United States customary units#Fluid volume

{{See also|Imperial and US customary measurement systems|Comparison of the imperial and US customary measurement systems}}

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 British imperial system on January 1, 1826."English units of measurement". The Columbia Encyclopedia 6th ed. 2001-2007. [https://web.archive.org/web/20080714214207/http://www.bartleby.com/65/en/Englsh-u.html archived copy]. Both systems are derived from English units, an older system of units which had evolved over the millennia before American independence, and which had its roots in both Roman and Anglo-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".{{cite book |author=Martin Gardner |title=Fads and Fallacies in the Name of Science |url=https://books.google.com/books?id=X0HCAgAAQBAJ&pg=PA180 |date=May 4, 2012 |publisher=Courier Corporation |isbn=978-0-486-13162-7 |pages=180–}}

The U.S. government passed the Metric Conversion Act of 1975, which made the metric 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 it voluntarily 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 the CIA World Factbook, the United States is one of three nations (along with Liberia and Myanmar (Burma)) that have not adopted the metric system as their official system of weights and measures.{{Cite book |chapter-url=https://www.cia.gov/the-world-factbook/references/weights-and-measures/ |title=The World Factbook |publisher=Central Intelligence Agency |location=Washington, D.C. |chapter=References - Weights and Measures |access-date=April 22, 2021}}

Executive Order 12770, signed by President George H. W. Bush on July 25, 1991, citing the Metric Conversion Act, directed departments and agencies within the executive branch of the United 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 the Secretary 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. See Metrication 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 the military. 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, rather than a measurement in millimeters,Robyn Williams (February 8, 1998) [http://www.abc.net.au/radionational/programs/ockhamsrazor/trouble-with-the-metric-system/3552102 "Trouble with the Metric System"]. Australian Radio National, Ockham's Razor. 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 the foot.Ed Tenner, (May 2005). [https://www.technologyreview.com/2005/05/01/39811/the-trouble-with-the-meter/ "The Trouble with the Meter"]. Technologyreview.com.

The term "United States customary units" was used by the former United States National Bureau of Standards,{{cite book |author= |date=1960 |title=Units of Weight and Measure (United States Customary and Metric}: Definitions and Tables of Equivalents |url=https://books.google.com/books?id=ejpUAAAAYAAJ&pg=PA1 |location=Washington DC |publisher=United States National Bureau of Standards |page=1 }} although "English units" is sometimes used in colloquial speech.{{cite journal |author= |title=Policy for NBS usage of SI Units |page=18 |date=January 1971 |journal=Technical News Bulletin |url=https://books.google.com/books?id=Efah8_3glg4C&pg=PA18 |volume=55 |issue=1 |location=Washington DC |publisher=United States National Bureau of Standards }}

File:Feet on tape measure 20180321.jpg with markings in inches, feet and centimeters]]

For measuring length, the U.S. customary system uses the inch, foot, yard, and mile, which are the only four customary length measurements in everyday use. From 1893, the foot was legally defined as exactly {{nowrap|{{frac|1200|3937}} {{val||ul=meter}}}} (approximately {{val|0.3048006|u=m}}). Since July 1, 1959, the units of length have been defined on the basis of {{val|1|ul=yard}} = {{val|0.9144|u=m}}, which makes the foot exactly {{val|0.3048|u=m}}. The U.S., the United Kingdom and other Commonwealth countries agreed on this definition per the International Yard and Pound Agreement of 1958. At the time of the agreement, the basic geodetic datum in North America was the North American Datum of 1927 (NAD27), which had been constructed by triangulation based on the definition of the foot in the Mendenhall Order of 1893, that is {{val|1|ul=foot}} = {{nowrap|{{frac|1200|3937}} {{val||ul=meter}}}}: this definition was retained for data derived from NAD27, but renamed the US survey foot to distinguish it from the international foot. For most applications, the difference between the two definitions is insignificant – one international foot is exactly {{val|0.999998}} of a US survey foot, for a difference of about {{cvt|1/8|in|mm|0}} per mile – but it affects the definition of the State Plane Coordinate Systems (SPCSs), which can stretch over hundreds of miles.{{Cite web|title=Frequently Asked Questions about the National Geodetic Survey|url=http://www.ngs.noaa.gov/faq.shtml#Feet|access-date=May 16, 2009|work=National Geodetic Survey}}

