United States customary units
United States customary units are a system of measurements commonly used in the United States. The US customary system developed from English units which were in use in the British Empire before American independence. Consequently most US units are virtually identical to the British imperial units. However, the British system was overhauled in 1824, changing the definitions of some units used there, so several differences exist between the two systems.
The majority of US 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. The US primarily uses customary units in its commercial activities, while science, medicine, government, and many sectors of industry use metric units. The International System of Units (SI), the modern form of the metric system, is preferred for many uses by the National Institute of Standards and Technology (NIST).
The US 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 United States-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 US government passed Omnibus Trade and Competitiveness Act of 1988, which made the metric system "the preferred system of weights and measures for US 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 US labeling requirements on food products, where SI units are almost always presented alongside customary units. According to the CIA Factbook, the US is one of three nations (the others being Liberia and Burma) that has not adopted the metric system as their official system of weights and measures.
US customary units are widely used on consumer products and in industrial manufacturing. Metric units are standard in science, medicine, and government, including the US Armed Forces, as well as many sectors of industry. 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.
Other nations had, or still have unofficially, customary units of their own, sometimes very similar in name and measure to US customary units, since they often share the same Germanic or Roman origins. Frequently, however, these units designate quite different sizes. For example, the mile ranged by country from one half to five US miles; foot and pound also had varying definitions. Until the twentieth century the customary units of measure in the United States were sometimes just as variable. Historically, a wide range of non-SI units were used in the United States and in Britain, but many have fallen into disuse. This article deals only with the units commonly used or officially defined in the United States.
Units of length
|Exact relationships shown in boldface|
|1 point (p)||352.777778 µm|
|1 pica (P̸)||12 p||4.233333 mm|
|1 inch (in)||6 P̸||25.4 mm|
|1 foot (ft)||12 in||0.3048 m|
|1 yard (yd)||3 ft||0.9144 m|
|1 mile (mi)||5280 ft or 1760 yd||1.609344 km|
|1 link (li)||33⁄50 ft or 7.92 in||0.2012 m|
|1 (survey) foot (ft)||1200⁄3937 m||0.30480061 m|
|1 rod (rd)||25 li or 16.5 ft||5.02921 m|
|1 chain (ch)||4 rd||20.11684 m|
|1 furlong (fur)||10 ch||201.1684 m|
|1 survey (or statute) mile (mi)||8 fur||1.609347 km|
|1 league (lea)||3 mi||4.828042 km|
|1 fathom (ftm)||2 yd||1.8288 m|
|1 cable (cb)||120 ftm or 1.091 fur||219.456 m|
|1 nautical mile (NM or nmi)||8.439 cb or 1.151 mi||1.852 km|
The system for measuring length in the United States customary system is based on 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. This definition was agreed with the UK and other Commonwealth countries, and so 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 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 the 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||0.09290341 m2|
|1 square chain (sq ch or ch2)||4356 sq ft (survey) or 16 sq rods||404.6873 m2|
|1 acre||43560 sq ft (survey) or 10 sq ch||4046.873 m2|
|1 section||640 acres or 1 sq mile (survey)||2.589998 km2|
|1 survey township (twp)||36 sections or 4 sq leagues||93.23993 km2|
The most widely used area unit with a name unrelated to any length unit is the acre. The National Institute of Standards and Technology 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)||16.387064 mL|
|1 cubic foot (cu ft) or (ft3)||1728 cu in||28.31685 L|
|1 cubic yard (cu yd) or (yd3)||27 cu ft||764.554857984 L
|1 acre-foot (acre ft)||43560 cu ft
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 material.
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 US 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||61.611519921875 μL|
|1 US fluid dram (fl dr)||60 min||3.6966911953125 mL|
|1 teaspoon (tsp)||80 min||4.92892159375 mL|
|1 tablespoon (Tbsp)||3 tsp or 4 fl dr||14.78676478125 mL|
|1 US fluid ounce (fl oz)||2 Tbsp or 1.0408 oz av of water||29.5735295625 mL|
|1 US shot (jig)||3 Tbsp||44.36029434375 mL|
|1 US gill (gi)||4 fl oz||118.29411825 mL|
|1 US cup (cp)||2 gi or 8 fl oz||236.5882365 mL|
|1 (liquid) US pint (pt)||2 cp or 16.65 oz av of water||473.176473 mL|
|1 (liquid) US quart (qt)||2 pt||0.946352946 L|
|1 (liquid) US gallon (gal)||4 qt or 231 cu in||3.785411784 L|
|1 (liquid) barrel (bbl)||31.5 gal or 1⁄2 hogshead||119.240471196 L|
|1 oil barrel (bbl)||42 gal or 2⁄3 hogshead||158.987294928 L|
|1 hogshead||63 gal or 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 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 (20 oz).
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 United States, 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 and gill are rarely used currently. The gill is often referred to as a "half-cup".
|1 (dry) pint (pt)||33.60 cu in||0.5506105 L|
|1 (dry) quart (qt)||2 pt||1.101221 L|
|1 (dry) gallon (gal)||4 qt or 268.8025 cu in||4.404884 L|
|1 peck (pk)||2 gal||8.809768 L|
|1 bushel (bu)||4 pk or 1.244 cu ft||35.23907 L|
|1 (dry) barrel (bbl)||7056 cu in or 3.281 bu||115.6271 L|
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 agricultural regions.
