The United States retains a non-decimal measurement system largely derived from traditional Roman and British units. Technically speaking, the federal government has observed the metric system since it ratified the Metre Convention in 1875, and indeed, the units described below are now legally defined through their metric equivalents rather than their historical origins. The metric system is also almost universally used by the scientific community, though Unit Confusion has sometimes arisen when American and international teams each assumed one was using the other's units. To further complicate matters, the American engineering community has held on to customary units, resulting in occasional disconnects between builders and users of scientific equipment (for example, the loss of the Mars Climate Orbiter, popularly attributed to Unit Confusion, was actually due in large part to this sort of thing, specifically a software error: the thruster-control algorithms used force-pounds, while the high-level software worked in newtons, a problem caused by lack of communication between NASA and its contractors and exacerbated by lack of proper testing). What has come to be known as the "customary system", for lack of a better term, has continued in usage in for a number of factors, including tradition, nationalism, concern over the economic costs of conversion (an attempt by Congress at metrication during The '70s was axed for this very reason, as well as lack of political will), and the reasoning that by the time the metric system was introduced in the late eighteenth century, the British units were better-regulated by law than those of its European neighbors, let alone France, whose unwieldy menagerie—around 250,000 units, many sharing the same name but varied by locale—was one of the reasons the metric system was created as a sort of Reset Button.
While the aforementioned mandatory metrication in the 70s ultimately failed, the bottling industry did make the switch, albeit for other reasons—Dr. Pepper advertised the small size gain from 2 quarts to 2 liters, and all other companies followed suit, which is why 2-liter bottles (and recently, 1-liter bottles) are in metric, but smaller units are in fluid ounces (which makes a Dirk Pitt Adventures novel look rather funny). It didn't hurt that the bottling industry wanted to switch from heavy, expensive and fragile glass to light, cheap and durable plastic bottles at just about that time; if you're going to be retooling everything anyway, might as well make it count.
In Canada, Britain, and other Anglophone countries, a cousin of this system also enjoys non-official use as the "imperial measurements", though their defining values may be slightly different; an imperial gallon, for example, is about 20% more voluminous than its American cousin. There's also some unofficial use in other countries, particularly Latin America, whose proximity to the USA means some prolific use of customary units; for example, where most metricated nations (such as those in Europe) use kilowatts and newtons-meter for power and torque, respectively, Latin American nations still use horsepower and pounds-foot, respectively. Car wheels (and inner dimensions of tires) are almost universally measured in inches. An abortive attempt was made to switch to metric in the early '80s; as of the present, even Michelin, the driving force behind the move, no longer supplies tires to fit these sizes with Michelin-branded ones now made by antique reproduction specialists Coker Tire in low volume at prices just short of exorbitant. Tire sizes are actually a complicated mix of units, which is fine because almost nobody outside the automotive industry knows, much less cares about, what they mean—the first number is in millimeters, the second a percentage (but may be millimeters), and the last in inches (but may also be millimeters). Fortunately, it's usually trivially obvious if the optional millimeter measurements are used, because the numbers will be much larger (around 23× as high for the second number, around 25× as high for the last number).)
Incidentally, many folks like to say that the United States uses the Imperial Measurement System, which is not the same as American Customary Units. The Imperial units were standardized in 1824, well after the USA stopped paying much heed to Westminster's declarations, while the United States customary units were based on older British units and standardized in the 1890s (the measurements now being defined by their metric equivalents, as mentioned above). Both systems are close enough to the same for most day-to-day usage that most Americans don't know or care about the difference anyways.
The following is intended as a primer for non-Americans to whom the customary units of measurement may seem foreign, cryptic or simply hard to visualize. In modern times, most of these units are defined by their metric equivalents.
Length and AreaThe basic unit of length is the foot (30.48 centimeters), which is divided into twelve inches (2.54 centimeters each). When greater precision is required, fractions of an inch are used (generally based on powers of 2, such as ½, ¼, ⅛, and so on), except in certain engineering disciplines which use decimal derivations. Feet and inches are commonly abbreviated by apostrophes (one for feet, two for inches — e.g., 6 feet and 4 inches [193 cm] becomes 6'4"). Three feet make up a yard (91.44 centimeters), which is more commonly encountered in American Football, as well as the basis of the current measurement system, defined in a 1959 agreement with Britain, Canada, Australia, New Zealand and South Africa as exactly 0.9144 meter.
