This shampoo label, showing contents as "23.7 FL OZ (700 mL)", illustrates the secondary status of metric units in the U.S. in the early 21st century. However the metric quantity is a round number.

Metrication in the United States is the process of introducing the International System of units (SI / Système International) to replace the customary units of measurement that are the primary units of measurement in the United States. This process has been largely completed in most other nations of the world but in the USA has been unsteady with no definite conclusion. The U.S. uses a mixture of units of measurement. In some fields, the metric system has been used in the United States since the early 1800s. The use of metric units instead of or in addition to United States customary units has been gradually increasing for many years, but much of the public momentum has been lost since the 1980s, except in schools, science, and manufacturing. While other countries use the metric system predominantly, the USA has continued largely with customary units. There has been little political will to continue conversion.

[edit] 19th century

In the early 1800s, the U.S. Coast and Geodetic Survey (the government's surveying and map-making agency) used meter and kilogram standards brought from France. In 1866, Congress authorized the use of the metric system and supplied each state with a set of standard metric weights and measures.

In 1875, the United States solidified its commitment to the development of the internationally recognized metric system by becoming one of the original seventeen signatory nations to the Metre Convention or the Treaty of the Meter. The signing of this international agreement concluded five years of meetings in which the metric system was reformulated, refining the accuracy of its standards. The Treaty of the Meter established the Bureau international des poids et mesures (BIPM, International Bureau of Weights and Measures) in Sèvres, France, to provide standards of measurement for worldwide use.

Under the Mendenhall Order in 1893, metric standards, developed through international cooperation under the auspices of BIPM, were adopted as the fundamental standards for length and mass in the United States. The U.S. customary units such as the foot and pound have been defined in relation to metric units ever since.

The 1895 Constitution of Utah, in Article X, Section 11, originally mandated that: "The Metric System shall be taught in the public schools of the State." This section was, however, later repealed.^[1]

[edit] 20th century

ANMC logo

The Conférence Générale des Poids et Mesures (General Conference on Weights and Measures) is the governing body for the metric system and comprises the signing nations of the Treaty of the Meter. The Conférence Génerale des Poids et Mesures approved an updated version of the metric system in 1960 named Le Système International d'Unités (International System of Units) and abbreviated SI.

On February 10, 1964, the National Bureau of Standards (former name of National Institute of Standards and Technology) issued a statement that it will use the metric system:

The metrication of the United Kingdom and the other Commonwealth nations, particularly Canada, increased the need of metrication in the United States.

U.S. metrication logo

In 1968, Congress authorized the U.S. Metric Study, a three-year study of systems of measurement in the U.S., with emphasis on the feasibility of metrication. The Department of Commerce conducted the study. A 45-member advisory panel consulted and took testimony from hundreds of consumers, business organizations, labor groups, manufacturers, and state and local officials. The final report of the study concluded that the U.S. would eventually join the rest of the world in the use of the metric system of measurement.^[3] The study found that metric units were already implemented in many areas and that it use was increasing. The majority of study participants believed that conversion to the metric system was in the best interests of the U.S., particularly in view of the importance of foreign trade and the increasing influence of technology in the U.S.

This measuring cup, manufactured and sold in the U.S. circa 1980 at the height of the metrication effort, features graduations in both metric and U. S. customary units, with the metric graduations in front for right handed users.

The U.S. Metric Study recommended that the United States implement a carefully planned transition to the principal use of the metric system over a decade. Congress passed the Metric Conversion Act of 1975 "to coordinate and plan the increasing use of the metric system in the United States". The Act did not stipulate the conversion happen over a decade. Voluntary conversion was initiated, and the U.S. Metric Board (USMB) was established for planning, coordination, and public education. The public education component led to public awareness of the metric system, but the public response included resistance, apathy, and sometimes ridicule (for example, the Saturday Night Live decabet sketch).^[4] In 1981, the USMB reported to Congress that it lacked the clear Congressional mandate necessary to bring about national conversion. Because of this ineffectiveness and an effort of the Reagan administration to reduce federal spending, the USMB was disbanded in autumn of 1982.