Units of mass
|Avoirdupois||1 grain (gr)||1⁄7000 lb||64.79891 mg|
|1 dram (dr)||27 11⁄32 gr or 8.859 carats||1.7718451953125 g|
|1 ounce (oz)||16 dr||28.349523125 g|
|1 pound (lb)||16 oz||453.59237 g|
|1 US hundredweight (cwt)||100 lb||45.359237 kg|
|1 long hundredweight||112 lb||50.80234544 kg|
|1 ton (short ton)||20 US cwt or 2000 lb||907.18474 kg|
|1 long ton||20 long cwt or 2240 lb||1016.0469088 kg|
|Troy||1 grain (gr)||1⁄7000 lb av or 1⁄5760 lb t||64.79891 mg|
|1 pennyweight (dwt)||24 gr or 7.776 carats||1.55517384 g|
|1 troy ounce (oz t)||20 dwt||31.1034768 g|
|1 troy pound (lb t)||12 oz t or 13.17 oz av||373.2417216 g|
|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 US, 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 United States. The imperial system, which is still used for some measures in the UK and other Commonwealth countries, is based on avoirdupois, with variations from US customary units larger than a pound.
The pound avoirdupois, which forms the basis of the US customary system of mass, is defined as exactly 453.59237 grams by agreement between the US, the UK 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 ounce avoirdupois.
For the pound and smaller units, the US 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 US customary system.
In the United States, only the ounce, pound and short ton—known in the country simply as the ton—are commonly used, though the hundredweight is still used in agriculture and shipping. The grain is used to describe the mass of propellant and projectiles in small arms ammunition. It was also used to measure medicine and other very small masses.
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.
|Teaspoon||5 mL||5 mL||4.74 mL||4.93 mL||5 mL|
|Dessertspoon||10 mL||—||9.47 mL||—||—|
|Tablespoon||20 mL||15 mL||14.21 mL||14.79 mL||15 mL|
|Fluid ounce||—||—||28.41 mL||29.57 mL||30 mL|
|Cup||250 mL||250 mL||284.13 mL||236.59 mL||240 mL|
|Pint||—||—||568.26 mL||473.18 mL||—|
|Quart||—||—||1136.52 mL||946.35 mL||—|
|Gallon||—||—||4546.09 mL||3785.41 mL||—|
The most common practical cooking measures for both liquid and dry ingredients in the United States (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 avdp. of chocolate), stick (e.g., 4 oz avdp. butter) or fruit/vegetable (e.g., a half lemon, two medium onions).
Some common volume measures in English-speaking countries are shown at right. The volumetric measures here are for comparison only.
Units of temperature
Degrees Fahrenheit are used in the United States 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 technical standards are expressed in Fahrenheit temperatures and US 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 is defined to freeze at 32 °F = 0 °C and boil at 212 °F = 100 °C at 1 atm; the conversion formula is easily shown to be:
or inversely as
- 1 board-foot = 1 ft × 1 ft × 1 in = 2.360 dm3
- 1 British thermal unit (Btu) ≈ 1055 J
- 1 calorie (cal) = 4.184 J
- 1 food calorie (kilocalorie, large calorie) (kcal, Cal) = 4.184 kJ
- 1 hand = 10.16 cm
- 1 horsepower ≈ 745.7 W
- 1 R-value (ft2·°F·h/Btu) ≈ 0.1761 RSI (K·m2/W)
- 1 slug (mass) = 1 lbf·s2/ft
- 1 U (rack unit) = 1.75 in
- Various combination units are in common use, including the foot-pound and the pound per square inch; these are straightforwardly defined based on the above basic units.
Other names for US customary units
The United States Code refers to these units as "traditional systems of weights and measures".
Other common ways of referring to these systems in the United States are: "Standard", "Customary", or, somewhat erroneously when considering volume/tonnage, "Imperial", or "English", which refers to the pre-1824 reform measures used throughout the British Empire. Another term is the "foot-pound-second" (FPS) system (as opposed to centimeter-gram-second (CGS) system).
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 originally developed fasteners standards using US units for the US auto industry; the organization now uses metric units.
- T. C. Mendenhall, Superintendent of Standard Weights and Measures, Order of April 5, 1893, published as Appendix 6 to the Report for 1893 of the Coast and Geodetic Survey.
- 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 it helps to bear in mind that 999,998 = 3937 × 254.
- Laws and Metric System, http://www.nist.gov/pml/wmd/metric/index.cfm, 2010
- "English units of measurement". The Columbia Encyclopedia 6th ed. 2001-2007. archived copy.
- Gardner, Martin (1957). "The Great Pyramid".
- "Appendix G - Weights and Measures". The World Factbook. Washington D.C.: Central Intelligence Agency. Retrieved August 28, 2012.
- 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" Technologyreview.com
- 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
- What are the 'official' conversions that are used by NGS to convert 1) meters to inches, and 2) meters to feet?. "Frequently Asked Questions about the National Geodetic Survey". National Geodetic Survey. Retrieved May 16, 2009.
- Astin, A. V. (July 27, 1968). Federal Register. 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 (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 (SI). Special Publication 811. http://physics.nist.gov/Pubs/SP811/sec06.html#6.1.2
- 93rd Conference on Weights and Measures. (2009). Specifications, Tolerances, and Other Technical Requirements for Weighing and Measuring Devices (NIST Handbook 44). National Institute of Standards and Technology.
- Summary of State Laws and Regulations in Weights and Measures. (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
- "Food and Drugs: FDA Food Labeling". US Government Printing Office. April 1, 2004.
For Nutrition facts labeling "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."
- 15 U.S.C. § 205b
- "Rules for SAE Use of SI (Metric) Units". Society of Automotive Engineers, Inc. May 1999. Retrieved July 2012.
- Rowlett's A Dictionary of Units of Measurement