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.{{Cite web| title = State Plane Coordinate System of 1983: legislation and foot version| date=n.d.| access-date = February 25, 2024| url = https://geodesy.noaa.gov/SPCS/images/spcs83-legislation-feet.png}}

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 in italics, and approximate values are shown in parentheses; values not in parentheses are exact.

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".{{Cite web |title=Deprecation of the United States (U.S.) Survey Foot |url=https://www.federalregister.gov/documents/2020/10/05/2020-21902/deprecation-of-the-united-states-us-survey-foot |access-date=October 19, 2023 |website=Federal Register |publication-date=October 5, 2020}}{{Cite web| title = U.S. Survey Foot| work = NIST| accessdate = March 29, 2023| date = c. 2022| url = https://www.nist.gov/pml/us-surveyfoot}}{{Cite web| title= U.S. Survey Foot: Revised Unit Conversion Factors| work= NIST| accessdate= December 14, 2023| date = September 23, 2019| url= https://www.nist.gov/pml/us-surveyfoot/revised-unit-conversion-factors |publisher = National Institute of Standards and Technology }} Updated January 4, 2023.

| {{val|8|u=fur}}

| {{ubl|{{nowrap|{{frac|6336|3937}} {{val||ul=km}}}}Roberts, R.W. (February 3, 1975). Federal Register republished in Barbrow, L.E. and Judson, L. V. (1976) [https://www.govinfo.gov/content/pkg/GOVPUB-C13-691a9b38e29a85d0925f4db586b60735/pdf/GOVPUB-C13-691a9b38e29a85d0925f4db586b60735.pdf Weights and Measures of the United States]. National Bureau of Standards Special Publication 447. p. 36|({{val|1.609347|ul=km}})}}

|{{val|1.609344|u=km}} (international mile)

|-

| {{val|1|u=lea}}

| league

| {{val|3|ul=mi}}

| {{ubl|{{nowrap|{{frac|19,008|3937}} {{val||ul=km}}}}Barton, John W.; Bowers, Shelby L.; Butcher, Tina G, Harshman, Richard A.; Lee, G. Diane & Warfield, Lisa, eds. [https://www.nist.gov/pml/weights-and-measures/publications/nist-handbooks/handbook-44-current-edition Specifications, Tolerances, and Other Technical Requirements for Weighing and Measuring Devices] 2022 Edition. n.p.: National Institute of Standards and Technology. pp. B-7, C-20.| ({{val|4.828|u=km}})}}

| {{val|4.828032|ul=km}}

|}

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, 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."

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File:Metric shampoo bottle.jpg

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 (based on the wine gallon and subdivisions of the fluid ounce), and one for measuring volumes of dry material, each with their own names and sub-units.

Although the units and their names are similar to the units in the imperial 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 the gallon, its subdivisions, and (in mass) higher combinations above the pound—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 ({{convert|231|in3|disp=comma}}) and Winchester bushel ({{convert|2150.42|in3|disp=comma}}), as opposed to British 1824 definition of {{convert|1|impgal}} {{=}} {{cvt|10|lb}} of water and the bushel as {{convert|8|impgal}}.