1980s Road Map mentioning an impending change to the metric system

The ending of the USMB increased doubts that metrication would really be implemented. Public and private sector metrication slowed even while competitiveness between nations and demands of global marketplaces increased.

The American National Metric Council (ANMC) was established in 1973 by the American National Standards Institute (ANSI) as a non-profit, tax-exempt organization for planning and coordinating metric activity in all sectors of the U.S. economy. The ANMC became a separately incorporated organization in 1976. The ANMC was intended to facilitate U.S. metrication by maintaining a voluntary and orderly process that minimizes costs and maximizes benefits, and to provide information, forums, individual assistance, and other services for its subscribers. The ANMC attempted to coordinate the metric planning of many industrial sectors, unlike the USMB, which only attempted to implement the policy set forth in the Metric Conversion Act of 1975. After the formation of the USMB, committees of the ANMC submitted conversion plans for the chemical sector and for the instruments sector. These sector conversion plans were later approved by the USMB. From 1975 through 1987, the ANMC held a series of well-attended annual conferences. Subsequently, a series of National Metric Conferences, jointly sponsored by the ANMC, the U.S. Metric Association (USMA), the U.S. Department of Commerce, and the National Institute of Standards and Technology (NIST), were held from 1989 through 1993.^[5]

A 1980s-vintage "dual" miles per hour and kilometers per hour sign, (formerly) located in Florida.

Congress included new encouragement for U.S. industrial metrication in the Omnibus Trade and Competitiveness Act of 1988. This legislation amended the Metric Conversion Act of 1975 and designated the metric system as "the Preferred system of weights and measures for United States 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 legislation required most federal agencies to use the metric system in their procurement, grants, and other business-related activities by the end of 1992. While not mandating metric use in the private sector, the Federal Government has sought to serve as a catalyst in the metric conversion of the country's trade, industry, and commerce. Exceptions were made for the highway and construction industries. The Department of Transportation planned to require metric units by 2000, but this plan was canceled by the 1998 highway bill TEA21.^[6] The U.S. military has generally high use of the metric system partly because of the need to work with other nations' militaries.^[7]

Some members of Congress attempted to ban use of the metric system on federal highways in 1992 and 1993.^[8][9] These bills were not popular in the House of Representives and failed before a vote.

[edit] Mars orbiter

The use of two different systems was the root cause in the loss of the Mars Climate Orbiter in 1998. NASA specified metric units in the contract. NASA and other organizations worked in metric units, but one subcontractor, Lockheed Martin, provided thruster performance data to the team in pound force seconds instead of newton seconds. The spacecraft was intended to orbit Mars at about 150 kilometers (93 mi) altitude, but the incorrect data meant that it probably descended instead to about 57 kilometers (35 mi), burning up in the Martian atmosphere.^[10]

[edit] 21st century

This section contains weasel words, vague phrasing that often accompanies biased or unverifiable information. Such statements should be clarified or removed. (March 2009)

[edit] Metrication efforts

The current effort toward national metrication is based on the claim that industrial and commercial productivity, mathematics and science education, and the competitiveness of U.S. products and services in world markets would be enhanced by completing the change to the international standard measurement system based on metric units. Gerard Iannelli, director of the U.S. Metric Program, gave the opinion that Americans have so far not adopted the Metric system because of the effort it takes to "translate" from Customary to Metric when both measurements are not available. He also cited what he perceived to be ineffective attempts at public education and awareness.^[11]

[edit] Current usage

[edit] Daily life

While most people in the U.S. have studied metric units at school, metric usage remains low in daily life. Weather reports on television are given in Fahrenheit and other customary units. Most speed limit and road signs are in miles per hour and miles. The BTU remains the common measure of heating and cooling, and clothing is measured in inches or sizes. Some of the more common SI prefixes are widely known, and are sometimes used with non-metric units. For example, computer memory storage capacity is measured in kilobytes, megabytes, or gigabytes. Nuclear explosions are measured in kilotons and megatons of TNT. The letter K is commonly used to denote "thousand" (e.g., "She ran a 5K race and earns $80K"). However, contrary to the SI system, this is often written with an uppercase K instead of a lowercase k. This usage has largely replaced the Roman numeral M that was commonly used for "thousand" before the 1960s.

