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U.S. customary units

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The U.S. customary units are the units of measurement currently used in the United States and they are mostly of British origin. Since 1959, the U.S. customary units are related to SI units. The directors of the national standards laboratories of Australia, Canada, New Zealand, South Africa, the United Kingdom, and the United States entered into agreement, effective July 1, 1959, whereby the equivalent of 1 yard = 0.9144 meter and 1 avoirdupois pound = 0.453 592 37 kilogram were adopted.[1][2]

As a broad generality, in the United States, the SI metric system of units is predominantly used in the fields of physics, chemistry and similar sciences. However, by and large, the so-called U.S. customary units are still widely used in industry, engineering, marketing and the general public of the United States.

This article selectively lists and defines the most commonly used U.S. customary units in terms of the corresponding SI units.

Units of length

The inch, foot, yard, and mile are the only four customary length measurements in everyday use by industry, engineers and the general public in the United States. In common usage, the "mile" is understood to be the statute mile (as differentiated from the "nautical mile").

Units of Length
Unit nameShort
form
DivisionsSI Equivalent
1 milmil--0.0254 mm (a)
1 inchin--2.54 cm (b)
1 footft12 in0.3048 m
1 yardyd3 ft0.9144 m
1 statute milemi5,280 ft1.609 km
1 nautical mile--6,076 ft 1.852 km
(a) 1 mil is 1/1000 of an inch
(b) 1 inch is 1/12 of a foot

It is quite common in the United States to use the word "feet" as the plural of the word "foot". One would say that the "A is one foot longer than B". However, one would say that "A is ten feet longer than B". That same usage applies to the words "square foot" and "square feet" as well as to "cubic foot" and "cubic feet".

Units of area

The square inch, square foot, square yard and acre are the four customary units of area measurement in everyday use by industry, engineers and the general public in the United States.

Units of Area
Unit nameShort
form
DivisionsSI Equivalent
1 square inchsq in--6.4516 cm²
1 square footsq ft144 sq in0.0929 m²
1 square yardsq yd9 sq ft0.836 m²
1 acre--43,560 sq ft4,047 m² (a)
(a) 1 acre = 0.4047 hectare (a non-SI metric unit)

Units of volume

The three units commonly used for measuring volumes in the United States by industry, engineers and the general public are the cubic inch, cubic foot and cubic yard. In addition, there is a group of units that define certain liquid volumes, and one that defines dry material volumes.

Other than the cubic foot, cubic inch and cubic yard, these units are differently sized from the units in the Imperial system once used in the United Kingdom, although the names of the units are similar. Also, while the United States has separate systems for defining certain volumes of liquids and dry material, the Imperial system has one set of units for both. By and large, the Imperial system has been replaced by the metric system in the United Kingdom.

Units of Volume (general)
Unit nameShort
form
DivisionsSI Equivalent
1 cubic inchcu in--16.387 mL
1 cubic footcu ft1,728 cu in 28.317 L
1 cubic yardcu yd 27 cu ft 0.7646 m³

It should be noted that in the United States, the natural gas and petroleum refining industries commonly use the letter M to denote one thousand (103 ) and the letters MM to denote one million (106 ) when stating gas volumes in cubic feet.

Units of Liquid Volume
Unit nameShort
form
DivisionsSI Equivalent
1 fluid ouncefl oz -- 29.574 ml
1 (liquid) pintpt 16 fl oz 0.4732 L
1 (liquid) quart qt 2 pt 0.9464 L
1 U.S. gallongal 4 qt 3.7854 L (a)
1 (oil) barrelbbl 42 gal 158.987 L (b)
1 (beer) barrelbbl 31 gal 117.348 L
(a) One U.S. gallon is exactly 231 cubic inches. An Imperial gallon (U.K.)

is approximately 20% larger than a U.S. gallon
(b) All countries define an (oil) barrel as 42 U.S. gallons

Units of Dry Volume
Unit nameShort
form
DivisionsSI Equivalent
1 (dry) pintpt 33.6 cu in 0.5506 L
1 (dry) quartqt 2 pt 1.1012 L
1 (dry) gallongal 4 qt 4.4048 L
1 peckpk 2 gal 8.8098 L
1 bushelbu 4pk 35.2391 L

Units of weight

In the everyday usage of the United States, the words weight and mass are interchangeable. In modern scientific usage however, weight and mass are two different quantities. Mass measures the amount of matter in an object whereas weight measures the amount of gravitational force acting on an object. Strictly speaking, one should make a distinction between pound-force and pound-mass. Thus, the units of weight in the tables below are in fact the units of force exerted by the action of standard gravity[3] on the listed units of mass.

