The imperial system of units, imperial system or imperial units (also known as British Imperial^[1] or Exchequer Standards of 1826) is the system of units first defined in the British Weights and Measures Act 1824 (5 Geo. 4. c. 74) 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.^[2] 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 (c. 72) (as amended).^[3]

Implementation

The Weights and Measures Act 1824 (5 Geo. 4. c. 74) was initially scheduled to go into effect on 1 May 1825.^[4] The Weights and Measures Act 1825 (6 Geo. 4. c. 12) pushed back the date to 1 January 1826.^[5] The 1824 act allowed the continued use of pre-imperial units provided that they were customary, widely known, and clearly marked with imperial equivalents.^[4]

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.^[6][7]

Imperial apothecaries' measures, based on the imperial pint of 20 fluid ounces, were introduced by the publication of the London Pharmacopoeia of 1836,^[8][9] the Edinburgh Pharmacopoeia of 1839,^[10] and the Dublin Pharmacopoeia of 1850.^[11] The Medical Act 1858 (21 & 22 Vict. c. 90) transferred to the Crown the right to publish the official pharmacopoeia and to regulate apothecaries' weights and measures.^[12]

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 0.914398415 metres.^[13]

Area

Volume

The Weights and Measures Act 1824 (5 Geo. 4. c. 74) 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 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).^[18] The 1824 act went on to give this volume as 277.274 cubic inches (4.54371 litres).^[18] The Weights and Measures Act 1963 (c. 31) refined this definition to be 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.^{[nb 1]} The Weights and Measures Act 1985 (c. 72) defined a gallon to be exactly 4.54609 L (approximately 277.4194 cu in).^[19]