[edit] Consumer and retail

A number of U.S. consumer products come in rounded metric sizes. The trend seems to be toward more product sizes based on metric units, owing to the international nature of manufacturing, distribution, and sales. A few of these products display the metric quantity first or more prominently. Some items are produced and sold in rounded metric quantities (e.g., Crest Glide dental floss is available in 35, 40, and 50 meter packages).

Perhaps the most common metric item sold is the two-liter bottle. Some supermarket chains also make their store brand soft drinks available in 3-liter sizes. Soft drink containers of 1 and 0.5 liters are sometimes sold alongside more popular 12 fl oz, 16 fl oz, 20 fl oz, and 24 fl oz (355, 473, 591, and 710 ml) sizes. Wine is also sold in standard bottles of 750 ml, as in the rest of the world. A fifth of liquor, once one fifth of a U.S. gallon, or 757 ml, is now commonly 750 ml, though it is still referred to as a 'fifth'. It is becoming increasingly common to find bottled water in half-liter and one-liter sizes, though 16 fl oz, 20 fl oz, and 24 fl oz sizes remain popular. However, attempts to sell 3- or 4-liter bottles of milk, instead of gallons (3.785 L), have been largely unsuccessful, and such bottles are almost never seen in the US.^{[citation needed]}

Household products such as shampoo, mouthwash, and dental floss have begun to be sold in metric sizes, and PowerBars and similar products have always been sold (but not marketed) by the gram. Consumer-size photographic film is commonly sold in a 35 mm standard (24x36 mm negative), although print sizes and large format films are defined in inches.^{[citation needed]}

Sometimes metric and non-metric are combined on the same product. The standard method for sizing tires combines millimeters for tread width but uses inches for rim diameter.^[12] Tire inflation for a typical passenger car is 30 psi (pounds per square inch), which is also given in its metric equivalent (207 kPa, or Kilopascal). In lighting, light bulbs use eighths of an inch for bulb diameter and full inches for fluorescent tube lengths, while the socket is always in millimeters (for example, the standard "medium Edison screw" is E26). On recently introduced Christmas lights, however, millimeters are often used with small globe-shaped bulbs (G30 and G40), and with miniature LED sets, where the standard T1¾ (⁷⁄₃₂-inch tube) ones are sometimes called M5 (5 mm miniature; not to be confused with M5 thread).^{[citation needed]}

Nutritional food labels typically report serving sizes in both systems but only list metric values (g or mg) for the breakdown of individual nutrients. The use of Calories (large calorie) instead of kilojoules is a case where a deprecated unit^[13] is used instead of the appropriate SI unit.^[14]

[edit] FPLA issues

Since 1992, the federal Fair Packaging and Labeling Act (FPLA) has required consumer goods to be labeled in both customary and metric units. There is a strong push by industry to amend this law to allow manufacturers to use metric-only labeling.^[15] Most states in the U.S. have already adopted laws allowing metric-only labeling.^[16] According to the European Community (now the EU) Directive 80/181/EEC issued on 20 December 1979, the European Union would have only allowed dual-unit labels to appear on products effective as of 31 December 2009, but problems experienced during metrication efforts in the UK forced the EU to abandon this deadline.^[17]

Pending the EU deadline, an amendment to the FPLA to allow metric-only labeling would allow manufacturers, both importers and exporters, to avoid the significant costs associated with having to produce two distinct types of package labels. A bill to permit metric-only labeling at the federal level was to have been introduced in 2005. Significant opposition from the Food Marketing Institute, representing U.S. grocers, has delayed the introduction of the bill. FMI expressed the fear that legislation to permit metric-only labels might also somehow mandate resizing of packaging to round metric sizes (a process already well underway), and this could require expensive remodeling or replacement of existing display cases. A compromise has been suggested whereby manufacturers in both the U.S. and the EU could opt for either metric-only or dual-unit labeling, but this idea has been viewed dimly by the EU negotiators.^{[citation needed]}

[edit] Construction

Out of all industries, construction has adopted metric units least. Dimensional lumber comes in standard nominal inch cross-section sizes (e.g., a "2-by-4" is really 1½ by 3½ in). Lengths are given in feet. Building codes are established at the local level and the U.S. has extensive building standards that are based on customary units. Because of Canada's proximity to the U.S., issues of common units still arise. This tendency in construction is similar to Japan, an otherwise metricated country, however traditional construction still uses Japanese units predating metrication.

