United States customary units are a system of measurements commonly
used in the United States. The
United States customary system (USCS or
USC) developed from
English units which were in use in the British
Empire before the U.S. became an independent country. However, the
United Kingdom's system of measures was overhauled in 1824 to create
the imperial system, changing the definitions of some units.
Therefore, while many U.S. units are essentially similar to their
Imperial counterparts, there are significant differences between the
The majority of U.S. customary units were redefined in terms of the
meter and the kilogram with the
Mendenhall Order of 1893 and, in
practice, for many years before. These definitions were refined by
the international yard and pound agreement of 1959. Americans
primarily use customary units in commercial activities, as well as for
personal and social use. In science, medicine, many sectors of
industry and some of government and military, metric units are used.
International System of Units
International System of Units (SI), the modern form of the metric
system, is preferred for many uses by the U.S. National Institute of
Standards and Technology (NIST). For newer units of measure where
there is no traditional customary unit, international units are used,
sometimes mixed with customary units, such as electrical resistance of
wire expressed in ohms (SI) per thousand feet.
2 Units of length
3 Units of area
4 Units of capacity and volume
4.1 Fluid volume
4.2 Dry volume
5 Units of weight
5.1 Grain measures
6 Cooking measures
7 Units of temperature
8 Other units
9 Other names for U.S. customary units
10 See also
12 External links
Imperial and US customary measurement systems
Imperial and US customary measurement systems and Comparison
of the imperial and US customary measurement systems
United States system of units is similar to the British imperial
system. Both systems are derived from English units, a system which
had evolved over the millennia before American independence, and which
had its roots in 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. Advocates of the customary system saw the French
Revolutionary, or metric, system as atheistic. An auxiliary of the
Institute in Ohio published a poem with wording such as "down with
every 'metric' scheme" and "A perfect inch, a perfect pint". One
adherent of the customary system called it "a just weight and a just
measure, which alone are acceptable to the Lord".
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 Factbook, the
United States is one of three nations (along
Liberia and Burma) that have not adopted the metric system as
their official system of weights and measures.
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 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 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 to the foot.
Units of length
Most common measures shown in italics
Exact relationships shown in boldface
1 point (p)
127/360 mm or 6996352778000000000♠352.778 μm
1 pica (P̸)
127/30 mm or 6997423300000000000♠4.233 mm
1 inch (in or ″)
1 foot (ft or ′)
1 yard (yd)
1 mile (mi)
7003160934400000000♠5280 ft or
1 link (li)
33/50 ft or 7.92 in
792/3937 m or 6999201170000000000♠20.117 cm
1 (survey) foot (ft)
1200/3937 m or 6999304800000000000♠30.480 cm
1 rod (rd)
25 li or 16.5 ft
19,800/3937 m or 7000502900000000000♠5.029 m
1 chain (ch)
4 rd or 66 ft
79,200/3937 m or 7001201160000000000♠20.116 m
1 furlong (fur)
792/3937 km or 7002201168000000000♠201.168 m
1 survey (or statute) mile (mi)
6336/3937 km or 7003160934700000000♠1.609347 km
1 league (lea)
19,008/3937 km or 7003482800000000000♠4.828 km
1 fathom (ftm)
1 cable (cb)
120 ftm or 1.091 fur
1 nautical mile (NM or nmi)
8.439 cb or 1.151 mi
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. Since July 1, 1959, these have been defined on the
basis of 1 yard = 0.9144 meters except for some
applications in surveying. The U.S., the
United Kingdom and other
Commonwealth countries agreed on this definition, and so it is often
termed international measure.
When international measure was introduced in the English-speaking
countries, 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 1 foot = 1200/3937 meters: 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 0.999998 of a
US survey foot, for a difference of about 1/8 inch (3 mm)
per mile – but it affects the definition of the State Plane
Coordinate Systems (SPCSs), which can stretch over hundreds of
The NAD27 was replaced in the 1980s by the
North American Datum
North American Datum of
1983 (NAD83), which is defined in meters. The SPCSs were also updated,
National Geodetic Survey
National Geodetic Survey left the decision of which (if any)
definition of the foot to use to the individual states. All SPCSs are
defined in meters, but seven states also have SPCSs defined in US
survey feet and an eighth state in international feet: the other
42 states use only meter-based SPCSs.
State legislation is also important for determining the conversion
factor to be used for everyday land surveying and real estate
transactions, although the difference (2 ppm) is of no practical
significance given the precision of normal surveying measurements over
short distances (usually much less than a mile). Twenty-four states
have legislated that surveying measures should be based on the US
survey foot, eight have legislated that they be made on the basis of
the international foot, and eighteen have not specified the conversion
factor from metric units.
Units of area
Exact relationships shown in boldface
1 square survey foot (sq ft or ft2)
144 square inches
1 square chain (sq ch or ch2)
7003435600000000000♠4356 sq ft (survey) or 16 sq rods
7004435600000000000♠43560 sq ft (survey) or 10 sq ch
640 acres or 1 sq mile (survey)
1 survey township (twp)
36 sections or 4 sq leagues
The most widely used area unit with a name unrelated to any length
unit is the acre. The National Institute of Standards and Technology
contends that customary area units are defined in terms of the square
survey foot, not the square international foot. Conversion factors
are based on Astin (July 27, 1968) and National Institute of
Standards and Technology (2008).