On land, 5,280 feet (1,760 yards) make up a (statute) mile (1.609344 kilometers); originally defined as exactly 5,000 feet, emulating the Roman mille passus, or a thousand paces of five feet each, a 1593 act by the British Parliament shortened the foot in order to accommodate the rod (5.0282 meters), chain (4 rods, 20.1168 meters), and furlong (10 chains, ⅛ mile, 201.168 meters), all of which were widely used in surveying at the time (in particular, the latter was originally defined as the distance a team of oxen can plow the land in one run and is still used in horse racing), allowing the inch, foot and yard to easily precede these units; while the rod would have an awkward 16.5 feet (5.5 yards) each, the shorter foot is nevertheless tidier than 15.625 pre-1593 feet (or around 5.208333 yards), and there would be exactly 168 inches to the rod.
In navigation, the nautical mile is used; unlike its land-based cousin, however, this mile is longer and unwieldy at around 6,076.1155 feet (1.1508 miles), or exactly 1,852 meters; as it was historically defined as one-sixtieth of an arc-degree of Earth's latitude (thus one arc-minute) and currently defined in metric terms, it is one of the few non-metric units officially tolerated for use among nations where the metric system is enforced. Water depth is also measured in fathoms (6 feet, 1.8288 meters).
For historical compatibility, real estate maps are still drawn in the older survey foot (1200/3937 meters, around 30.48006 centimeters).
Two smaller units, the point (1/72 inch, around 0.3528 millimeters) and pica (12 points, 1/6 inch, around 4.2333 millimeters), are used in graphic design and typesetting. Outside those fields, one is liable to run into points in the context of font sizes (though usage for font sizes on a computer screen is mostly traditional and will not usually reflect the actual physical size of a font — technically, a 12-point font should render in the same physical size regardless of the physical size of the screen and screen resolution).
One notable exception is the automotive industry, which standardized on metric fasteners in the '80s, thus it's common to see both fractional-inch and metric sockets and wrenches in toolkits. Also, in some circles, lengths of less than a half-inch or so are given in millimeters because they're easier to work with; things like the tips on writing instruments and the width of photo film have been measured in millimeters for decades.
In area, an acre (4,046.8564224 square meters, just over 0.4 hectare) is defined as the area of a rectangle measuring one chain by one furlong (66×660 feet), or 43,560 square feet (4,840 square yards). Outside agriculture, the square mile (640 acres, 258.9988110336 hectares) is used.
MassThe basic unit of mass is the (avoirdupois) pound (453.59237 grams), which is divided into sixteen ounces (28.349523125 grams each)note . The pound can be further divided into 7,000 grains (64.79891 milligrams each), though it is seldom used except in certain specialized fields, mostly archery and firearms.
2,000 pounds make a (short) ton (907.18474 kilograms), while the heavier (imperial) ton, used in Britain, weighs 2,240 pounds (1,016.0469088 kilograms); the discrepancy has to do with a pair of units more-or-less unique to the British system, the stone (14 pounds, 6.35029318 kilograms), the hundredweight (112 pounds, 1/20 ton, 50.80234544 kilograms; the even more uncommon American version has been trimmed to exactly 100 pounds).
Those who deal with precious metals use the troy pound (144⁄175 avoirdupois pound or 373.2417216 grams), defined as 5,760 grains. The troy pound consists of twelve troy ounces (31.1034768 grams each), in contrast to sixteen for its avoirdupois version. When one refers to both units, the standard ounce and pound are sometimes called ounce avoirdupois and pound avoirdupois, respectively, to avoid confusion.
The pound is often abbreviated as "lb", from the Roman equivalent libra (which is also the source of the £ symbol).
VolumeThe basic unit of liquid volume is the gallon (3.785411784 liters), defined as 231 cubic inches. A gallon is divided into four quarts (946.352946 milliliters each), which is divided into two pints (473.176473 milliliters each), divided in turn into two cups (236.5882365 milliliters each), each divided again into eight fluid ounces (29.5735295625 milliliters each), not to be confused with the ounce discussed above (note, however, that a fluid ounce of a water-based liquid weighs close enough to an ounce avoirdupois for government work; hence the saying "A pint's a pound the world around", which turns a blind eye to every [ex-]British part of the world).