[edit] Aviation

Aircraft heights for air traffic control and related purposes are measured in feet in the U.S. In fact, most of the world continues to use feet for aviation altitudes, with the notable exceptions of China, North Korea, and Russia and many other CIS countries.

[edit] Education

Most students are taught the metric system in elementary, middle, and high school. Instruction primarily centers on the concepts of powers of 10, the associated prefixes, and the conversion from one prefix form to another. Units of length, volume, and mass are typically introduced through comparison with ordinary objects. The metric system is not reinforced much outside the classroom due to lack of popular metric usage. Consequently, while students may understand some of the concepts underlying the metric system, they do not necessarily have an intuitive sense of the value of the units.

In science education, students predominantly use metric measures in experiments and equations, allowing those interested in the sciences to become more familiar with the metric system. Science classes are requisites for undergraduate degree in the U.S. which ensures most educated people have some familiarity with the metric system. Within higher education, the metric system is universal for classes in the sciences (see below). Some non-science textbooks in the U.S. use only the metric system without giving conversions in order to promote metrication. Older students accustomed more to the U.S. customary units may be perplexed by unfamiliar metric measures.

[edit] Electricity and energy

There are no U.S. customary units for electric current, potential difference, or charge since these concepts were developed after the international adoption of metric in science. The metric units ampere, volt, and coulomb have been the only units used. Energy is often measured in watt-hours, BTUs, or calories rather than the SI joule. The rated power of engines, electric motors, and power plant steam turbines is frequently described in horsepower. Residential and commercial electrical energy use is metered and billed in kilowatt hours, kW·h, which is a measure of the watts used over a period of time. Power plant output is generally expressed in millions of watts, or megawatts.

Heating, cooling, and combustion power are often measured in BTUs per hour. Fuel prices are mostly given in customary units, such as dollars per gallon, barrel, thousand cubic feet, or long ton . The SI term hertz has replaced the term cycles per second as a unit of frequency.

[edit] Financial services

The United States was one of the first nations to adopt decimal currency. Until 2001, U.S. stocks were traded in fractions of dollars (½, ¼, ⅛) based on the old Spanish pieces of eight. Some commodity market prices are quoted in customary units (such as barrels of oil, troy ounces of gold, pounds of frozen pork bellies, etc.). The federal government reports international production figures in metric units (for instance, wheat in metric tons) but in bushels for domestic production figures.

[edit] Firearms

The U.S. continues to use the inch for caliber on many civilian and law enforcement firearms.^[18] Except for the 9 mm, most widely recognizable calibers are in inches, such as the .45, .357, or .22. The arms industry is an international one, however, and many foreign gun manufacturers design, build, and sell firearms measured in inches (a good example is the .357 SIG caliber). Also, inch measurements of caliber may refer to the diameter of the bullet and not that of the barrel, like the .308 Winchester, which fires the same projectile as the .30-06 Springfield and the .300 Savage. Often, the caliber is just a number close to the diameter of the bullet, like the .220 Swift, .223 Remington and .222 Remington Magnum, all of which actually have .224 inch diameter bullets. The U.S. military, reflecting its need to ensure interoperability with its NATO allies, uses metric measurements for almost all weapons calibers, even when the caliber in question originated as or was derived from a different measurement. (e.g. 7.62 mm rather than .308, or 5.56 mm instead of .223).