Units of capacity and volume
Volume in general
1 cubic inch (cu in) or (in3)
1 cubic foot (cu ft) or (ft3)
7003172800000000000♠1728 cu in
1 cubic yard (cu yd) or (yd3)
27 cu ft
1 acre-foot (acre ft)
7004435600000000000♠43560 cu ft
7003161333300000000♠1613.333 cu yd
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, and one for measuring volumes of dry
Other than the cubic inch, cubic foot and cubic yard, these units are
differently sized from the units in the imperial system, although the
names of the units are similar. Also, while the U.S. has separate
systems for measuring the volumes of liquids and dry material, the
imperial system has one set of units for both.
Most common measures shown in italic font
Exact conversions in bold font
1 minim (min)
~1 drop or 0.95 grain of water
1 US fluid dram (fl dr)
1 teaspoon (tsp)
1 tablespoon (Tbsp)
3 tsp or 4 fl dr
1 US fluid ounce (fl oz)
2 Tbsp or 1.0408 oz av of water
1 US shot (jig)
1.5 fl oz or 3 Tbsp
1 US gill (gi)
22⁄3 jig or 4 fl oz
1 US cup (cp)
2 gi or 8 fl oz
1 (liquid) US pint (pt)
2 cp or 16.65 oz av of water
1 (liquid) US quart (qt)
1 (liquid) US gallon (gal)
4 qt or 231 cu in
1 (liquid) barrel (bbl)
31.5 gal or 1⁄2 hogshead
1 oil barrel (bbl)
11⁄3 (liquid) barrel or 42 gal or 2⁄3 hogshead
1.5 oil barrels or 63 gal or 7000842187500000000♠8.421875 cu ft
or 524.7 lb of water
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. The fluid ounce derives its name originally from being the
volume of one ounce avoirdupois of water, 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.
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
A 23.7 US fl oz (700 mL) bottle displaying both US and metric units.
There are varying standards for barrel for some specific commodities,
including 31 gal for beer, 40 gal for whiskey or kerosene, and 42 gal
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
Minims, drams and gill are rarely used currently. The gill is often
referred to as a "half-cup".
1 (dry) pint (pt)
7001336003125000000♠33.6003125 cu in
1 (dry) quart (qt)
1 (dry) gallon (gal)
1 peck (pk)
1 bushel (bu)
1 (dry) barrel (bbl)
7003705600000000000♠7056 cu in or
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. 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. However pecks, or bushels are
sometimes used—particularly for grapes, apples and similar fruits in
Units of weight
Pound (force) and Pound (mass)
1 grain (gr)
1 dram (dr)
27 11⁄32 gr or 8.859 carats
1 ounce (oz)
1 pound (lb)
1 US hundredweight (cwt)
1 long hundredweight
1 ton (short ton)
20 US cwt or 2000 lb
1 long ton
20 long cwt or 2240 lb
1 grain (gr)
1⁄7000 lb or 1⁄5760 lb t
1 pennyweight (dwt)
24 gr or 7.776 carats
1 troy ounce (oz t)
1 troy pound (lb t)
12 oz t or 13.17 oz
Most common measures shown in italics
Exact conversions shown in bold
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.,
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 Commonwealth 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
7002453592370000000♠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 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
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.
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
In agricultural practice, a bushel is a fixed volume of 2150.42 cubic
inches. 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
In trade terms a bushel is a term used to refer to these nominal
weights, although even this varies. With oats,
Canada uses 34 lb
bushels and the USA uses 32 lb bushels.
Main article: Cooking weights and measures
Common volume measures in English-speaking countries
(Comparable measures listed for comparison purposes.)
The most common practical cooking measures for both liquid and dry
ingredients in the U.S. (and many other countries) are the teaspoon,
tablespoon, and cup, along with halves, thirds, quarters, and eighths
of these. Pounds, ounces, fluid ounces, and common sizes are also
used, such as can (presumed size varies depending on product), jar,
square (e.g., 1 oz of chocolate), stick (e.g., 4 oz butter), or
fruit/vegetable (e.g., a half lemon, two medium onions).[citation
Units of temperature
Fahrenheit are used in the U.S. to measure temperatures in
most non-scientific contexts. The
Rankine scale of absolute
temperature also saw some use in thermodynamics. Scientists worldwide
use the kelvin and degree Celsius. Several U.S. technical standards
are expressed in
Fahrenheit temperatures and American medical
practitioners often 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 32 °F =
0 °C and boils at 212 °F = 100 °C at 1 atm. The
conversion formula is:
displaystyle [^ circ text F ]= frac 9 5 [^ circ text C
or inversely as
displaystyle [^ circ text C ]= frac 5 9 ([^ circ text F
1 board-foot = 1 ft × 1 ft × 1 in =
British thermal unit (Btu) ≈ 1055 J
1 ton of refrigeration (12,000 Btu/h) = 3.517 kW
1 calorie (cal) = 4.184 J
1 food calorie (kilocalorie, large calorie) (kcal, Cal) =
1 foot-pound (energy) ≈ 1.356 J
1 hand = 4 in = 10.16 cm
1 horsepower ≈ 745.7 W
1 R-value (ft2⋅°F⋅h/Btu) ≈ 0.1761 RSI (K⋅m2/W)
1 slug = 1 lbf⋅s2/ft
1 U (rack unit) = 1.75 in = 44.45 mm
Various combination units are in common use, including the foot-pound
(ft⋅lbf), the acre-foot, and the pound per square inch (psi); 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 (0.4 mm).
