United States customary units
United States customary units (often incorrectly referred to as
The majority of U.S. customary units were redefined in terms of the meter and kilogram with the Mendenhall Order of 1893 and, in practice, for many years before.[4] These definitions were refined by the international yard and pound agreement of 1959.[5]
The United States uses customary units in commercial activities, as well as for personal and social use. In science, medicine, many sectors of industry, and some government and military areas, metric units are used. The
History
The United States system of units of 1832 is based on the system in use in Britain prior to the introduction to the British
The customary system was championed by the U.S.-based International Institute for Preserving and Perfecting Weights and Measures in the late 19th century. Some advocates of the customary system saw the French Revolutionary, or metric, system as atheistic. The president of an Ohio auxiliary of the Institute wrote that the traditional units were "a just weight and a just measure, which alone are acceptable to the Lord". His organization later went so far as to publish music for a song proclaiming "down with every 'metric' scheme".[8]
The U.S. government passed the
U.S. customary units are widely used on consumer products and in industrial manufacturing. Metric units are standard in science, medicine, as well as many sectors of industry and government, including the
Length
For measuring length, the U.S. customary system uses the
The NAD27 was replaced in the 1980s by the North American Datum of 1983 (NAD83), which is defined in meters. The SPCSs were also updated, but the U.S. National Geodetic Survey left the decision of which (if any) definition of the foot to use to the individual states (and other jurisdictions). All SPCS 1983 systems are defined in meters, but forty jurisdictions also use the survey foot, six use the international foot, and ten do not specify which, if any, foot type should be used.[14]
In 2019, the NIST, working with the National Geodetic Survey (NGS), National Ocean Service (NOS), National Oceanic and Atmospheric Administration (NOAA) and Department of Commerce (DOC), issued a Federal Register Notice (FRN) indicating the deprecation of the U.S. survey foot and U.S. survey mile units from December 31, 2022.[15]
In the following tables in this and subsequent sections, the most common measures are shown in italics, and approximate values are shown in parentheses; values not in parentheses are exact.
International units
Unit | Name | Divisions | SI equivalent |
---|---|---|---|
1 twip | twip |
|
(17.6388888 μm) |
1 mil | 1⁄1000 in | 25.4 μm | |
1 p | point | 1⁄72 in | |
1 P | pica | 12 p | |
inch | 6 P | 25.4 mm | |
|
foot
|
12 in | 0.3048 m[16] |
1 yd | yard | 3 ft | 0.9144 m |
1 mi | mile | 1.609344 km | |
1 le | league | 4.828032 km |
International nautical units
Unit | Name | Divisions | SI equivalent |
---|---|---|---|
1 ftm | fathom | ||
1 cb
|
cable
|
| |
nautical mile |
|
US survey units
Note that as announced by the National Institute of Standards and Technology, the US survey foot, and other units defined in terms of it, have been deprecated since 2023, "except for historic and legacy applications".[15][12][17]
Unit | Name | Divisions | SI equivalent | |
---|---|---|---|---|
Before 2023 | From 2023 | |||
1 li | link | 0.201168 m | ||
1 ft | U.S. survey foot (deprecated since 2023)
|
— | — | |
1 rd |
|
5.0292 m | ||
1 ch | chain | 20.1168 m | ||
1 fur | furlong | 10 ch | 201.168 m | |
1 mi |
|
8 fur | 1.609344 km (international mile) | |
1 lea | league | 3 mi | 4.828032 km |
Area
Unit | Name | Divisions | SI equivalent | |
---|---|---|---|---|
Before 2023 | From 2023 | |||
|
square survey foot
|
144 in2 | (0.09290341 m2) | — |
|
square chain |
|
(404.6873 m2) | 404.68564224 m2 |
1 acre | acre |
