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U.S. Traditional Units

U.S. customary units

The U.S. customary units, commonly known in the United States as English units or standard units, are the non-SI (non-metric) units of measurement that are currently used in the U.S., in some cases alongside the International System of Units. This system of units is similar to, but not to be confused with, the Imperial system still used in the United Kingdom. Both systems derive from the evolution of local units over the centuries as a result of standardization efforts in England; the local units themselves mostly trace back to Roman and Anglo-Saxon units. 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, and is evident in labeling requirements on food products, for example. However, for various reasons, customary units are still widely used on consumer products and in industrial manufacturing; only in specific scientific contexts are SI units always preferred. Since everyday weights and measures are mostly non-SI, children in U.S. public schools are generally taught customary units before SI, although many schools are now attempting to teach SI units at an earlier age.

In most Commonwealth countries, such as Australia and New Zealand, where metrication has been more forcefully imposed and has encountered less resistance from industry and consumer market forces, metrication is relatively complete, although some informal usage of non-SI units remains, particularly in Canada. Although increasingly metric, the UK has taken some time to change over, and retains use of non-SI units, particularly in informal situations. In more populated and developed British colonies, such as India, existing local weights and measures had been redefined based upon English units; some of which are still in limited use.

Other countries have had customary units of their own, sometimes very similar in root (Germanic, Roman), name and value to the ones listed below—foot or pound, for instance—but frequently, similar terms designate quite different sizes. For example, miles ranged from 1 to 10 kilometers. The standardization of these units was of varying quality, but was often comparable to or better than the U.S. customary system of the 19th century. Most were later redefined in terms of kilogram and meter. Often, unlike English units, they were rounded to "nice" SI values, leading to their use in colloquial speech, alongside SI terms, into the present day.

Historically, a wide range of non-SI units have been used in the United States. and United Kingdom, and in England before that, but many of these have fallen into disuse. This article only deals with the units commonly used or officially defined in the United States.

Units of length

The system for measuring length in the United States' 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 millimeters, while one inch U.S. survey measure is defined so that 39.37 inches is exactly 1 meter. For most applications, the difference is insignificant (about 3 millimeter per mile). International measure is used for everyday use, engineering, and commerce in the United States, while survey measure is used only 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 = 30.48 cm
  • 1 yard (yd) = 3 ft = 91.44 cm
  • 1 mile (mi) = 5280 ft = 1760 yd = 1.609344 km
  • 1 rod (unit) (rd) (also called pole or perch) = 16.5 ft = 5.0292 m
  • 1 furlong (fur) = 40 rd = 660 ft = 201.168 m
  • 1 mile (survey) = 8 fur = 5280 ft ≈ 1.609347 km

Sometimes, for surveying purposes, units known as Gunther's Chain Measure (or equivalently Surveyor's Chain Measure) are used. These units are defined by survey feet as follows:

  • 1 link (li) = 7.92 in = 0.66 ft = 0.001 fur (approx. 201.168 mm)
  • 1 chain (ch) (Gunter's) = 100 li = 66 ft (approx. 20.1168 m)

To measure depths at sea, fathoms are used:

  • 1 fathom = 6 feet = 1.8288 m
  • 1 cable length = 120 fathoms = 720 feet = 219.456 m (US Navy definition, there are others)

Units of area

The units of area in the U.S. customary system are mostly based on the units of length squared, e.g., square inch (sq in, 645.16 mm²). Since the U.S. customary system has two differing definitions of the foot (international and survey), there are also two differing definitions for the square foot.

  • 1 square foot (sq ft) = 144 sq in = 929.0304 cm²
  • 1 square rod (sq rd) = 272.25 sq ft = 25.29285264 m²
  • 1 acre = 10 sq ch = 1 fur × 1 ch = 160 sq rd = 43,560 sq ft = 4046.8564224 m²
  • 1 square mile (sq mi) = 27,878,400 sq ft = 3,097,600 sq yd = 640 acres = 2.589988110336 km²

The Public Land Survey System, used to divide land in most of the United States, uses the survey mile to divide land into regular square parcels, such "sections" and "townships." However, these terms aren't used for general area measurement, instead used to indicate particular parcels of land in a rectangular grid.

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 ml (or 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 ≈ 12,334.818 m³

Liquid volume

  • 1 minim (min) = 1/480 fl oz ≈ 61.612 µL
  • 1 fluid dram (fl dr) = 1/8 fl oz = 60 min ≈ 3.697 mL
  • 1 fluid ounce (fl oz) = 1/128 gal = 8 fl dr ≈ 29.574 mL
  • 1 gill (gi) = 7.21875 cu in = 4 fl oz ≈ 118.294 mL
  • 1 cup = 2 gi = 8 fl oz ≈ 236.588 mL
  • 1 pint (pt) = 2 cups = 4 gi = 16 fl oz ≈ 473.176 mL
  • 1 fifth = 25.6 fl oz ≈ 757.082 mL
  • 1 quart (qt) = 2 pt = 32 fl oz ≈ 946.353 mL
  • 1 gallon (gal) = 231 cu in = 4 qt = 128 fl oz = 3.785411784 L

The gill is basically a paper unit which is never actually used in the United States. Distilled liquor and wine were bottled in fifths until 1973; when the term "fifth" is used now it almost always means 750 mL.

Dry volume

  • 1 pint (pt) ≈ 550.610 mL
  • 1 quart (qt) = 2 pt ≈ 1.101 L
  • 1 gallon (gal) = 4 qt = 268.8025 in³ = 4.404 842 803 2 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.

The Avoirdupois units are legally defined as measures of mass, but the names of these units are sometimes applied to measures of force. For instance, in most contexts, the pound avoirdupois is used as a unit of mass, but in the realm of 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, which is the same in all three systems. However, while each system has some overlap in the names of their units of measure (all have ounces and pounds), the relationship between the grain and these other units within each system varies. For example, in apothecaries and troy weight, the pound and ounce are the same, but are different from the pound and ounce in avoirdupois, in terms of their relationships to grains and to each other. The systems also have different units between the grain and ounce (apothecaries has scruple and dram, troy has pennyweight, and avoirdupois has just dram). The dram was once known as the drachm.

To alleviate confusion, it is typical when publishing non-avoirdupois weights to mention the name of the system along with the unit. Precious metals, for example, are often weighed in "troy ounces", because just "ounce" would be more likely to be assumed to mean an ounce avoirdupois.

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

  • 1 grain (gr) = 64.79891 mg
  • 1 dram (dr) = 27 11/32 gr = 1/16 oz ≈ 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 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 has limited use in the United States.

  • 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

The most common cooking weights and measures in the U.S. are as follows:

  • 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) = 1/4 lb = 4 oz ≈ 115 g

Grain measures

These are derived from the volume measures and there have been more for other crops.

  • 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 and boils at 212 °F at 1 atm.
  • Water saturated with common salt freezes at -6.02 °F.
  • Conversion formula: F=\frac{9}{5}C+32

This article is available under the terms of the GNU Free Documentation License. Original article can be found here.


 

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