United States customary units

Not to be confused withImperial units.

Countries using themetric(SI),imperial,and US customary systems as of 2019

United States customary unitsform a system ofmeasurement unitscommonly used in theUnited Statesand mostU.S. territories,since being standardized and adopted in 1832.TheUnited States customary systemdeveloped fromEnglish unitsthat were in use in theBritish Empirebefore the U.S. became an independent country. The United Kingdom's system of measures was overhauled in 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 themeterandkilogramwith theMendenhall Orderof 1893 and, in practice, for many years before.These definitions were refined by theinternational yard and poundagreement of 1959.

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).For newer types of measurement where there is no traditional customary unit, international units are used, sometimes mixed with customary units: for example,electrical resistanceof wire expressed inohms(SI) per thousand feet.

History

See also:Imperial and US customary measurement systemsandComparison of the imperial and US customary measurement systems

The United States system of units of 1832 is based on the system in use in Britain prior to the introduction to the Britishimperial systemon January 1, 1826.Both systems are derived fromEnglish units,a system which had evolved over the millennia beforeAmerican independence,and which had its roots in bothRomanandAnglo-Saxonunits.

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".

The U.S. government passed theMetric Conversion Actof 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 itvoluntarilyconverts 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 withLiberiaandMyanmar (Burma)) that have not adopted the metric system as their official system of weights and measures.

Executive Order 12770,signed by PresidentGeorge H. W. Bushon July 25, 1991, citing the Metric Conversion Act, directed departments and agencies within theexecutive branchof theUnited States Governmentto "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."

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 themilitary.There are anecdotal objections to the use of metric units in carpentry and the building trades, on the basis that it is easier to remember an integer number of inches plus a fraction than a measurement in millimeters,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.

The term "United States customary units" was used by the formerUnited States National Bureau of Standards,although "English units" is sometimes used in colloquial speech.

Length

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(approximately0.3048006m).Since July 1, 1959, the units of length have been defined on the basis of1yd=0.9144 m.The U.S., theUnited Kingdomand otherCommonwealthcountries agreed on this definition per theInternational Yard and Pound Agreementof 1958. At the time of the agreement, the basicgeodetic datumin North America was theNorth American Datumof 1927 (NAD27), which had been constructed bytriangulationbased on the definition of the foot in theMendenhall Orderof 1893, that is1ft=1200⁄3937m:this definition was retained for data derived from NAD27, but renamed theUS survey footto distinguish it from the international foot.For most applications, the difference between the two definitions is insignificant – one international foot is exactly0.999998of a US survey foot, for a difference of about1⁄8in (3 mm) per mile – but it affects the definition of theState Plane Coordinate Systems(SPCSs), which can stretch over hundreds of miles.

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 Surveyleft 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.

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.

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.

International units

List of international units

Unit	Name	Divisions	SI equivalent
1twip	twip	1⁄20p 1⁄1440in	(17.6388888μm)
1mil	thousandth of an inch mil	1⁄1000in	25.4 μm
1 p	point	1⁄72in	127⁄360mm (352.778μm)
1 P	pica	12 p	127⁄30mm (4.233mm)
1in 1″	inch	6 P	25.4mm
1ft 1′	foot	12in	0.3048m
1yd	yard	3ft	0.9144m
1mi	mile	5280ft 1760yd	1.609344km
1le	league	3mi 15840ft	4.828032km

International nautical units

List of international nautical units

Unit	Name	Divisions	SI equivalent
1ftm	fathom	2yd 6ft	1143⁄625m 1.8288m
1cb	cable	120ftm 1.091fur	3429⁄15625km 0.219456km
1nmi 1NM	nautical mile	1.151mi 8.439cb	1852m 1.852km

US survey units

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".

List of US Survey units

Unit	Name	Divisions	SI equivalent
			Before 2023	From 2023
1li	link	33⁄50ft 7.92in	792⁄3937m (0.20117m)	0.201168m
1 ft	U.S. survey foot(deprecated since 2023)	—	1200⁄3937m (0.3048006m)	—
1rd	rod "pole" "perch"	25li 16.5ft	19,800⁄3937m (5.029m)	5.0292m
1ch	chain	4rd 66ft	79,200⁄3937m 20.117m	20.1168m
1fur	furlong	10ch	792⁄3937km (201.168m)	201.168m
1mi	U.S. survey mile "statute mile"	8fur	6336⁄3937km (1.609347km)	1.609344km(international mile)
1lea	league	3mi	19,008⁄3937km (4.828 km)	4.828032km

Area

List of US units of area

Unit	Name	Divisions	SI equivalent
			Before 2023	From 2023
1sq ft 1 ft2	square surveyfoot	144in2	(0.09290341m2)	—
1 sq ch 1 ch2	squarechain	4356 sq ft (survey) 16 sq rod	(404.6873 m2)	404.68564224m2
1acre	acre	43560sq ft (survey) 10 sq ch	(4046.873 m2)	4046.8564224m2
1section	section	640acres 1 sq mi (survey)	(2.589998km2)	—
1 twp	survey township	36sections 16 sq lea	(93.23993km2)	—

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,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."

