Units::Imperial units


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Metric equivalents in this article usually assume the latest official definition. Before this date, the most precise measurement of the imperial Standard Yard was {{safesubst:#invoke:val|main}} metres.<ref>Sears et al. 1928. Phil Trans A, 227:281.</ref>

Table of length equivalent units
Unit Relative to previous Feet Millimetres Metres Notes
thou (th) 1/12000 0.0254 0.0000254
Also 25.4 μm
inch (in) 1000 thou 1/12 25.4 0.0254
foot (ft) 12 inches 1 304.8 0.3048
yard (yd) 3 feet 3 914.4 0.9144
Defined as exactly 0.9144 metre by the International yard and pound agreement of 1959
chain (ch) 22 yards 66 20,116.8 20.1168
The distance between the two wickets on a cricket pitch
furlong (fur) 10 chains 660 201.168
220 yards
mile (mi) 8 furlongs 5,280 1,609.344
1,760 yards
league (lea) 3 miles 15,840 4,828.032
No longer an official unit in any nation.
Maritime units
fathom (ftm) 2.0266 yards 6.08 1,828.8 1.8288
The British Admiralty in practice used a fathom as 6 feet. This was despite its being 1/1000 of a nautical mile (i.e. 6.08 feet) until the adoption of the international nautical mile.<ref name="fath">The exact figure was 6.08 feet, but 6 feet 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 5 fathoms 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.</ref>
cable 100 fathoms 608 185.3184
One tenth of a nautical mile. When in use it was approximated informally as 100 fathoms.
nautical mile 10 cables 6,080 1,853.184
Used for measuring distances at sea. Until the adoption of the international definition of 1,852 metres in 1970, the British nautical (Admiralty) mile was defined as 6,080 feet.<ref>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).</ref>
Gunter's survey units (17th century onwards)
link 7.92 inches 66/100 201.168 0.201168
1/100 of a chain
rod 25 links 66/4 5,029.2 5.0292
The rod is also called pole or perch and equal to 5.5 yards
chain 4 rods 66 20.1168
100 links or 1/10 of a furlong


Unit Relation to
units of length
Square feet Square rods Square miles Square metres Hectares Notes
perch 1 rod × 1 rod 272.25 1 1{{safesubst:#invoke:val|main}} {{safesubst:#invoke:val|main}} {{safesubst:#invoke:val|main}}
Although the proper term is square rod, for centuries this unit has been called a pole or perch or, more properly, square pole or square perch.
rood 1 furlong × 1 rod<ref>{{#invoke:citation/CS1|citation CitationClass=web


{{safesubst:#invoke:val|main}} 40 1{{safesubst:#invoke:val|main}} {{safesubst:#invoke:val|main}} 0.1012
The rood is {{safesubst:#invoke:val|main}} square yards.
acre 1 furlong × 1 chain {{safesubst:#invoke:val|main}} 160 1640 {{safesubst:#invoke:val|main}} 0.4047
One acre is {{safesubst:#invoke:val|main}} square yards
Note: All equivalences are exact except hectares, which are accurate to 4 significant figures.


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 {{safesubst:#invoke:convert|convert}} of distilled water weighed in air with brass weights with the barometer standing at {{safesubst:#invoke:convert|convert}} at a temperature of {{safesubst:#invoke:convert|convert}}. In 1963, the gallon was redefined as the volume of 10 pounds of distilled water of density {{safesubst:#invoke:val|main}} g/mL weighed in air of density {{safesubst:#invoke:val|main}} against weights of density 8.136 g/mL, which works out to {{safesubst:#invoke:val|main}} or {{safesubst:#invoke:val|main}}. The Weights and Measures Act of 1985 switched to a gallon of exactly {{safesubst:#invoke:val|main}} (approximately {{safesubst:#invoke:val|main}}).<ref></ref>

Table of volume units
Unit Imperial
Millilitres Cubic inches US ounces US pints
fluid ounce (fl oz) 1     120     convert}}
gill (gi) 5     14     convert}}
pint (pt) 20     1     convert}}
quart (qt) 40     2     convert}}
gallon (gal) 160     8     convert}}
Note: The millilitre equivalences are exact, but cubic-inch and US measures are correct to 5 significant figures.