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One US fluid ounce is {{frac|1|16}} of a US pint, {{frac|1|32}} of a US quart, and {{frac|1|128}} of a US gallon. The teaspoon, tablespoon, and cup are defined in terms of a fluid ounce as {{frac| 1|6}}, {{frac|1|2}}, and 8 fluid ounces respectively. The fluid ounce derives its name originally from being the volume of one ounce avoirdupois of water,{{Cite web |date=November 25, 2024 |title=Ounce {{!}} Imperial, Avoirdupois & Metric {{!}} Britannica |url=https://www.britannica.com/science/ounce |access-date=December 1, 2024 |website=www.britannica.com |language=en}} but in the US it is defined as {{frac|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{{mdash}}precisely {{val|5|u=mL}} and {{val|15|u=mL}} respectively.21CFR101.9(b)(5)(viii) [http://edocket.access.gpo.gov/cfr_2004/aprqtr/21cfr101.9.htm 2 1CFR101.9]

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 ({{val|20|u=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".

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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.109th National Conference on Weights and Measures (2025). [https://www.nist.gov/document/2025-nist-handbook-44 Specifications, Tolerances, and Other Technical Requirements for Weighing and Measuring Devices]. National Institute of Standards and Technology. §4.45 "Dry Measures". Pecks and bushels are sometimes used, particularly for grapes, apples and similar fruits in agricultural regions.

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{{main|Pound (force)|Pound (mass)}}

There have historically been five different English systems of mass: tower, apothecaries', troy, avoirdupois, and metric. 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. The imperial 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 {{convert|1|lb|g|5|disp=out|lk=out|abbr=off}} 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 mass,{{Cite web|url=https://www.nist.gov/document/2025-nist-handbook-44|title=NIST Handbook 44, Appendix C, General Tables of Units of Measurement, page C-7 Avoirdupois Units of Mass}} but the name pound is also applied to measures of force. For instance, in many contexts, the pound avoirdupois is used as a unit of mass, but in some contexts, the term "pound" is used to refer to "pound-force". The slug 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 and dram, troy has pennyweight, and avoirdupois has just dram, sometimes spelled drachm). The dram in avoirdupois weighs just under half of the dram in apothecaries'. The fluid 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{{snd}} known in the country simply as the ton{{snd}} 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.

In agricultural practice, a bushel is a fixed volume of {{convert|2150.42|in3|L|abbr=off|sp=us}}. The mass of grain will therefore vary according to density. Some nominal weight examples are:{{Cite web|title=United States Department of Agriculture|url=https://www.ers.usda.gov/data-products/feed-grains-database/documentation/|date=August 20, 2019|website=United States Department of Agriculture|access-date=May 28, 2020}}{{Cite web|title=Weights and Measures of agricultural commodities|url=https://www.nass.usda.gov/Statistics_by_State/Delaware/Publications/Annual_Statistical_Bulletin/2009/Page29.pdf|website=United States Department of Agriculture-National Agricultural Statistics Service|access-date=May 28, 2020}}

• 1 bushel (corn) = {{cvt|56|lb|kg|4}}
• 1 bushel (wheat) = {{cvt|60|lb|kg|4}}
• 1 bushel (barley) = {{cvt|48|lb|kg|4}}

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{{main|Cooking weights and measures}}

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).{{Cite book |last=Graham |first=Eunice |chapter=Common Measurements with Factor Labeling |chapter-url=https://openwa.pressbooks.pub/culinarymath/chapter/common-measurements-with-factor-labeling/ |title=Culinary Math |publisher=Open Washington Pressbooks |language=en}}

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Degrees Fahrenheit are used in the U.S. to measure temperatures in most non-scientific contexts. The Rankine scale of absolute temperature is used in engineering thermodynamics when the Fahrenheit scale is employed. (0 °Ra = absolute zero = –459.67 °F) Scientists worldwide use the kelvin and degree 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 is linear but the scales have different zero points, so conversion is not simply multiplication by a factor. Pure water freezes at {{nowrap|1=32 °F = 0 °C}} and boils at {{nowrap|1=212 °F = 100 °C}} at 1 atm. The conversion formula is:

$$\backslash \{T\backslash \}\_\backslash mathrm\{^\backslash circ\; F\}\; =\; \backslash tfrac\{9\}\{5\}\; \backslash \{T\backslash \}\_\backslash mathrm\{^\backslash circ\; C\}\; +\; 32$$

or inversely as

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

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Length

• 1 hand = 4 in = {{cvt|4.000|in|disp=out}}
• 1 U (rack unit) = 1.75 in = {{cvt|1.7500|in|disp=out}}

