livestock auction markets, cattle are sold in dollars per hundredweight (short), whereas hogs are sold in dollars per hundred kilograms. Imperial units still dominate in recipes, construction, house renovation and gardening.^{[56][57][58][59][60]} Land is now surveyed and registered in metric units, although initial surveys used imperial units. For example, partitioning of farm land on the prairies in the late 19th and early 20th centuries was done in imperial units; this accounts for imperial units of distance and area retaining wide use in the Prairie Provinces. The size of most apartments, condominiums and houses continues to be described in square feet rather than square metres, and carpet or flooring tile is purchased by the square foot. Motor-vehicle fuel consumption is reported in both litres per 100 km and statute miles per imperial gallon,^[61] leading to the erroneous impression that Canadian vehicles are 20% more fuel-efficient than their apparently identical American counterparts for which fuel economy is reported in statute miles per US gallon (neither country specifies which gallon is used). Canadian railways maintain exclusive use of imperial measurements to describe train length (feet), train height (feet), capacity (tons), speed (mph), and trackage (miles).^[62]

Imperial units also retain common use in firearms and ammunition. Imperial measures are still used in the description of cartridge types, even when the cartridge is of relatively recent invention (e.g., .204 Ruger, .17 HMR, where the calibre is expressed in decimal fractions of an inch). However, ammunition that is already classified in metric is still kept metric (e.g., 9×19mm). In the manufacture of ammunition, bullet and powder weights are expressed in terms of grains for both metric and imperial cartridges.

As in most of the western world, air navigation is based on nautical units, e.g., the nautical mile, which is neither imperial nor metric, though altitude is still measured in imperial feet^[63] in keeping with the international standard.

Metrication in Australia has largely ended the official use of imperial units, though for particular measurements, international use of imperial units is still followed. In licensed venues, draught beer and cider is sold in glasses and jugs with sizes based on the imperial fluid ounce, though rounded to the nearest 5 mL.

Australians regularly use imperial units for birth weights, heights and screen sizes.

Property size is frequently advertised in acres as well as square metres.

Navigation is done in nautical miles, and water-based speed limits are in nautical miles per hour.

Although New Zealand completed metrication in the 1970s, a study of university students undertaken in 1992 found a continued use of imperial units for birth weight and human height alongside metric units.^[64]

In aviation, altitude and airport elevation are measured in feet whilst navigation is done in nautical miles; all other aspects (fuel quantity, aircraft weight, runway length, etc.) use metric units.

Screen sizes for devices such as televisions,^[65] monitors^[66] and phones,^[67] and wheel rim sizes for vehicles,^[68] are stated in inches, as is the convention in the rest of the world.

Ireland has officially changed over to the metric system since entering the European Union, with distances on new road signs being metric since 1997 and speed limits being metric since 2005. The imperial system remains in limited use – for sales of beer in pubs (traditionally sold by the pint). All other goods are required by law to be sold in metric units, although old quantities are retained for some goods like butter and sausages, which are sold in 454-gram (1 lb) packaging. The majority of cars sold pre-2005 feature speedometers with miles per hour as the primary unit, but with a kilometres per hour display as well.

Some imperial measurements remain in limited use in Malaysia, the Philippines, Sri Lanka and South Africa. Measurements in feet and inches, especially for a person's height, are frequently encountered in conversation and non-governmental publications.

Prior to metrication, it was a common practice in Malaysia for people to refer to unnamed locations and small settlements along major roads by referring to how many miles the said locations were from the nearest major town. In some cases, these eventually became the official names of the locations; in other cases, such names have been largely or completely superseded by new names. An example of the former is Batu 32 (literally "Mile 32" in Malay), which refers to the area surrounding the intersection between Federal Route 22 (the Tamparuli-Sandakan highway) and Federal Route 13 (the Sandakan-Tawau highway). The area is so named because it is 32 miles west of Sandakan, the nearest major town.

Petrol is still sold by the imperial gallon in Anguilla, Antigua and Barbuda, Belize, Myanmar, the Cayman Islands, Dominica, Grenada, Montserrat, St Kitts and Nevis and St. Vincent and the Grenadines. The United Arab Emirates Cabinet in 2009 issued the Decree No. (270 / 3) specifying that, from 1 January 2010, the new unit sale price for petrol will be the litre and not the gallon. This in line with the UAE Cabinet Decision No. 31 of 2006 on the national system of measurement, which mandates the use of International System of units as a basis for the legal units of measurement in the country.^{[69][70][71][72]} Sierra Leone switched to selling fuel by the litre in May 2011.^[73]

In October 2011, the Antigua and Barbuda government announced the re-launch of the Metrication Programme in accordance with the Metrology Act 2007, which established the International System of Units as the legal system of units. The Antigua and Barbuda government has committed to a full conversion from the imperial system by the first quarter of 2015.^[74]