|
(4046.873 m2) | 4046.8564224 m2 |
1 section | section |
|
(2.589998 km2) | — |
1 twp | survey township |
|
(93.23993 km2) | — |
The most widely used area unit with a name unrelated to any length unit is the acre. The National Institute of Standards and Technology formerly contended that customary area units are defined in terms of the square survey foot, not the square international foot,[16] but from 2023 it states that "although historically defined using the U.S. survey foot, the statute mile can be defined using either definition of the foot, as is the case for all other units listed in this table. However, use of definitions based on the U.S. survey foot should be avoided after December 31, 2022 except for historic and legacy applications."[17]
Volume
Unit | Name | Divisions | SI equivalent |
---|---|---|---|
|
cubic inch | — | 16.387064 mL[19]
|
|
cubic foot | 1,728 cu in
|
28.316846592 L |
|
cubic yard | 27 cu ft
|
|
1 acre⋅ft | acre-foot |
|
|
The
Although the units and their names are similar to the units in the
As a non-participant in that reform, the U.S. retained the separate systems for measuring the volumes of liquids and dry material, whereas the imperial system had unified the units for both under a new imperial gallon. The U.S. uses the pre-1824 gallon (231 cubic inches, 3,790 cm3) and Winchester bushel (2,150.42 cubic inches, 35,239.1 cm3), as opposed to British 1824 definition of 1 imperial gallon (4.5 L; 1.2 US gal) = 10 lb (4.5 kg) of water and the bushel as 8 imperial gallons (36 L; 9.6 US gal).
Fluid volume
Unit | Name | Divisions | Metric equivalent |
---|---|---|---|
1 min | minim |
|
61.611519921875 μL
|
1 fl dr
|
US fluid dram
|
60 min | 3.6966911953125 mL
|
1 tsp
|
teaspoon
|
80 min | 4.92892159375 mL
|
1 tbsp | tablespoon |
|
14.78676478125 mL
|
|
US fluid ounce
|
|
29.5735295625 mL
|
1 jig | US shot |
|
44.36029434375 mL
|
1 gi | US gill |
|
118.29411825 mL
|
1 c | US cup |
|
236.5882365 mL
|
US pint (liquid) |
|
0.473176473 L | |
US quart (liquid) | 2 US pt | 0.946352946 L | |
1 pot
|
US pottle (liquid)
|
2 US qt | 1.892705892 L |
US gallon (liquid) |
|
3.785411784 L | |
1 bbl | barrel (liquid) | 31.5 US gal
|
119.240471196 L |
1 bbl
|
oil barrel |
|
158.987294928 L |
1 hogshead | hogshead |
|
238.480942392 L |
One US fluid ounce is 1⁄16 of a US pint, 1⁄32 of a US quart, and 1⁄128 of a US gallon. The teaspoon, tablespoon, and cup are defined in terms of a fluid ounce as 1⁄6, 1⁄2, and 8 fluid ounces respectively. The fluid ounce derives its name originally from being the volume of one ounce avoirdupois of water,[citation needed] but in the US it is defined as 1⁄128 of a US gallon. Consequently, a fluid ounce of water weighs about 1.041 ounces avoirdupois.
For nutritional labeling and medicine in the US, the teaspoon and tablespoon are defined as a metric teaspoon and tablespoon—precisely 5 mL and 15 mL respectively.[20]
The saying, "a pint's a pound the world around", refers to 16 US fluid ounces of water weighing approximately (about 4% more than) one pound avoirdupois. An imperial pint of water weighs a pound and a quarter (20 oz).
There are varying standards for barrel for some specific commodities, including 31 gallons for beer, 40 gallons for whiskey or kerosene, and 42 gallons for petroleum. The general standard for liquids is 31.5 gal or half a hogshead. The common 55-gallon size of drum for storing and transporting various products and wastes is sometimes confused with a barrel, though it is not a standard measure.
In the U.S., single servings of beverages are usually measured in fluid ounces. Milk is usually sold in half-pints (8 fluid ounces), pints, quarts, half gallons, and gallons. Water volume for sinks, bathtubs, ponds, swimming pools, etc., is usually stated in gallons or cubic feet. Quantities of gases are usually given in cubic feet (at one atmosphere).
