The United States Customary System for units of measurement, also known in the United States as English, Imperial or standard units, is the primary and most commonly-used system of units of measurement in the United States. In some cases SI (International System of Units — the modern metric system) are used alongside customary units within the United States. All units are defined in terms of SI base units, but at ratios inconvenient for conversion.

English units in the global context refers to the historical system of units that originated in England, and there are many points of difference between present-day non-metric English units (Imperial units) and the U.S. customary units, as detailed in the comparison of the imperial and US customary measurement systems.

The U.S. system of units is similar to the Imperial system (in reference to the British Empire^[1]) which was used officially in the United Kingdom until 1995^[2] (and which still has official usage there for a few purposes, and unofficial usage for many others). Both systems derive from the evolution of local units over the centuries, as a result of standardization efforts in England; the local units themselves mostly trace back to Roman and Anglo-Saxon units. Today, these units are defined in terms of SI units.

In the Omnibus Trade and Competitiveness Act of 1988, the United States government designated the metric system of measurement as "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, especially small business, as it voluntarily converts to the metric system of measurement. This process of legislation and conversion is known as metrication, and in the U.S. is most evident in labeling requirements on food products, where SI units are almost always presented alongside customary units.

However, metrication in the United States has been less forcefully imposed than in other countries,^[3] and has encountered more resistance from industrial and consumer market forces, so customary units are still widely used on consumer products and in industrial manufacturing; only in military, medical, and scientific contexts are SI units generally the norm.

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,^[4] or that inch measurements are more suitable when distances are frequently divided by two.^[3]

Other countries had (or still have, unofficially), customary units of their own, sometimes very similar in name and measure to the U.S. customary units, since they often shared the same Germanic or Roman origins. Frequently, however, these units designated quite different sizes. For example, in different countries, the mile ranged from one half to five U.S. miles, and even foot and pound had varying definitions. Even within the United States, at least through the 19th century, the customary units of measure were sometimes just as variable. Eventually, most countries, including the United States, redefined their customary units in terms of SI units like kilogram and meter. Often, though, unlike English units, they were rounded to "nice" SI values, leading to their use in colloquial speech, alongside SI terms, into the present day.

Historically, a wide range of non-SI units has been used in the United States, and on the British Isles before that, but many of these have fallen into disuse. This article deals only with the units commonly used or officially defined in the United States.

As of 2007, the U.S. customary units remain the last widely used official non-metric system of measurement as besides the United States only Liberia and Myanmar have not adopted the International System of Units as their primary or sole system of measurement.^[5]

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. However, for each of these units there exist two slightly different definitions, yielding two different systems of measure - international measure, and U.S. survey measure. The relationships between the different units within each measure is the same, but each measure has a slightly different definition in terms of metric units.

One inch international measure is exactly 25.4 millimeters, while one inch U.S. survey measure is defined so that 39.37 inches is exactly 1 meter. For most applications, the difference is insignificant. (One international inch is exactly 0.999998 of a U.S. survey inch,^[6] for a difference of about 3 millimeters per mile). International measure is used for everyday use, engineering, and commerce in the United States, while survey measure is used only for surveying.

International measure, agreed in 1959, uses the same definition of the units involved as is used in the UK and other Commonwealth countries. Before that date, those other countries still used separate standards. U.S. survey measure uses an older definition of the units (specified by the Mendenhall Order in 1893) which the United States used prior to adopting international measure. Previous to this agreement, the U.S. standard was identical to survey measure.

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 foot, cubic inch 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.

Technically speaking, since these units are defined in terms of the inch, it would make a difference whether international or survey measure was used. However, in practice, the difference between the two definitions would be imperceptible, and in any case in defining volumes international measure is used.

Most important measures shown in bold font

One fluid ounce is 1/16 of a U.S. pint, 1/32 of a U.S. quart, and 1/128 of a U.S. gallon.

The fluid ounce derives its name originally from being the volume of one ounce avoirdupois of water, but in the U.S. it is defined as 1/128 of a U.S. gallon — consequently, a fluid ounce of water weighs about 1.043 ounces avoirdupois.

