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The U.S. customary units (more commonly known in the US as English units or standard
units) are the non-metric units of measurement that are presently used in the
United States, in some cases alongside the metric system of units. This system of units is similar to the Imperial system (sometimes called the British system) once used in the United Kingdom and The Commonwealth, but has some differences. Both systems derive from the units which developed over the
centuries in England. Today U.S. customary units are defined in terms of SI units.
The official policy of the United States government is to designate the metric system of measurement as the preferred system
of weights and measures for U.S. trade and commerce. This process is known as metrication. In practice, this process in the United States is not very far advanced: the customary units are
taught to children before metric units (although many schools are now attempting to teach metric units at an early age),
customary units are preferably chosen in informal situations, and only in specific scientific contexts are metric units always
preferred. The United Kingdom is more advanced in the process than the United States, and in most Commonwealth countries (such as
Australia and New Zealand)
the process is largely completed, although some informal usage of non-metric units remains.
Historically a wide range of non-metric units have been used in the US and UK, and in England before that, but many of these
have fallen into disuse. This article will mainly deal with those commonly used or officially defined in the US.
Units of length
The system for measuring length in the U.S. customary system is based on the inch,
foot, yard and
mile. However, for each of these units there exist two slightly different definitions,
yielding two different systems of measure - international measure, and U.S. survey measure. The relationships between the
different units within each measure is the same, but each measure has a slightly different definition in terms of metric
units.
One inch international measure is exactly 25.4 mm, while one inch U.S.
survey measure is defined so that 39.37 inches is exactly one metre. For most
applications, the difference is insignificant (about 3 millimetres per mile). International measure is used for everyday use,
engineering, and commerce in the United States, while survey measure is only used for surveying.
International measure uses the same definition of the units involved as is used in the UK and other Commonwealth countries.
U.S. survey measure uses an older definition of these units which the United States used prior to adopting international
measure.
- 1 inch (in) == 25.4 mm
- 1 foot (ft) == 12 in == 304.8 mm
- 1 yard (yd) == 3 ft == 914.4 mm
- 1 mile (mi) == 1760 yd == 1.609344 km
- 1 rod (rd) (also called pole or perch) == 16.5 ft == 5.0292 m
- 1 furlong (fur) == 40 rd == 660 ft == 201.168 m
- 1 mile == 8 fur == 5280 ft = 1.609347 km (survey)
Sometimes, for surveying purposes, units known as Gunther's Chain Measure (or equivalently Surveyor's Chain
Measure) are used. These units are defined as follows:
- 1 link (li) == 7.92 in == 0.001 rd = 201.168 mm
- 1 chain (ch) == 100 li == 66 ft = 20.117 m
To measure depths at sea, fathoms are used:
Units of area
The units of area in the U.S. customary system are based on the square inch (sq in). Since the U.S. customary system has two differing definitions of the
inch, there are also two differing definitions for the square inch. But presuming international measure is used, 1 square inch is
exactly 645.16 mm².
- 1 square foot (sq ft) == 144 sq in = 929.03 cm²
- 1 square rod (sq rd) == 272.25 sq
ft = 25.316 m²
- 1 acre == 10 sq ch == 1 fur × 1 ch == 160 sq rd == 43,560 sq ft = 4046.9 m²
- 1 square mile (sq mi) == 640 acres = 2.59 km²
- 1 section == 1 sq mi (survey) = 2.58999847 km²
Units of capacity and volume
The cubic inch, cubic foot and cubic yard are commonly used for measuring volume. In addition, there is one group of units for
measuring volumes of liquids, and one for measuring volumes of dry material.
Other than the cubic foot, cubic inch and cubic yard, these units are differently sized from the units in the Imperial system,
although the names of the units are similar. Also, while the U.S. has separate systems for measuring the volumes of liquids and
dry material, the Imperial system has one set of units for both.
Technically speaking, since these units are defined in terms of the inch, it would make a difference whether international or
survey measure was used. However, in practice, the difference between the two definitions would be imperceptible, and in any case
in defining volumes international measure is used.