For historical reasons, a few dry goods are sold using the dry volume system, whose basic measure is the (dry) quart (1.101220942715 liters), four of which create a (dry) gallon (4.40488377086 liters); crops are typically sold by the bushel (8 dry gallons or 35.23907016688 liters) or peck (2 dry gallons, ¼ bushel or 8.80976754172) — thus Peter Piper picked two dry gallons of pickled peppers. The Commonwealth, on the other hand, standardized the gallon, for both wet and dry goods, as exactly 4.54609 liters, having been originally defined in 1824 as the volume of ten pounds of water at 62°F (17°C); this also results in larger imperial pints (568.26125 milliliters each), so the counterpart to the "pint's a pound..." thing is an imperial gallon weighing ten pounds; the "cup" does not exist as a unit per se, and a Brit seeing "cups" in an American recipe is likely to think "but what size of cup?".
42 liquid gallons equal a petroleum barrel (158.987294928 liters), the standard international unit in which oil is sold, but most other liquids sold by the barrel use a definition of 31.5 gallons. A barrel of whiskey, however, is larger than a petroleum or "standard" barrel, at 53 gallons (200.62682455 liters). The buttload (476.961884784 liters) is now used just to mean "a lot", but it used to be two hogsheads of 63 gallons each (238.480942392 liters), used to measure wine.
The acre-foot (43,560 cubic feet) is used for measuring large-scale water resources such as lakes, aqueducts, canals, rivers, and irrigation flows. Its definition is what you'd expectthe amount of water needed to cover one acre to a depth of one foot. The metric equivalents vary by a tiny percentage based on whether the international or US survey foot is used, but both translate to a hair less than 1,233.5 cubic meters.
Spaces you don't expect to fill with liquid (car trunks, apartments, warehouses, etc.) are expressed in cubic feet (28,316.846592 cubic centimeters).
Cooks use three additional units: the teaspoon (~4.9289 milliliters), tablespoon (3 tablespoons, ½ fluid ounce or 14.78675 milliliters), and kitchen cup (16 tablespoons, 8 fluid ounces, or 236.588 milliliters), which is slightly more voluminous than the fluid cup above. Two cups make a pint (473.176 milliliters).
Ingredients in recipes are usually measured by volume, not weight (save meat and produce, which are bought by the pound). Most households do not possess a kitchen scale, thus these are generally priced only for the upscale or gourmet market, and can cost significantly more than an accurate measuring cup (e.g. 50¢ for a cup vs. $30+ for the cheapest scale at large chain houseware stores). Americans therefore tend to see British recipes as inaccessible and "only for snooty gourmets."
Prior to the early '70s, American automobile engines listed their displacement in cubic inches (The "409" that The Beach Boys sung about was a car with a 409-cubic-inch engine). American cars have been built using metric measurements since then, when the energy crisis led to downsizing of engines, and it became preferable to advertisers to describe the size in liters rather than cubic inches. but since some cubic inch-based sizes are closely related to former performance they still pop up now and then. When Vanilla Ice rapped about his "five-oh," he was referring to the 5.0l Mustang, which was a heavily redesigned "302" that is really closer to 4.9l. Small-block Chevrolet V8s are still closely associated with 350 c.i., although the company hasn't made one (or at least a 5.7l) in years.
TemperatureThe standard scale of temperature is the Fahrenheit, where the boiling and freezing points of water at sea level are respectively set at 212° and 32°, a gap of exactly 180°, as intended by the unit's inventor and namesake, German physicist/engineer Daniel Gabriel Fahrenheit, as it would make it easier to mark a rotary-gauge thermometer. As for 0°F, it is an arbitrary point, based on the approximate freezing point of seawater, then the lowest measurable temperature at the time the scale was developed.
The mean body temperature of a healthy adult is approximately 98°100°F (an oft-quoted figure of 98.6° is really just an overprecise conversion from 37°C). Temperatures less than 0° are colloquially spoken as "X below [zero]"; thus, 10°F would be "ten below". The Fahrenheit and Celsius scales coincide at 40°, and absolute zero is exactly 459.67°F. To derive a Celsius temperature from a Fahrenheit reading, subtract 32 from it and divide by 1.8 (9⁄5); conversely, to derive Fahrenheit from Celsius, multiply the reading by 1.8 and then add 32.