There are two systems of weight used in the United States, the Avoirdupois system and the Troy system. The Avoirdupois system is the one primarily used in industry, engineering and the basic avoirdupois unit is the pound.

The Troy system is used for weighing precious metals (such as gold, silver and platinum) and gemstones.

The Apothecary system once used by pharmacists has been largely replaced by the SI metric system of units.

Avoirdupois Units of Weight
Unit nameShort
form
DivisionsSI Equivalent
1 graingr -- 64.799 mg (a)
1 ounceoz -- 28.3495 g (b)
1 poundlb 16 oz 453.59237 g
1 (short) tontn 2,000 lb 907.1847 kg
1 (long) tontn 2,240 lb 1,016.0469 kg
(a) 1 grain is 1/7000 of a pound
(b) 1 ounce is 1/16 of a pound
Troy Units of Weight
Unit nameShort
form
DivisionsSI Equivalent
1 graingr -- 64.799 mg (a)
1 pennyweightdwt 24 gr 1.55517384 g
1 troy ounceoz t 20 dwt 31.1035 g
1 troy poundlb t 12 oz t 373.24172 g
(a) The grain is the same weight in avoirdupois and troy weight

Bushels of grain

Although the bushel is a dry volume unit, most grains in the United States are sold commercially as bushels defined by weight. The weight of a bushel of grain depends upon the specific grain and the percentage of moisture in the grain. The table below lists the weight of a bushel of grain for some example grains, each at their "standard" amount of moisture.[4]

Weight of a Bushel of Grain
GrainStandard
moisture,
weight %
(wet basis)
Pounds Kilograms
wheat13.56027.22
shelled corn15.556 25.40
barley14.54821.77
oats14.03214.52
rye14.05625.40
soybean13.060 27.22

Units of pressure

The most commonly used units of pressure by industry, engineers and the general public in the United States are pound-force per square inch, foot of water and inches of mercury.

Units of Pressure
Unit nameShort
form
DivisionsSI Equivalent
1 pound-force/foot2psf -- 47.880 Pa
1 pound-force/inch2psi 144 psf 6,894.757 Pa
1 poundal/foot2pdl/ft2 -- 1.4882 Pa
1 atmosphereatm --101,325 Pa
1 inch of mercury (a)in Hg -- 3376.85 Pa
1 inch of water (a)in H2O -- 248.84 Pa
1 foot of water (a)ft H2O in H2O 2986.080 Pa
(a) At 60 °F (15.56 °C)

Units of energy and work

The most widely used unit of energy in the United States is the British thermal unit (Btu), which has a number of versions (the International Table Btu, the thermochemical Btu, the mean Btu and others). Of those various version, the most widely used is the International Table Btu and the thermochemical Btu listed in the table below.

The therm (105 Btu) is used by gas distribution companies. It is approximately equal to the heat energy released by burning 100 cubic feet of natural gas.

Units of Energy and Work
Unit nameShort
form
DivisionsSI Equivalent
1 British thermal unit
(International Table) (a)
Btu -- 1055.056 J
1 British thermal unit
(thermochemical)
Btu -- 1054.350 J
1 British thermal unit
(mean)
Btu -- 1055.87 J
1 therm thm 105 Btu1055.056 × 105 J
watt-hourWh -- 3600.000 J
1 foot-pound-forceft•lbf -- 1.3558 J
1 foot-poundalft•pdl --0.04214 J
(a) The International (Steam) Table value of the Btu is by far the most
commonly used value of the Btu

Units of power

The two most commonly used units of power in the United States customary units are mechanical horsepower and Btu/hour. The watt, an SI metric unit of power, is also widely used.

The boiler horsepower is the rate of energy required to evaporate 34.5 lb (15.65 kg) of water at 212 °F (100 °C) in one hour. It is a rarely used unit.

Units of Power
Unit nameShort
form
DivisionsSI Equivalent
1 British thermal unit/hour
(International Table Btu)
Btu/h -- 0.29307 W
1 mechanical horsepower (a)hp-- 745.6999 W
1 boiler horsepower hp--9,809.50 W
1 foot-pound-force/secondft•lbf/s60 ft•lbf/min1.3558 W
(a) 1 mechanical horsepower is 550 ft•lbf/s. Brake horsepower (BHP or bhp) is defined as
mechanical horsepower delivered directly to and measured at the engine's or motor's shaft.

References

  1. Section 8 of Louis E. Barbrow and Lewis V. Judson Weights and Measures Standards of the United States. A brief history (1976)
  2. Appendix 5 of NIST publication SP447
  3. The International System of Units (SI), NIST Special Publication 330, 2001 Edition (pdf page 29 of 77 pdf pages)
  4. Publication AE-945 Agricultural Engineering Department, North Dakota State University, December 1987


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