Volume

• 1 board-foot = 1 ft × 1 ft × 1 in = {{cvt|144.0|in3|L|disp=out|lk=on}}

Mass

• 1 slug = 1 lbf⋅s²/ft ≈ 14.59390 kg

Force

• 1 poundal = force to accelerate 1 pound mass 1 foot/second/second ≈ {{convert|1|pdl|N|3|disp=out|lk=on|abbr=off}}.
• 1 kip = 1000 lbf ≈ {{cvt|1000.000|lbf|kN|disp=out|lk=on}}

Energy

• 1 foot-pound ≈ {{cvt|1.000|ft.lbf|J|disp=out}}
• 1 British thermal unit (Btu) ≈ {{cvt|1.000|Btu|kJ|disp=out}} (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 ≈ {{cvt|1.0000|hp|W|disp=out|lk=on}}
• 1 ton of refrigeration (12,000 Btu/h) = {{cvt|12000.|Btu/h|kW|disp=out}}

Pressure

• 1 inch of mercury = the pressure produced by 1 inch height of mercury = {{convert|1|inHg|Pa|2|disp=out|abbr=off}} (33.8639 hPa, millibars)
• 1 pound per square inch (psi) ≈ {{cvt|1.0000|psi|Pa|disp=out}}

Torque

• 1 pound-foot ≈ {{cvt|1.000|lbf.ft|N.m|disp=out|lk=on}}

Insulation

• 1 R-value (ft²⋅°F⋅h/Btu) ≈ 0.1761 R_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 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 {{convert|1/64|in}}.{{cite book|last1=Wells|first1=Larry J.|title=Makin' Leather: A Manual of Primitive and Modern Leather Skills|date=1981|publisher=Cedar Fort|isbn=0882908359|page=13}}
• 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 foil is measured in mils ({{frac|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 {{val|5.067|e=−4|u=mm2}} (or {{val|7.854|e=−7|u=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, {{frac|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.

The United States Code refers to these units as "traditional systems of weights and measures".{{usc|15|205b}}

Other common ways{{fact|date=February 2024}} of referring to the system are: customary, standard, English, or imperial (which refers to the post-1824 reform measures used throughout the British Empire & Commonwealth countries).{{Cite web| last = Kelechava| first = Brad| title = US Customary System: An Origin Story| work = The ANSI Blog| access-date = February 25, 2024| date = June 18, 2018| url = https://blog.ansi.org/2018/06/us-customary-system-history-units/}} Another term is the foot–pound–second (FPS) system, as opposed to centimeter–gram–second (CGS) and meter–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. The Society of Automotive Engineers (SAE) originally developed fasteners standards using U.S. units for the U.S. auto industry; the organization now largely uses metric units.{{cite web |url= http://www.sae.org/exdomains/standardsdev/global_resources/SAE%20Metric%20Users%20Guide.pdf |title= Rules for SAE Use of SI (Metric) Units |date= May 1999 |publisher= Society of Automotive Engineers, Inc |access-date=July 1, 2012}}

• Board foot
• Conversion of units
• History of measurement and 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

{{Reflist}}

• [https://web.archive.org/web/20080316180643/http://www.unc.edu/~rowlett/units/index.html Rowlett's Dictionary of Units of Measurement (archived)]

{{United States Customary Units|state=expanded}}

{{systems of measurement|sp=us}}

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{{DEFAULTSORT:United States Customary Units}}

Category:Systems of units