Volume

A 23.7 US fl oz (700 mL) bottle displaying both U.S. and metric units

List of US units of volume

Unit	Name	Divisions	SI equivalent
1 cu in 1in3	cubic inch	—	16.387064mL
1 cu ft 1 ft3	cubic foot	1,728cu in	28.316846592L
1 cu yd 1 yd3	cubic yard	27cu ft	764.554857984 L 0.764554857984m3
1acre⋅ft	acre-foot	43,560cu ft (1,613.333cu yd)	1.23348183754752ML (1,233.482m3)

Thecubic inch,cubic footandcubic yardare commonly used for measuring volume. In addition, there is one group of units for measuring volumes of liquids (based on thewine gallonandsubdivisions 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).

Fluid volume

List of US units of volume for liquids

Unit	Name	Divisions	Metric equivalent
1min	minim	(1drop) of water (0.95grain) of water	61.611519921875μL
1fl dr	US fluid dram	60min	3.6966911953125mL
1tsp	teaspoon	80min	4.92892159375mL
1tbsp	tablespoon	3tsp 4fl dr	14.78676478125mL
1floz 1US fl oz	US fluid ounce	2tbsp (1.0408imp fl ozof water)	29.5735295625mL
1jig	US shot	1.5US fl oz 3tbsp	44.36029434375mL
1gi	US gill	22⁄3jig 4US fl oz	118.29411825mL
1c	US cup	2gi 8US fl oz	236.5882365mL
1pint 1US pt	US pint(liquid)	2c (16.65imp fl ozof water)	0.473176473L
1qt 1US qt	US quart(liquid)	2US pt	0.946352946L
1pot	US pottle(liquid)	2US qt	1.892705892L
1gal 1US gal	US gallon(liquid)	4US qt 231cu in	3.785411784L
1bbl	barrel(liquid)	31.5US gal	119.240471196L
1bbl	oil barrel	11⁄3bbl (liquid) 42US gal	158.987294928L
1hogshead	hogshead	2bbl (liquid) 1.5bbl (oil) 63US gal 8.421875cu ft	238.480942392L

One US fluid ounce is1⁄16of a US pint,1⁄32of a US quart, and1⁄128of a US gallon. The teaspoon, tablespoon, and cup are defined in terms of a fluid ounce as1⁄6,1⁄2,and 8 fluid ounces respectively. The fluid ounce derives its name originally from being the volume of one ounceavoirdupoisof water,but in the US it is defined as1⁄128of 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 mLand15 mLrespectively.

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 forbarrelfor 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 ofdrumfor 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". Thepottleis often referred to as a "half-gallon".

Dry volume

List of US units of volume for dry goods

Unit	Name	Divisions	Metric equivalent
1pt	pint(dry)	33.6003125cu in	(0.55061047135749 L)
1qt	quart(dry)	2pt	(1.1012209427149 L)
1gal	gallon(dry)	4qt	(4.4048837708599 L)
1pk	peck	2gal	(8.8097675417199 L)
1bu	bushel	4pk	(35.239070166879 L)
1bbl	barrel(dry)	7056 cu in 26.25gal (3.281bu)	(115.62712358400 L)

Dry volumeis 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. The US dry gallon is less commonly used, and was not included in the handbook that many states recognize as the authority on measurement law.Howeverpecks,orbushelsare sometimes used—particularly forgrapes,applesand similarfruitsin agricultural regions.

Mass and weight

Main articles:Pound (force)andPound (mass)

Conversions

Type	Unit	Name	Divisions	SI equivalent
Avoirdupois	1gr	grain	1⁄7000lb	64.79891mg
	1dr	dram	27+11⁄32gr (8.859carats)	1.771845195g
	1oz	ounce	16drams	28.349523125g
	1lb	pound	16oz	453.59237g
	1cwt 1US cwt	US (short)hundredweight	100lb	45.359237kg
	1cwt 1long cwt	longhundredweight	112lb	50.80234544kg
	1ton 1short ton	short ton	20US cwt 2,000lb	907.18474kg
	1ton	long ton	20long cwt 2,240lb	1,016.0469088kg
Troy	1gr	grain	1⁄7000lb 1⁄5760lbt	64.79891mg
	1dwt	pennyweight	24gr (7.776carats)	1.55517384g
	1ozt	troy ounce	20dwt	31.1034768g
	1lbt	troy pound	12ozt (13.16571429oz)	373.2417216g

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. Theimperialsystem, 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 453.59237gramsby 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,but the name pound is also applied to measures offorce.For instance, in many contexts, thepoundavoirdupois is used as a unit of mass, but in some contexts, the term "pound" is used to refer to "pound-force".Theslugis 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 dramunit 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.