[edit] Manufacturing

Globalization of manufacturing has led to wide adoption of metric standards, although this is not yet universal. After a confusing period where automobiles were assembled with both customary and metric fasteners in each vehicle, cars are now universally built with metric parts. Automobile engines, once named after their displacement in cubic inches, e.g. the 426 Hemi, are currently named after their rounded measurement in liters (e.g., the 6.1 L Hemi). Technical publications by industry publishers and American automakers often give engine displacements in both cubic inches and cubic centimeters (or liters).^[19][20] For example, the specifications for the Dodge SRT-8 6.1 L Hemi state the displacement as 370 cubic inches (6,059 cc).^[21]

The electronics and computer industries largely converted to metric standards as new technologies were introduced toward the end of the 20th century. Thus, while 5¼ inch floppy disks were truly in 5¼ inch wide packages, the nominal design diameter of the replacement "3½ inch" disks was actually 90 mm. CDs and DVDs are 120 mm in diameter, but their size is rarely mentioned; Mini CDs are sometimes called "80 mm CDs". Computer fan sizes are exclusively referred to by metric units (120 mm or 80 mm case fans, for example), while hard disk drive and CD drive bay widths are customary measurements, typically 3½ inches and 5¼ inches, respectively. Additionally, screen sizes are measured in inches, such as a 24-inch widescreen LCD.

[edit] Military

The U.S. military uses metric measurements extensively to ensure interoperability with allied forces, particularly NATO STANAGs, "standardization agreements". Ground forces measure distances in "klicks", slang for kilometers. Most military firearms are measured in metric units, beginning with the M-14 which was introduced in 1957,^[18] although a few legacy exceptions exist, such as .50-caliber guns. Aircraft ordnance is normally measured in pounds. Heavy weapon caliber is measured in millimeters. Military vehicles are generally built to metric standards. An exception is the U.S. Navy whose guns are measured in inches and whose undersea fleet measures distances in terms of "kiloyards" (equivalent to 914.4 m), depth as "feet", and velocity, in some cases, as "feet per second". The Navy and Air Force continue to measure distance in nautical miles and speed in knots; these units are now accepted for use with SI by the BIPM.^[22]

[edit] Illegal drugs and controlled substances

Illegal drugs and controlled substances are often measured in metric quantities.^[23] The federal law prohibiting them defines the penalties in metric masses.^[24]

In everyday usage within its subculture, marijuana is sold using a combination of metric and American customary units. At small scales the basic unit is the gram (or fractions thereof), but for larger orders fractions of ounces and pounds are used generally starting at ⅛ ounce, which is often defined in turn as 3.5 grams and referred to simply as "an eighth."^[25] The term "kilo" is used quite commonly by those in law enforcement to describe the amount of drugs (in kilograms) seized during a drug bust. A kilo of illegal drugs (in particular cocaine) is often referred to by the slang term "key".

[edit] Science and medicine

In science, metric usage is essentially universal, consistent with worldwide usage, although additional specialized units are often utilized for specific purposes in various disciplines (such as the light-year in astronomy). Medicine is generally metric, but where interaction with patients is required, usage is often mixed. Doctors often measure a patient's weight in pounds to compute a dosage of medicine administered in grams or milliliters. Heights are measured in inches but head circumferences in centimeters. Even in many cases for which metric units are otherwise used, U.S. practice often varies from the rest of the world. For example, U.S. diabetes patients measure their blood sugar levels in milligrams per deciliter, whereas most other countries use millimoles per liter.

In hospital settings, patient data is typically recorded using metric quantities, but all interaction with patients is done with reference to U.S. customary units (i.e., patient weights might be recorded in kilograms but discussed with patients in pounds). Because of online access to BMI calculators and concern over rising obesity rates, the international standard of body mass index based on the relationship between weight in kilograms and height in meters has been widely adopted by American healthcare workers and patients.

Meteorologists are required to have substantial knowledge of the metric system, since most observing, analysis, synoptic weather charts, and computer forecast models are all done in metric. Because most weather forecasts are issued to the public in customary units, meteorologists must also know how to convert between the two systems.

Usage among veterinarians varies, but because of the heavy emphasis on public interaction, animal weights (e.g., for cats or dogs) are nearly always recorded and reported in pounds and ounces. Usage at veterinary teaching hospitals, by contrast, favors the SI units.