Bolts and screws follow the
Unified Thread Standard
Unified Thread Standard rather than the
ISO metric screw thread
ISO metric screw thread standard.
Knitting needles in the
United States are measured according to a
non-linear unitless numerical system.
Aluminium foil is measured in mils (1/100th of an 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
6990506700000000000♠5.067×10−4 mm2 (or
6990506708663999999♠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
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.
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
United States Code refers to these units as "traditional systems
of weights and measures".
Other common ways of referring to the system in the U.S. are:
"Standard", "Customary", or, erroneously: "Imperial", or "English"
(which refers to the post-1824 reform measures used throughout the
British Empire). 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 uses metric units.
Conversion of units
Celsius vs. Kelvin)
History of measurement, systems, and units of measurement
Plan for Establishing Uniformity in the Coinage, Weights, and Measures
United States (1790)
Mars Climate Orbiter
Standard cubic foot
^ T.C. Mendenhall, Superintendent of Standard Weights and Measures.
Order of April 5, 1893 Archived September 30, 2012, at the Wayback
Machine., published as Appendix 6 to the Report for 1893 of the
Coast and Geodetic Survey.
^ a b c Astin, A.V., Karo, H.A. and Mueller, F.H. (June 25, 1959). Doc
59-5442, "Refinement of Values for the
Yard and the Pound." Federal
Register. When reading the document note that 999,998 = 3937 × 254.
^ Laws and Metric Program. U.S. National Institute of Standards and
English units of measurement". The Columbia Encyclopedia 6th ed.
2001-2007. archived copy.
^ a b c Gardner, Martin (1957). "The Great Pyramid".
^ a b "Appendix G - Weights and Measures". The World Factbook.
Washington, D.C.: Central Intelligence Agency. Retrieved August 28,
^ Robyn Williams (February 8, 1998) "Trouble with the Metric System".
Australian Radio National, Ockham's Razor.
^ Ed Tenner, (May 2005). "The Trouble with the Meter"
^ a b c d e f Roberts, R.W. (February 3, 1975). Federal Register
republished in Barbrow, L.E. and Judson, L. V. (1976) Weights and
Measures of the United States. National Bureau of Standards Special
Publication 447. p. 36
^ a b c "Frequently Asked Questions about the National Geodetic
Survey". National Geodetic Survey. Retrieved May 16, 2009.
contribution= ignored (help)
^ Astin, A. V. (July 27, 1968).
Federal Register Archived 2006-10-05
at the Wayback Machine.. Republished in Barbrow, L.E and Judson, L.V.
Weights and Measures of the United States: A Brief History. National
Bureau of Standards
Special Publication 447. pp. 34–35.
^ National Institute of Standards and Technology. (2008). Guide for
the Use of the
International System of Units
International System of Units (SI).
^ The recommended symbol for the liter in the
United States is 'L' per
National Institute of Standards and Technology. (1995.) Guide for the
Use of the
International System of Units
International System of Units (SI).
^ a b 101st Conference on Weights and Measures 2016. (2017).
Specifications, Tolerances, and Other Technical Requirements for
Weighing and Measuring Devices. National Institute of Standards and
Technology. p. C-6, C-11, C-16.
^ Summary of State Laws and Regulations in Weights and Measures
Archived December 5, 2011, at the Wayback Machine.. (2005) National
Institute of Standards and Technology.
^ NIST Handbook 44, Appendix C, General Tables of Units of
Measurement, page C-6
Avoirdupois Units of
Mass Archived 2011-10-18 at
the Wayback Machine.
^ "Title 21, 101.9 Nutrition labeling of food" (PDF). Code of Federal
Regulations. US Government Printing Office. 2012-04-12. Retrieved
2014-11-02. 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
^ Wells, Larry J. (1981). Makin' Leather: A Manual of Primitive and
Leather Skills. Cedar Fort. p. 13.
^ 15 U.S.C. § 205b
^ "Rules for SAE Use of SI (Metric) Units" (PDF). Society of
Automotive Engineers, Inc. May 1999. Retrieved July 2012. Check
date values in: access-date= (help)
Rowlett's A Dictionary of Units of Measurement
Systems of measurement
International System of Units
International System of Units (SI)
UK imperial system
US customary units
French (Trad. • Mesures usuelles)
Biblical and Talmudic
Humorous (FFF system)