There also exists the Rankine scale, which is largely a historical curiosity at this point. Like the Kelvin scale, it starts at absolute zero and uses the same size degree as the Fahrenheit scale, so water freezes around 491.67° and boils at 671.64°. It's mainly used by engineers and scientists working in American Customary Units as a shortcut to avoid converting everything to metric and then back again.
American cookbooks specify temperatures exclusively in °F. Oven temperatures are always given in 25° increments, e.g. "Preheat oven to 425". The "gas mark" notation seen in British cookbooks is not used.
Dates and TimeAmerica almost exclusively uses the 12-hour clock, which is divided into two intervals — "AM" (ante meridiem, before noon) and "PM" (post meridiem, after noon). "8 o'clock" therefore is not a specific enough time in certain circumstances, and would be clarified as "8 o'clock in the morning/evening." The 24-hour clock is mostly limited to the military, thus the sobriquet "military time". In the Army and Air Force, 18:00 would be called "eighteen hundred hours", whereas the Navy (and by extension, the Marine Corps) drops "hours" and would simply say "eighteen hundred". Do note that in writing, military time does not use a colon (e.g., 1800). To a lesser extent, 24-hour time is used by professionals in round-the-clock industries, such as hospitals, transportation, and (increasingly) restaurants.
As to dates, they are written in the month/day/year format instead of the European day/month/year. December 20th, 2010 would be written as 12/20/2010, or just 12/20/10. This mimicks the way the date is written and said in full in English (unless you follow the "20 December" format), but it has the quirk of going back and forth in size, instead of being strictly hierarchical.
EnergyA few units of energy are in common use. The first and most basic is the foot-pound, which, as the name suggests, is the amount of work done by one pound of force over one foot (~1.36 joules). Since nearly anyone who has a use for measurements of energy outside the following applications is primed on SI units, this is mostly used to give laymen and/or schoolchildren an intuitive idea of what energy and work are.
The second is the kilowatt-hour (~3.6 megajoules), mostly used by electric companies. Most Americans know what a watt is through electrical device ratings, so to make things more intuitive, power meters use this composite unit (the amount of energy used by a 1,000-watt appliance in an hour) instead of measuring things directly in joules.
The third is the calorie (~4.184 joules), used in heating, nutrition and kinesiology. An odd duck of sorts, the unit, based on the specific heat of waternote . It was introduced as a metric unit in 1824, and was later displaced by the joule — but not before it found usage within the United States. The unit most Americans refer to when they say "calorie" — the nutritional one, see below — is actually the kilocalorie (formerly known as "large calorie").
There is also the British Thermal Unit (BTU), which is mainly used for things like specifying the amount of heat put out by furnaces. There are various definitions of the BTU, which range between 1,054-1,060 joules. To make things even more confusing to outsiders, American engineers measure cooling in tons (12,000 BTU per hour, equivalent to a ton of ice per day or ~3,516.85 watts) and heating in MBH (a thousand BTU per hour, or ~293.07 watts; the "M" is the Roman numeral for "1,000", not the expected million); both units are also technically measures of power, not energy, but the BTU is an odd enough bird that it's better to keep everything together.
PowerThe common unit of power is horsepower, of which there are at least seven variants, just to be even more confusing. The most common is mechanical horsepower (AKA brake horsepower, from the method used to derive it), used by auto manufacturers to tell the public that their car is more powerful, and is equivalent to 550 foot-pounds per second (~745.7 watts). There is also indicated horsepower (an old measurement used for reciprocating steam engines), shaft horsepower, metric horsepower, electrical horsepower, boiler horsepower, and hydraulic horsepower, all of which can be safely ignored, with the sole exception of the metric version, which is sometimes used to rate power outputs of automobiles and other engines in Europe, and always used for Japanese cars. The metric horsepower is sometimes called the Pferdestaerke (German for "horse strength"), and abbreviated "PS" or (rarely) "cv". The metric horsepower is slightly smaller than its American equivalent, at 735.49875 watts — for instance, the old Japanese output limit of 280 PS for sports cars is around 276 BHP. You'll also hear references to "SAE net" or "gross" horsepower; specific to car engines, these terms have to do with the testing standard, not the units themselves. SAE gross horsepower, the standard for the auto industry until 1972 and still used in the trucking industry, is measured on a test stand in a pressurized room, with coolant and oil pumped through from outside sources so the engine doesn't have to spin its own pumps. In other words, rigged to give the highest number possible.