Volume in general
- 1 cubic inch (in³ or cu in) == 16.387064 cm³
- 1 cubic foot (ft³ or cu ft) == 1728 cu in = 28.317 L
- 1 cubic yard (yd³ or cu yd) == 27 cu ft = 7.646 hL
- 1 acre-foot == 43,560 cu ft = 325,851 gallons = 13,277.088 m³
Liquid volume
- 1 minim (min) = 61.612 µL
- 1 fluid dram (fl dr) == 60 min =
3.697 mL
- 1 fluid ounce (fl oz) == 8 fl dr = 29.574 mL
- 1 gill (gi) == 7.21875 cu in == 4 fl oz = 118.294 mL
- 1 pint (pt) == 4 gi == 16 fl oz = 473.176 mL
- 1 quart (qt) == 2 pt = 946.353 mL
- 1 gallon (gal) == 231 cu in == 4 qt == 3.785411784 L
Dry volume
- 1 pint (pt) = 550.610 mL
- 1 quart (qt) == 2 pt = 1.101 L
- 1 gallon (gal) == 4 qt = 268.8 cu in = 4.405 L
- 1 peck (pk) == 8 qt == 2 gal = 8.81 L
- 1 bushel (bu) == 2150.42 cu in == 4 pk = 35.239 L
Units of mass
There have historically been four different English systems of mass: Tower weight, Troy weight, avoirdupois weight, and
apothecaries weight. Tower weight fell out of use (due to legal prohibition) centuries ago, and was never used in the United
States. Troy weight is still used to weigh precious metals. Apothecaries weight, once used in pharmacy, has been largely replaced
by metric measurements. Avoirdupois weight is the primary system of mass in the U.S. customary system.
There is some confusion as to whether these are units of mass, or of force. The pound avoirdupois is
legally defined as a unit of mass, though in physics the term "pound" can represent "pound-force" (a unit of force properly abbreviated as "lbf").
Troy weight, avoirdupois weight and apothecaries weight are all defined in terms of the same basic unit, the grain. However,
they consist of various units (pounds, ounces, etc.) with the same name but different definitions in terms of the grain and in
terms of each other. The pound and ounce in apothecaries and troy weight are the same, but each system has different
subdivisions.
The pound avoirdupois, which forms the basis of the U.S. customary system of mass, is defined as exactly 453.59237 grams. All the other units of mass are defined in terms of it.
For the pound and smaller units, the U.S. customary system and the British Imperial system are identical. However, they differ
when dealing with units larger than the pound. The definition of the pound avoirdupois in the British Imperial system is
identical to that in the U.S. customary system.
Avoirdupois weight
See Avoirdupois
- 1 grain (gr) == 64.79891 mg
- 1 dram (dr) == 27.34375 gr = 1.772 g
- 1 ounce (oz) == 16 dr == 437.5 gr = 28.35 g
- 1 pound (lb) == 16 oz == 7000 gr == 453.59237 g
- 1 hundredweight (cwt) == 100 lb == 45.359 kg
- 1 ton (t) == 20 cwt == 2000 lb == 907.18474 kg = 0.907 t
The ton and hundredweight above are sometimes referred to as the short ton, and the short hundredweight, to
distinguish them from the British Imperial ton and hundredweight, which are larger and hence are referred to as the long
ton and long hundredweight. The long ton and hundredweight have limited use in the United States, although they are
officially recognized as units. Any unqualified reference in the U.S. to "ton" or "hundredweight" means the short ton and
hundredweight mentioned above. Sometimes the terms "net" and "gross" are used instead of "short" and "long", respectively.
- 1 long
hundredweight == 112 lb = 50.802 kg
- 1 long ton == 20 long cwt == 2240 lb = 1016.047 kg = 1.016 t
Apothecaries weight
The grain has the same definition as for Avoirdupois weight
- 1 scruple (s ap) == 20 gr = 1.296 g
- 1 dram apothecaries (dr ap) == 3 s ap = 3.888 g
- 1 ounce apothecaries (oz ap) == 1 oz t = 8 dr ap = 480 gr = 31.103 g
- 1 pound apothecaries (lb ap) == 1 lb t == 12 oz ap == 5760 gr = 373.242 g
The pound and ounce apothecaries are identical to the pound and ounce Troy.
Troy weight
The grain has the same definition as for Avoirdupois weight
- 1 pennyweight (dwt) == 24 gr = 1.555 g
- 1 ounce troy (oz t) == 20 dwt == 480 gr = 31.103 g
- 1 pound troy (lb t) == 12 oz t == 5760 gr = 373.242 g
Cooking measures
Measures of volume and weight used in
cooking. Note that differing cooking measures are used in other countries.
- 1 teaspoon == 1/3 tbsp == 1/6 fl oz = 5 ml
- 1 tablespoon (tbsp) == 1/2 fl oz == 4 fl dr = 15 ml
- 1 cup == 8 fl oz = 240 ml
- 1 stick (of butter) == ¼ lb == 4 oz = 115 g
Grain measures
These are derived from the volume measures.
- 1 bushel (maize) == 56 lb = 25.401 kg
- 1 bushel (wheat) == 60 lb = 27.216 kg
Units of temperature
- Traditionally, degrees Fahrenheit are used in the United States to measure
temperatures.
- Pure water freezes at 32 °F.
- Water saturated with common salt freezes at -6.02 °F.
- Water boils at 212 °F at 1 atmospheric pressure.
- Conversion formula:
Other units
See also
Source: Appendix C, NIST Handbook 44, 2002 edition .
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