In early 2007, NASA announced that it will use metric units for all operations on the lunar surface when it returns to the Moon, projected for 2020.^[26]

[edit] Sports

U.S. citizens are frequently exposed to metric units through coverage of international sporting events, particularly the Olympic Games.

Races in the U.S. are usually run in metric distances (e.g., the 100 meter dash and 5 km ("5K") races), although the mile distance remains popular. Note, however, particularly for local events, even a "5K" race is often promoted as a "3.1-mile" event, and the course markers will be at one-mile (not one kilometer) intervals. The marathon is referred to as a 26-mile race (actually 26.22 miles) rather than 42.195 km.

Measured distances as used in events such as shot put, high jump, and discus throw are, however, often in feet and inches, except at the national and international levels. The field and court sizes for most popular team sports (even soccer) were originally set in non-metric units. This is reflected in American football, in which the playing field is divided into yards, and many important statistics are measured in yards. Some sporting equipment, such as skis and poles, is sold in metric units. The lengths of automobile and horse races in the U.S. are generally set in miles.

Bicycles, especially at the high end, are increasingly being sold and sized in metric (for example, a frame described as "21 inch" size in the past is now often labeled as a "53 cm" frame instead). Some bicycle wheels are labeled in metric as "700c" for a nominal 700 mm diameter road bike tire, but mountain bikes tend to use 26-inch wheels.

Common swimming pool dimensions are 25-yard, 25-meter, and 50-meter. High schools and the NCAA conduct 25-yard competitions. USA Swimming (USA-S) swims in both metric and non-metric pools.

[edit] Transportation

"Valencia Rd. Tucson. 500 m" on Interstate 19

Highway speed limits are posted in miles per hour and distances are largely displayed in miles, yards, or feet, although a few dual mile/km signs can be found, mostly left over from demonstration projects that are no longer supported.^[27] One exception is Interstate 19 in Arizona, because of its direct link to Mexico,^{[citation needed]} which is almost completely signed in metric, except for speed limit signs. Signage on this road is, however, being converted back to customary units as they are replaced. Another exception is the turnpike section of Delaware Highway 1, which uses a kilometer-based system, in anticipation of the mid-1990s conversion in the U.S. to the metric system (which did not happen). Distances were originally signed in kilometers, but have since been replaced with standard milemarkers. The exit signs have remained metric.

Since 2000, the Manual on Uniform Traffic Control Devices has been published in both metric and American Customary Units. Metric signs, including speed signs in kilometers per hour, are now displayed in the manual even though they are not normally used on the roads and there is no definite plan to go metric yet. For visual distinction, regulatory and advisory metric speed signs have black circles around the numbers followed by "km/h" and other regulatory metric signs have yellow plates reading "METRIC" in black on top of the main signs. Officially, this means that, although not widely used, metric road signs are allowed in the United States. The UK Metric Association considers this one way in which metrication is ahead of the United Kingdom, where metric road signs are usually illegal.^[28]

Distance sign near Bowling Green, Ohio

Due to their proximity to Canada, the states of Washington, Vermont, New Hampshire, Maine, and parts of Upstate New York include distances in both miles and kilometers on many highway signs,^{[citation needed]} especially in municipalities close to the border, although some older signs still have miles only. Newer signs on state highways on the island of Kaua‘i in Hawai‘i also include distances in both miles and kilometers.^[29] Mile markers along highways on the Island of Hawai'i will indicate both units whenever the whole kilometer corresponds with the whole mile (ex.: 5mi/8 km). At least one route (U.S. Route 31) in Alabama has both mileposts and kilometer-posts.^{[citation needed]} In Houston, Texas many speed limits have km/h signs underneath the MPH signs. These are located near both airports and in the Texas Medical Center. The U.S. territory of Puerto Rico generally uses American-style (MUTCD) signs with metric measurement on highways, displaying kilometer posts and distances in km, yet speed limit signs are in miles per hour.