All that said, you're unlikely to see horsepower used outside of the automotive (and the propeller aviation and helicopter aviation) realm or certain kinds of electrical motors. As noted above, all electrical appliances in the US specify their rated consumption in watts (or the equivalent in volts and amperes), especially light bulbs and pretty much anything with a heating element in it. Ads for things electronic also tend to boast about how many watts they can output, especially high-power audio amplifiers and, sometimes, even the radio stations themselves.
ForceMeasured in pounds, as stated above. Often expressed as "pounds of force", "lb-force", or "lbf" for disambiguation from the mass unit.
TorqueTorque, or rotational force, is the cross-product of the measured linear force vector and the scalar radius at which it is measured. In conventional units, this is expressed in either pounds-feet or foot-pounds. Both are correct and interchangeable, but some engineers prefer one over the other.
PressureAir pressure is usually given in psi, or pounds per square inch (~6.894 kilopascals). Atmospheric pressure is usually given in atmospheres or millimeters of mercury (760 mmHg = 1 atm = 101.325 kilopascals ~ 14.7psi (all at sea level)). Aviators still measure pressure in inches of mercury (29.92" = 1 atm), both for altimeter settings and engine manifold pressures. Televised weather reports on the news also commonly use inches of mercury for the barometric pressure.
Food LabelingAs mentioned above, the food industry was one of the few to be legally required to use the metric system during the failed attempt at forced metrication, so food packaging is, outside school, the place where most Americans encounter the metric system. Since there are few laws regarding the use of metric units, different foods tend to use metric and customary units in different ways on packaging. Most of the time, either system will be indicated first, with its equivalent indicated thereafter in parentheses. Some examples are as follows:
Nutritional labels indicate energy content in kilocalories (confusingly, always labeled "Calories" — note the capitalization; that's important), and amounts of other nutrients and additives in grams or milligrams. Vitamin content is typically indicated only as a percentage of the federally-recommended daily allotment. Since the format of nutritional labeling is set by the FDA, this is harmonious across all food products.
Dry and refrigerated packaged, canned, and frozen foods indicate net weight in ounces, followed by the metric equivalent in (parentheses).
Fresh meat, fish, and produce is always sold by the pound, and metric measurements are seldom indicated. (Cheese is also sometimes sold by the pound.)
Beverages sold in plastic bottles (soft drinks and most fruit juices) are labeled in liters for sizes 1 liter and up, or fluid ounces for smaller bottles, commonly at — common sizes are 12, 16 or 20 fluid ounces, 450 milliliters, 0.5, 1, 1.5, 2, or (less commonly) 3 liters. Soft drinks less than 1 liter are either sold individually or in "six packs", "eight packs", "12-packs" or "24-packs", held together by a plastic spine called a yoke, by a plastic bag, or packed in cardboard.
Canned beverages are labeled in fluid ounces — common sizes are 8, 10, 12, 18, or 24 fluid ounces. Juices and other more expensive beverages may have a little bit removed to save money, leading to 11.5-ounce juice cans. (Incidentally, twelve fluid ounces comes out to ~354.88 milliliters, but beverage canners almost always round that up to exactly 355.)
Beverages sold in cartons or polyurethane jugs (dairy products and some fruit juices) are sold by the gallon, half-gallon, or quart.
Prescription drugs, on the other hand, are labeled in metric units, because the amounts in question are usually so tiny as to render the conventional units meaningless, plus the fact that we're deep into scientist territory here. One may occasionally find a medicine bottle with weight indicated in grains, though they're today extremely rare.
