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United States customary units form a system ofmeasurement units commonly used in theUnited States and mostU.S. territories[1] since being standardized and adopted in 1832.[2] TheUnited States customary system developed fromEnglish units that were in use in theBritish Empire before the U.S. became an independent country. The United Kingdom's system of measures evolved by 1824 to create the imperial system (withimperial units), which was officially adopted in 1826, changing the definitions of some of its units. Consequently, while many U.S. units are essentially similar to their imperial counterparts, there are noticeabledifferences between the systems.
The majority of U.S. customary units were redefined in terms of themeter andkilogram with theMendenhall Order of 1893 and, in practice, for many years before.[3] These definitions were refined by theinternational yard and pound agreement of 1959.[4]
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. TheInternational System of Units (SI), the modern form of themetric system, is preferred for many uses by the U.S.National Institute of Standards and Technology (NIST).[5] For newer types of measurement where there is no traditional customary unit, international units are used, sometimes mixed with customary units: for example,electrical resistance of wire expressed inohms (SI) per thousand feet.
The United States customary system of units of 1832 is based on the system in use in the United Kingdom prior to the introduction to the Britishimperial system on January 1, 1826.[6] Both systems are derived fromEnglish units, an older system of units which had evolved over the millennia beforeAmerican independence, and which had its roots in bothRoman andAnglo-Saxon units.
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".[7]
The U.S. government passed theMetric Conversion Act of 1975, which made themetric system "the preferred system of weights and measures for U.S. trade and commerce". The legislation states that the federal government has a responsibility to assist industry as itvoluntarily converts to the metric system, i.e.,metrification. This is most evident in U.S. labeling requirements on food products, where SI units are almost always presented alongside customary units. According to theCIA World Factbook, the United States is one of three nations (along withLiberia andMyanmar (Burma)) that have not adopted the metric system as their official system of weights and measures.[8]
Executive Order 12770, signed by PresidentGeorge H. W. Bush on July 25, 1991, citing the Metric Conversion Act, directed departments and agencies within theexecutive branch of theUnited States Government to "take all appropriate measures within their authority" to use the metric system "as the preferred system of weights and measures for United States trade and commerce" and authorized theSecretary of Commerce "to charter an Interagency Council on Metric Policy ('ICMP'), which will assist the Secretary in coordinating Federal Government-wide implementation of this order." Implementation has been limited.SeeMetrication in the United States.
U.S. customary units are widely used on consumer products and in industrial manufacturing. Metric units are standard in the fields of science, medicine, and engineering, as well as many sectors of industry and government, including themilitary.[8] There are anecdotal objections to the use of metric units incarpentry and the building trades, on the basis that it is easier to remember an integer number of inches plus a fraction, rather than a measurement in millimeters,[9] or that foot-inch measurements are more suitable when distances are frequently divided into halves, thirds, and quarters, often in parallel. The metric system also lacks a parallel measurement to thefoot.[10]
The term "United States customary units" was used by the formerUnited States National Bureau of Standards,[11] although "English units" is sometimes used in colloquial speech.[12]
For measuring length, the U.S. customary system uses theinch,foot,yard, andmile, which are the only four customary length measurements in everyday use. From 1893, the foot was legally defined as exactly1200⁄3937m (approximately0.3048006 m).[13] Since July 1, 1959, the units of length have been defined on the basis of1 yd =0.9144 m, which makes the foot exactly0.3048 m.[4] The U.S., theUnited Kingdom and otherCommonwealth countries agreed on this definition per theInternational Yard and Pound Agreement of 1958. At the time of the agreement, the basicgeodetic datum in North America was theNorth American Datum of 1927 (NAD27), which had been constructed bytriangulation based on the definition of the foot in theMendenhall Order of 1893, that is1 ft =1200⁄3937m: this definition was retained for data derived from NAD27, but renamed theUS survey foot to distinguish it from the international foot.[4] For most applications, the difference between the two definitions is insignificant – one international foot is exactly0.999998 of a US survey foot, for a difference of about1⁄8 in (3 mm) per mile – but it affects the definition of theState Plane Coordinate Systems (SPCSs), which can stretch over hundreds of miles.[14]
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 theU.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.[15]
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.[16]
In the following tables in this and subsequent sections, the most common measures are shown initalics, and approximate values are shown in parentheses; values not in parentheses are exact.
| Unit | Name | Divisions | SI equivalent |
|---|---|---|---|
| 1 twip | twip |
| 127/7200mm (17.638 μm) |
| 1 mil | 1/1000in | 25.4 μm | |
| 1 p | point | 1/72 in | 127/360mm (352.7 μm) |
| 1 P | pica | 12 p | 127/30mm (4.23 mm) |
| inch | 6 P | 25.4 mm | |
| foot | 12 in | 0.3048 m[17] | |
| 1 yd | yard | 3 ft | 0.9144 m |
| 1 mi | mile | 1.609344 km | |
| 1 le | league | 4.828032 km |
| Unit | Name | Divisions | SI equivalent |
|---|---|---|---|
| 1 ftm | fathom | ||
| 1 cb | cable |
| |
| nautical mile |
Note that as announced by theNational 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".[16][13][18]
| 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 | |
| Unit | Name | Divisions | SI equivalent | |
|---|---|---|---|---|
| Before 2023 | From 2023 | |||
| squarefoot | 144 in2 (international) | 0.0929034 m2 | 0.0929034 m2 |
| squareyard | 9 sq ft | 0.83612736 m2 | 0.83612736 m2 |
| squarechain |
| (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 theacre. 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,[17] 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."[18]
| Unit | Name | Divisions | SI equivalent |
|---|---|---|---|
| cubic inch | — | 16.387064 mL[20] |
| cubic foot | 1,728 cu in | 28.316846592 L |
| cubic yard | 27 cu ft |
|
| 1 acre⋅ft | acre-foot |
Thecubic inch,cubic foot andcubic yard are commonly used for measuring volume. In addition, there is one group of units for measuring volumes of liquids (based on thewine gallon andsubdivisions of the fluid ounce), and one for measuringvolumes of dry material, each with their own names and sub-units.