Minims, drams, gill, and pottle are rarely used currently. The gill is often referred to as a "half-cup". The pottle is often referred to as a "half-gallon".
Dry volume
Unit | Name | Divisions | Metric equivalent |
---|---|---|---|
1 pt | pint (dry) | 33.6003125 cu in
|
(0.55061047135749 L) |
1 qt | quart (dry) | 2 pt | (1.1012209427149 L) |
1 gal | gallon (dry) | 4 qt | (4.4048837708599 L) |
1 pk | peck | 2 gal | (8.8097675417199 L) |
1 bu | bushel | 4 pk | (35.239070166879 L) |
1 bbl | barrel (dry) | (115.62712358400 L) |
Mass and weight
Type | Unit | Name | Divisions | SI equivalent |
---|---|---|---|---|
Avoirdupois | 1 gr | grain | 1⁄7000 lb | 64.79891 mg
|
1 dr | dram | 1.771845195 g | ||
1 oz | ounce | 16 drams | 28.349523125 g | |
1 lb | pound | 16 oz | 453.59237 g | |
US (short) hundredweight | 100 lb | 45.359237 kg | ||
long hundredweight | 112 lb | 50.80234544 kg | ||
short ton | 907.18474 kg | |||
1 ton | long ton | 1,016.0469088 kg | ||
Troy | 1 gr | grain |
|
64.79891 mg
|
1 dwt | pennyweight | 1.55517384 g | ||
1 ozt
|
troy ounce
|
20 dwt | 31.1034768 g | |
1 lbt
|
troy pound
|
|
373.2417216 g |
There have historically been five different English systems of mass:
The pound avoirdupois, which forms the basis of the U.S. customary system of mass, is defined as exactly 453.59237 grams by agreement between the U.S., the United Kingdom, and other English-speaking countries in 1959. Other units of mass are defined in terms of it.
The avoirdupois pound is legally defined as a measure of
Troy weight, avoirdupois weight, and apothecaries' weight are all built from 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 apothecary 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
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 avoirdupois ounce.
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 imperial system is identical to that in the U.S. customary system.
In the U.S., only the ounce, pound and short ton – known in the country simply as the ton – are commonly used, though the hundredweight is still used in agriculture and shipping. The grain is used to describe the mass of propellant and projectiles in small arms ammunition. It was also used to measure medicine and other very small masses.
Grain measures
In agricultural practice, a bushel is a fixed volume of 2,150.42 cubic inches (35.2391 liters). The mass of grain will therefore vary according to density. Some nominal weight examples are:[24][25]
- 1 bushel (corn) = 56 lb (25.4012 kg)
- 1 bushel (wheat) = 60 lb (27.2155 kg)
- 1 bushel (barley) = 48 lb (21.7724 kg)
Cooking measures
Measure | Australia | Canada | UK |
US | US FDA[26] |
---|---|---|---|---|---|
Teaspoon | 5 mL |
5 mL |
5 mL |
4.93 mL |
5 mL
|
Dessertspoon |
10 mL |
— | 10 mL |
— | — |
Tablespoon | 20 mL |
15 mL |
15 mL |
14.79 mL |
15 mL
|
Fluid ounce | — | — | 28.41 mL |
29.57 mL |
30 mL
|
Cup |
250 mL |
250 mL |
— | 236.59 mL |
240 mL
|
Pint | 570 mL |
— | 568.26 mL |
473.18 mL |
— |
Quart | — | — | 1.14 L | 0.95 L | — |
Gallon | — | — | 4.55 L | 3.79 L | — |
The most common practical cooking measures for both liquid and dry ingredients in the U.S. are teaspoon, tablespoon, and cup, along with halves, thirds, quarters, and eighths of each. Units used are pounds, ounces, and fluid ounces. Common sizes are also used, such as can (presumed size varies depending on product), jar, square (e.g. of chocolate), stick (e.g. of butter), or portion of fruit or vegetable (e.g. a half lemon, two medium onions).[27]
Temperature
Degrees
The relationship between the different temperature scales is
or inversely as
Other units
Length
Volume
- 1 dm3
Mass
- 1 lbf⋅s2/ft ≈ 14.59390 kg
Force
- 1 poundal = force to accelerate 1 pound mass 1 foot/second/second ≈ 0.138 newtons.