British apothecaries' volume measures

These measurements were in use from 1824, when the new imperial gallon was defined, but were officially abolished in the United Kingdom on 1 January 1971.<ref>The Weights and Measures (Equivalents for dealings with drugs) Regulations 1970</ref><ref>Museum of the Royal Pharmaceutical Society, London, Information Sheet: 11</ref> In the USA, though no longer recommended, the apothecaries' system is still used occasionally in medicine, especially in prescriptions for older medications.<ref name="Zentz2010">{{#invoke:citation/CS1|citation |CitationClass=book }}</ref><ref name="Boyer2009">{{#invoke:citation/CS1|citation |CitationClass=book }}</ref>

Table of British apothecaries' volume unitsUnknown extension tag "ref"
Unit Symbols &
Relative to
metric value<ref group=note>The vinculum over numbers (e.g. 3) represents a repeating decimal.</ref>
minim ♏, Mx, a symbol for minim in the apothecaries' system.svg, m, m., min   {{#invoke:Gaps|main}}
fluid scruple fl ℈, fl s 20 minims {{#invoke:Gaps|main}}
fluid drachm
(fluid dram, fluidram)
ʒ, fl ʒ, fʒ, ƒ 3, fl dr 3 fluid scruples {{safesubst:#invoke:val|main}}
fluid ounce ℥, fl ℥, f℥, ƒ ℥, fl oz 8 fluid drachms {{safesubst:#invoke:val|main}}
pint O, pt 20 fluid ounces {{safesubst:#invoke:val|main}}
gallon C, gal 8 pints {{#invoke:Gaps|main}}
Unknown extension tag "references"

Mass and weight

In the 19th and 20th centuries, the UK used three different systems for mass and weight:<ref>The distinction between mass and weight is not always clearly drawn. In certain contexts, the term pound may refer to a unit of force rather than mass.</ref>

The troy pound ({{safesubst:#invoke:val|main}}) was made the primary unit of mass by the 1824 Act; however, its use was abolished in the UK on 1 January 1879,<ref name="Britain1878">{{#invoke:citation/CS1|citation |CitationClass=book }}</ref> with only the troy ounce ({{safesubst:#invoke:val|main}}) and its decimal subdivisions retained.<ref name="Chisholm1911">{{#invoke:citation/CS1|citation |CitationClass=book }}</ref> The Weights and Measures Act 1855 (18 & 19 Victoria C72) made the avoirdupois pound the primary unit of mass.<ref name="Britain1855">{{#invoke:citation/CS1|citation |CitationClass=book }}</ref> 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 grams kilograms Notes
grain (gr) 1/7000 {{safesubst:#invoke:val|main}}
Exactly {{safesubst:#invoke:val|main}} milligrams.
drachm (dr) 1/256 1.7718451953125
ounce (oz) 1/16 {{safesubst:#invoke:val|main}}
pound (lb) 1 {{safesubst:#invoke:val|main}} {{safesubst:#invoke:val|main}}
Exactly {{safesubst:#invoke:val|main}} grams by definition.
stone (st) 14 {{safesubst:#invoke:val|main}} {{safesubst:#invoke:val|main}}
A person's weight is often 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.
slug (slug) {{safesubst:#invoke:val|main}} {{safesubst:#invoke:val|main}} {{safesubst:#invoke:val|main}}
Slug is US Customary unit of mass. Force = Mass • g. Pounds = Slugs • 32.174 (ft./s^2) <ref>*C.Slug-_*Unit-</ref>
quarter (qr/qtr) 28 {{safesubst:#invoke:val|main}}
{{safesubst:#invoke:anchor|main}}One quarter is equal to two stone or a quarter of a hundredweight. The term quarter was also commonly used to refer to a quarter of a pound in a retail context.
hundredweight (cwt) 112 {{safesubst:#invoke:val|main}}
One imperial hundredweight is equal to eight stone. This is the long hundredweight as opposed to the short hundredweight of 100 pounds as used in the United States and Canada.<ref name="justice1">Weights and Measures Act</ref>
ton (t) 2240 {{safesubst:#invoke:val|main}}
As with the US and Canadian<ref name="justice1"/> systems, twenty hundredweights equal a ton. The imperial hundredweight is 12% greater than the US and Canadian equivalent. The imperial ton (or long ton) is 2,240 pounds, which is much closer to a metric tonne (about 2,204.6 pounds), compared to the short ton of {{safesubst:#invoke:convert|convert}}.

Imperial units sections
Intro  Implementation  Units   Natural equivalents    Relation to other systems    Current use of imperial units    See also   Notes   References    External links   

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