Although the units and their names are similar to the units in theimperial system, and many units are shared between the two systems as a whole; with respect to volume, however, this is quite the contrary. The independence of the U.S. from the British Empire decades prior to the reformation of units in 1824—most notably thegallon, its subdivisions, and (in mass) higher combinations above thepound—is the cause of the differences in values.
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).
| 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 is1⁄16 of a US pint,1⁄32 of a US quart, and1⁄128 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 ounceavoirdupois of water,[21] but in the US it is defined as1⁄128 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—precisely5 mL and15 mL respectively.[22]
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 forbarrel 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 ofdrum 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". Thepottle is often referred to as a "half-gallon".
| Unit | Name | Divisions | Metric equivalent |
|---|---|---|---|
| 1 pt | pint (dry) | 33.6003125 cu in | 0.5506104713575 L |
| 1 qt | quart (dry) | 2 pt | 1.101220942715 L |
| 1 gal | gallon (dry) | 4 qt | 4.40488377086 L |
| 1 pk | peck | 2 gal | 8.80976754172 L |
| 1 bu | bushel | 4 pk | 35.23907016688 L |
| 1 bbl | barrel (dry) | (115.627123584 L) |
Dry volume is measured on a separate system, although many of the names remain the same. Small fruits and vegetables are often sold in dry pints and dry quarts.
While the US dry gallon is no longer used, and is no longer included in the handbook that many states recognize as the authority on measurement law, it implicitly exists since the US dry measures of bushel, peck, quart and pint are still in use.[23]Pecks andbushels are sometimes used, particularly forgrapes,apples and similarfruits in agricultural regions.
| Type | Unit | Name | Divisions | SI equivalent |
|---|---|---|---|---|
| Avoirdupois | 1 gr | grain | 1⁄7000lb | 64.79891 mg |
| 1 dr | dram | 1.771845195 g | ||
| 1 oz | ounce | 16drams | 28.349523125 g | |
| 1 lb | pound | 16 oz | 453.59237 g | |
| US (short)hundredweight | 100 lb | 45.359237 kg | ||
| longhundredweight | 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:tower,apothecaries',troy,avoirdupois, andmetric. Of these, the avoirdupois weight is the most common system used in the U.S., although Troy weight is still used to weigh precious metals. Apothecaries' weight—once used by pharmacies—has been largely replaced by metric measurements. Tower weight fell out of use in England (due to legal prohibition in 1527) centuries ago, and was never used in the U.S. Theimperial system, which is still used for some measures in the United Kingdom and other countries, is based on avoirdupois, with variations from U.S. customary units larger than a pound.
The pound avoirdupois, which forms the basis of the U.S. customary system of mass, is defined as exactly 453.59237grams 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 ofmass,[24] but the name pound is also applied to measures offorce. For instance, in many contexts, thepound avoirdupois is used as a unit of mass, but in some contexts, the term "pound" is used to refer to "pound-force". Theslug is another unit of mass derived from pound-force.
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 anddram, troy haspennyweight, and avoirdupois has just dram, sometimes spelleddrachm). The dram in avoirdupois weighs just under half of the dram in apothecaries'. Thefluid dram unit of volume is based on the weight of 1 dram of water in the apothecaries' system.
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 armsammunition. It was also used to measure medicine and other very small masses.
In agricultural practice, abushel 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:[25][26]
| Measure | Australia | Canada | UK | US | USFDA[27] |
|---|---|---|---|---|---|
| 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. areteaspoon,tablespoon, andcup, along with halves, thirds, quarters, and eighths of each. Units used arepounds,ounces, andfluid ounces. Common sizes are also used, such ascan (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).[28]
DegreesFahrenheit are used in the U.S. to measuretemperatures in most non-scientific contexts. TheRankine scale ofabsolute temperature is used in engineeringthermodynamics when the Fahrenheit scale is employed. (0 °Ra =absolute zero = –459.67 °F) Scientists worldwide use thekelvin anddegree Celsius. Several U.S. technical standards are expressed in Fahrenheit temperatures, and some American medical practitioners use degrees Fahrenheit for body temperature.
The relationship between the different temperature scales islinear but the scales have different zero points, so conversion is not simply multiplication by a factor. Pure water freezes at32 °F = 0 °C and boils at212 °F = 100 °C at 1atm. The conversion formula is:
or inversely as
Length
Volume
Mass
Force
Energy
Power
Pressure
Torque
Insulation
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:
The United States Code refers to these units as "traditional systems of weights and measures".[30]
Other common ways[citation needed] of referring to the system are:customary,standard,English, orimperial (which refers to the post-1824 reform measures used throughout the British Empire & Commonwealth countries).[31] Another term is thefoot–pound–second (FPS) system, as opposed tocentimeter–gram–second (CGS) andmeter–kilogram–second (MKS) systems.
Tools and fasteners with sizes measured in inches are sometimes called "SAE bolts" or "SAE wrenches" to differentiate them from their metric counterparts. TheSociety of Automotive Engineers (SAE) originally developed fasteners standards using U.S. units for the U.S. auto industry; the organization now uses metric units.[32]
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.