- 1 kip = 1000 lbf ≈ 4.44822 kN
Energy
- 1 foot-pound ≈ 1.356 J
- 1 British thermal unit (Btu) ≈ 1.055 kJ (1,054–1,060 J, depending on which of several definitions of BTU is used)
- 1 Quad 1015 BTU, one quadrillion BTU (short-scale) or 1.055×1018 joule (1.055 exajoules or EJ)
Power
- 1 horsepower ≈ 745.7 W
- 1 ton of refrigeration (12,000 Btu/h) = 3.517 kW
Pressure
- 1 inch of mercury = the pressure produced by 1 inch height of mercury = 3,386.39 pascals (33.8639 hPa, millibars)
- 1 pound per square inch (psi) ≈ 6,895 Pa
Torque
- 1 pound-foot ≈ 1.356 N⋅m
Insulation
- 1 R-value (ft2⋅°F⋅h/Btu) ≈ 0.1761 RSI (K⋅m2/W)
Various combination units are in common use; these are straightforwardly defined based on the above basic units.
Sizing systems are used for various items in commerce, several of which are U.S.-specific:
- US standard clothing size
- American wire gauge is used for most metal wire.
- Scoop (utensil) sizes, numbered by scoops per quart
- Thickness of leather is measured in ounces, 1 oz equals 1⁄64 inch (0.40 mm).[28]
- Bolts and screws follow the Unified Thread Standard rather than the ISO metric screw thread standard.
- Knitting needles in the United States are measured according to a non-linear unitless numerical system.
- Thickness of aluminum foilis measured in mils (1⁄1000 inch, or 0.0254 mm) in the United States.
- Cross-sectional area of electrical wire is measured in circular mils in the U.S. and Canada, one circular mil (cmil) being equal to 5.067×10−4 mm2 (or 7.854×10−7 in2). Since this is so small, actual wire is commonly measured in thousands of a cmil, called either kcmil or MCM.
- The mil or thou is also sometimes used to mean thousandth of an inch.
- Sheet metal in the U.S. is commonly measured in gauge (not to be confused with the American wire gauge), which is derived from weight and thus differs by material.
- Nominal Pipe Size is used for the outside diameter of pipes. Below NPS14, the NPS number is not consistent with the pipe diameter in inches.
- Copper tubing, however, is measured in nominal size, 1⁄8 inch less than the outside diameter.
- The Schedule system is used for standard pipe thicknesses.
- Alcohol content is frequently given in proof, 2 × percent alcohol by volume
- The cord is used for volume of firewood.
- The square is used to mean 100 square feet in construction.
- Heat flux in the U.S. is measured in langleys.
Other names for U.S. customary units
The United States Code refers to these units as "traditional systems of weights and measures".[29]
Other common ways[
Tools and fasteners with sizes measured in inches are sometimes called "SAE bolts" or "SAE wrenches" to differentiate them from their metric counterparts. The Society of Automotive Engineers (SAE) originally developed fasteners standards using U.S. units for the U.S. auto industry; the organization now uses metric units.[31]
See also
- Board foot
- Conversion of units
- units of measurement
- Plan for Establishing Uniformity in the Coinage, Weights, and Measures of the United States (1790)
- Mars Climate Orbiter, which failed due to a measurement-units-related software bug
- Standard cubic foot
References
- from the original on June 3, 2018. Retrieved February 25, 2024.
While virtually all scientists use metric units, many US engineers, both inside and outside the space programme, use imperial units, converting them when necessary.
For nutrition labeling purposes, a teaspoon means 5 milliliters (mL), a tablespoon means 15 mL, a cup means 240 mL, 1 fl oz means 30 mL, and 1 oz in weight means 28 g.
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