The saying "a pint's a pound the world around" refers to 16 fluid ounces of water weighing approximately one pound avoirdupois in the United States, but that is true only in the United States. In the rest of the English-speaking world, 20 fluid ounces (one pint) of water will weigh one and a quarter pounds.

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, and gallons. Water volume for sinks, bathtubs, ponds, swimming pools, etc., is usually stated in gallons.

The gill is basically a paper unit which is not actually used in the United States any more. Minims and drams are also rarely used.

Most important measures shown in bold font

Small fruits and vegetables are often sold in dry pints and dry quarts. The U.S. dry gallon is less commonly used, and is not listed in the relevant statute, which jumps from the dry quart to the peck.^[8] However pecks, or bushels are sometimes used particularly for grapes, apples and similar fruits in agricultural regions.

There have historically been four different English systems of mass: Pound, Troy weight, Avoirdupois system, and Apothecaries' system. Tower weight fell out of use (due to legal prohibition) centuries ago, and was never used in the United States. Troy weight is still used to weigh precious metals. Apothecaries weight, once used in pharmacy, has been largely replaced by metric measurements. Avoirdupois mass, being directly related to the kilogram, is used to define mass in the U.S. customary system today.

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 the some contexts, the term "pound" is almost exclusively used to refer to "pound-force".

Troy weight, avoirdupois weight, and apothecaries weight are all defined in terms of 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 apothecaries 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). The dram was once known as the drachm.

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.

The pound avoirdupois, which forms the basis of the U.S. customary system of mass, is defined as exactly 453.59237 grams. All the other units of mass are defined in terms of it.

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 British Imperial system is identical to that in the U.S. customary system.

• bold indicates commonly used units

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 has the same definition as for Avoirdupois weight.

The most common cooking weights and measures in the United States are as follows:

Note: SI equivalents have been rounded to reasonable values in the table above, because cooking is not a precise practice.

In agricultural practice, a bushel is a fixed mass of grain, nominally based on dry volume units.

• 1 bushel (maize) = 56 lb ≈ 25.401 kg
• 1 bushel (wheat) = 60 lb ≈ 27.216 kg

Degrees Fahrenheit are used in the United States to measure temperatures in most non-scientific contexts.

• Pure water freezes at 32 °F and boils at 212 °F at 1 atm.
• Water saturated with common salt freezes at −6.02 °F.
• Conversion formula: ${\displaystyle F={\frac {9}{5))C+32}$
• For a rough conversion formula that is easier to perform in one's head, American travelers to regions that use Celsius may prefer ${\displaystyle F\approx 2C+30\,}$ which is identical to the real formula when F=50 and is accurate to within 5 degrees Fahrenheit when ${\displaystyle 5\leq F\leq 95}$.

For scientific matters the Rankine scale, an absolute temperature scale based upon Fahrenheit, saw some use. Modern scientists almost always use the Kelvin and Celsius scales, though U.S. medical practitioners often use the U.S. customary units for body temperature and weight.

• British thermal unit (~ 1055 J)
• Horsepower (~ 746 W)
• Hand (10.16 cm)
• Calorie (4.184 J or kJ, depending on usage)
• Board-foot (2.360 dm³)
• R-value (0.176 K·m²/W)
• Slug (mass) (1 lb_f·s²/ft)
• Various combination units are in common use, including the foot-pound and the PSI; these are straightforwardly defined based on the above basic units.

• History, systems and units of measurement
• Comparison of the imperial and US customary measurement systems
• Metric system in general and the International System of Units (SI) in particular
• Conversion of units and approximate conversion of units
• Plan for Establishing Uniformity in the Coinage, Weights, and Measures of the United States

• Source: Appendix C, NIST Handbook 44, 2006 edition (http://ts.nist.gov/WeightsAndMeasures/upload/AppendC-06-HB44-Final.pdf Appendix C PDF]).
• Judson, Lewis B., Weights and Measures Standards of the United States: A brief history, NBS Special Publication 447, orig. iss. October 1963, updated March 1976 (46 page PDF file or table of contents).
• Jacques J. Proot's Anglo-Saxon weights & measures page.
• Rowlett's A Dictionary of Units of Measurement