Drugs and AlcoholBeer is generally sold in 6-packs, with 6 bottles or cans measuring 12 fl. oz. each. Some beers may be sold in loose bottles of larger sizes from 16 oz. (colloquially, a "tallboy") to 40 oz. (a "forty") Many bars often have pints available as well as pitchers for larger parties; glass mugs of approximately one liter are also fairly common, particularly in areas settled by people from southern Germany (e.g. the Upper Midwest). A pitcher of beer contains roughly 60 oz, or five bottles' worth. Some bars and restaurants specializing in beer will sell drafts in 20 oz "English" pints. Beer purchased to-go from a bar is typically poured into a "growler", which may be either 32 or 64 ounces. Wine and spirits are typically sold in 750 mL bottles, 750 mL being the round metric number that most closely approximates the pre-metric bottle size of ⅕ gallon (a "fifth"). These bottles, along with the smaller 375 mL and 200 mL bottles, are still colloquially referred to as "fifths", "pints", and "half-pints" despite their volume being slightly less than either. (These terms are more commonly used with spirits; many people just call 750 mL of wine a "bottle", though there are various archaic names for the larger sizes, including "Magnum" for a 1.5L, "Jeroboam" for a 3L, or "Methuselah" for a 6L, all the way up to the 30L "Melchizedek".) Spirits are also sold in 1.75 L bottles, which are colloquially called "handles" after the carrying handle such bottles usually have, or "half-gallons" or "half-g's" because they hold slightly less than half a gallon of spirits. Single-serving size bottles of spirits are 50 mL and are often referred to as "minis" or "airplane bottles" due to their widespread use for mixing drinks on commercial flights.
The alcoholic strength of spirits is described as a percentage of alcohol by volume, and, in the case of spirits, by "degrees proof", where 1 proof = 0.5% ABV. (Contrast British degrees proof, wherein 100 proof equals the point at which gunpowder moistened by the spirit is still capable of ignition, i.e. 57.15% a.b.v, or 114.3 American proof). Several US states regulate alcoholic beverages based on alcohol by weight, which is roughly equivalent to 4/5 of a given product's alcohol by volume; 3.2% ABW beer, the strongest beer that may be sold in grocery stores or convenience stores in several of those states, is roughly 4.0% ABV.
Marijuana is usually purchased in weights measuring ⅛th of an ounce, or it is measured in grams for smaller quantities. The same goes for "magic" psilocybin mushrooms. Cocaine is sold by the gram or 8-ball, which is 3.5 grams (just under ⅛ of an ounce hence the name). LSD can come in liquid form, measured in micrograms, or is sold on cardboard strips known as "blotters." Drugs like ecstasy typically come in quarter-gram pills, which are sold individually (though the weight of active MDMA in the pill will be substantially lower than that). There's an old joke that "drugs are God's way of teaching Americans the metric system."
Note that since drugs like marijuananote , cocaine, LSD, and ecstasy are illegal, they are by definition sold on an unregulated black market. The only guarantee a buyer has for the weight and purity of a particular drug is the good faith of the seller.
Abbreviations: The words are usually not spelled out, so here are the relevant abbreviations (and symbols):
- $: Dollars (confusingly, $ is also the abbreviation for Pesos in Mexico, which borders the U.S.). Appears before the number (except in Quebec).
- Doubly confusing, as some areas that operate with the US dollar (such as Puerto Rico or the Virgin Islands) may refer to the currency as a "peso".
- ¢: Cents. Appears after the number. Used instead of, not in addition to, the dollar sign, and only if an integer number of cents are specified (rather than a decimal value of dollars). Almost never used these days, as (thanks to inflation) most goods worth buying cost more than a dollar. Even as early as the 1960s, when computer character sets were becoming standardized, neither the ASCII nor the EBCDIC character set contained a cents sign (American computer keyboards replaced it with the caret, i.e, ^).
- fl oz: Fluid ounces
- ft or ': Feet
- hp or bhp: (Brake) horsepower
- in or ": Inches
- lb or #: Pounds (although # is dying as an abbreviation, it is occasionally used after a number to denote weight. 5# = 5lb. It's most commonly used by voicemail systems: "Please enter [multi-digit number], followed by the pound sign.")
- mi: Miles
- Except for mph (miles per hour)
- Not to be confused with the metric "m" for "meter". note
- Except for mph (miles per hour)
- oz: Ounces
- yd: Yards
For those wondering, a hogshead is 63 U.S. liquid gallons, and a rod is 16.5 feet. 40 rods to a hogshead, as mentioned in the quote above, works out to approximately 120,000 liters per 100 km. In miles per gallon that's statistical zero.note A gas-guzzler even by American standards of the quote's era, then.