Gasoline and diesel fuel are sold by the U.S. gallon, and fuel economy is rated in miles per gallon (MPG). However, some border towns neighboring Canada display the equivalent cost in Canadian dollars per liter because of the many Canadians who cross the border to buy cheaper gasoline.^{[citation needed]} Automobile crankcase oil is sold by the quart, antifreeze by the gallon, and brake fluid and A/C refrigerant by the fluid ounce. Windshield wiper replacement blades are measured in inches.

Domestic airline flights are assigned altitudes in feet and measure speed in knots. Nautical charts show depth in fathoms and use the nautical mile for distance. (One minute of arc of latitude at the radius of the earth at sea level was the standard for one nautical mile until ca. 1929. The nautical mile is now defined as exactly 1852 meters.) Railroads use the standard gauge of 4 feet 8½ inches, as does most of Europe (where it is expressed instead as 1435 mm). U.S. Federal Motor Vehicle Safety Standard 101, which governs vehicle controls and displays, permit speedometers to display miles per hour, kilometers per hour, or both. Odometers are permitted to record miles or kilometers, but must be clearly labeled as to which unit they record. In practice, virtually all U.S.-market vehicles have mile odometers and dual labeled speedometers with mile-per-hour as the primary calibration. Some vehicles with electronic speed and distance readouts can be switched between U.S. and metric units.

[edit] Hybrid units

Some measurements are done in units derived from both customary and metric units. For example:

• Power per unit length from a nuclear reactor fuel rod: kilowatts per foot
• Heat rate from a power plant: BTUs per hour per kilowatt
• Federal automobile exhaust emission standards: grams per mile
• Caffeine in beverages: milligrams per ounce
• Running load limits specified in Boeing airplane flight manuals: kilograms per inch.
• Energy Efficiency Ratio (EER), which is the ratio of an air conditioner's cooling capacity in BTU/hr and its input power in watts. It is used instead of the coefficient of performance, which is a dimensionless ratio that is the same in any system of units as long as they are consistent.

Any measurement that combines customary units with electrical units is necessarily hybrid, since there are no U.S. customary electrical units. An example can be found in Table 8, chapter 9 of the National Electrical Code Handbook (8th ed.) where resistance of conductors per unit length is given in ohms per thousand feet.

Other units are based on customary units but use metric prefixes. For example, distance to target for a U.S. submarine is expressed in kiloyards.

[edit] Cultural impact

The continuing usage of U.S. customary units has been a cause for speculation in fiction set in the future. Many such works of fiction have assumed that the United States and the human race in general will use metric units in the future; however, others have considered the continuing persistence of the U.S. customary system to be more likely. Additionally, writers will sometimes use customary units simply because American readers and watchers will understand the measurements; other times, they are simply used by accident. The 1966 show Star Trek, for instance, initially used U.S. customary units despite multiple Earth national origins and a setting in the 23rd century. Later, starting with "The Changeling", metric measures were used, albeit inconsistently. Later sequels to the show, such as Star Trek: The Next Generation, used the metric system exclusively.

Both the metric system and attitudes in the U.S. toward the metric system are a target of humorous jabs in shows such as The Simpsons^[30] and Futurama^{[citation needed]}. The Saturday Night Live sketch Decabet, released in the height of the metrication movement, lampooned the different metric measurements by introducing a new Metric Alphabet consisting of only ten letters.

[edit] See also

• Plan for Establishing Uniformity in the Coinage, Weights, and Measures of the United States, 1790 report including proposal for decimal system based on the foot
• The Metric Marvels
• Metrication controversy

[edit] References

[edit] External links

• The United States and the Metric System (LC 1136) nist.gov
• The Metric System in the United States
• Gometric.us
• A Metric America: A Decision Whose Time Has Come on nist.gov
• Metric Conversion Act of 1975 on nist.gov
• US Metric Association (mirror link)
• Mean Mr Metric satire about the (non)conversion to metric units. One of the highest scoring submissions to the Internet Oracle. Originally written by Jim Palfreyman in October 1991.
• Waits & Measures
• Examples of metric roadsigns in the U.S.
• P-Metric and Euro Metric Tire Sizing on TireRack.com
• Metrication.us site
• Metric 4 US site
• Metrication matters site