Grain measures

In agricultural practice, abushelis 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:

• 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

Main article:Cooking weights and measures

Common volume measures in English-speaking countries

(Comparable measures listed for comparison purposes.)

Measure	Australia	Canada	UK	US	USFDA
Teaspoon	5mL	5mL	5mL	4.93mL	5mL
Dessertspoon	10mL	—	10mL	—	—
Tablespoon	20mL	15mL	15mL	14.79mL	15mL
Fluid ounce	—	—	28.41mL	29.57mL	30mL
Cup	250mL	250mL	—	236.59mL	240mL
Pint	570mL	—	568.26mL	473.18mL	—
Quart	—	—	1.14L	0.95L	—
Gallon	—	—	4.55L	3.79L	—

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).

Temperature

DegreesFahrenheitare used in the U.S. to measuretemperaturesin most non-scientific contexts. TheRankine scaleofabsolute temperaturealso saw some use inthermodynamics.Scientists worldwide use thekelvinanddegree 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 islinearbut the scales have different zero points, so conversion is not simply multiplication by a factor. Pure water freezes at32 °F = 0 °Cand boils at212 °F = 100 °Cat 1atm.The conversion formula is: $$\{T\}_{\mathrm {^{\circ }F} }={\tfrac {9}{5}}\{T\}_{\mathrm {^{\circ }C} }+32$$

or inversely as $$\{T\}_{\mathrm {^{\circ }C} }={\tfrac {5}{9}}{\bigl (}\{T\}_{\mathrm {^{\circ }F} }-32{\bigr )}.$$

Other units

Length

• 1hand= 4 in = 101.6 mm
• 1 U (rack unit) = 1.75 in = 44.45 mm

Volume

• 1board-foot= 1 ft × 1 ft × 1 in = 2.360dm³

Mass

• 1slug= 1lbf⋅s²/ft ≈ 14.59390 kg

Force

• 1poundal= force to accelerate 1 pound mass 1 foot/second/second ≈ 0.138newtons.
• 1kip= 1000 lbf ≈ 4.44822kN

Energy

• 1foot-pound≈ 1.356 J
• 1British thermal unit(Btu) ≈ 1.055 kJ (1,054–1,060 J, depending on which of several definitions of BTU is used)
• 1Quad10¹⁵BTU, one quadrillion BTU (short-scale) or 1.055×10¹⁸joule (1.055 exajoules or EJ)

Power

• 1horsepower≈ 745.7W
• 1ton of refrigeration(12,000 Btu/h) = 3.517 kW

Pressure

• 1inch of mercury= the pressure produced by 1 inch height of mercury = 3,386.39 pascals (33.8639 hPa, millibars)
• 1pound per square inch(psi) ≈ 6,895 Pa

Torque

• 1pound-foot≈ 1.356N⋅m

Insulation

• 1R-value(ft²⋅°F⋅h/Btu) ≈ 0.1761R_SI(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 gaugeis used for most metal wire.
• Scoop (utensil)sizes, numbered by scoops per quart
• Thickness ofleatheris measured in ounces, 1 oz equals1⁄64inch (0.40 mm).
• Bolts and screws follow theUnified Thread Standardrather than theISO metric screw threadstandard.
• Knitting needlesin the United States are measured according to anon-linear unitless numerical system.
• Thickness ofaluminum foilis measured inmils(1⁄1000inch, or 0.0254 mm) in the United States.
• Cross-sectional area of electrical wire is measured incircular milsin the U.S. and Canada, one circular mil (cmil) being equal to5.067×10⁻⁴mm²(or7.854×10⁻⁷in²). Since this is so small, actual wire is commonly measured in thousands of a cmil, called either kcmil or MCM.
• Themilorthouis also sometimes used to meanthousandth of an inch.
• Sheet metalin the U.S. is commonly measured ingauge(not to be confused with the American wire gauge), which is derived from weight and thus differs by material.
• Nominal Pipe Sizeis 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 innominal size,1⁄8inch less than the outside diameter.
• TheSchedulesystem is used for standard pipe thicknesses.
• Alcohol content is frequently given inproof,2 × percentalcohol by volume
• Thecordis used for volume of firewood.
• Thesquareis used to mean 100 square feet in construction.
• Heat fluxin the U.S. is measured inlangleys.

Other names for U.S. customary units

The United States Code refers to these units as "traditional systems of weights and measures".

Other common waysof referring to the system are:customary,standard,English,orimperial(which refers to the post-1824 reform measures used throughout the British Empire & Commonwealth countries).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.

See also

• Board foot
• Conversion of units
• History of measurementandunits 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

External links

• Rowlett's Dictionary of Units of Measurement (archived)