American Customary Measurements

"The metric system is the tool of the devil! My car gets forty rods to the hogshead, and that's the way I likes it!" —Grandpa, The Simpsons

36-24-36. (Maybe, if she's 5'3".)

Whereas in most of the world the metric system has come into near-universal use by ordinary citizens, the US retains a non-decimal system of measurement largely derived from traditional English units. Technically speaking, the US government has observed the metric system since it ratified the Convention on the Meter in the late 19th century, and indeed, the customary units described below are now legally defined in terms of 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 the builders and users of scientific equipment (for example, loss of the Mars Climate Orbiter, popularly attributed to unit confusion between American and European space agencies, was actually due in large part to this sort of thing ). What has come to be known as the "customary system", for lack of a better term, has continued in usage in the US for a number of factors, including tradition, national pride, and the expense of conversion. An attempt by Congress to mandate metric conversion in the 1970s failed primarily because of the last reason (and the lack of political will to insist). Only the bottling industry switched, but not for that reason. One company (Dr Pepper) advertised the small size gain from 2 quarts to 2 liters, and all other companies followed, which is why 2 liter bottles (and more recently, 1 liter bottles) are in metric but smaller units are in Fluid Oz.

In Canada, Britain and other English-speaking countries, these units also enjoy non-official use (known as Imperial measurements, after the British Empire), though their defining values may be slightly different; an Imperial gallon, for example, is about 1.2 American gallons. This is, combined with mixing randomly with metric units, of course part of the secret British plan to confuse all foreign visitors. There's also some unofficial use in other countries, particularly in Latin America, where the close proximity to the USA means Imperial units are sometimes used instead of Metric units; engine power and torque, for example, is often measured in Europe in kilowatts and newtons-meter (kW and Nm), while in Latin America the former is sometimes measured in horsepower and the latter in pounds-foot. Car wheels (and tire inner dimensions) are almost universally measured in inches, worldwide. An abortive attempt was made to switch to metric in the early 80s; at this writing (2008) only Michelin supplies tires to fit these sizes, in low volume at prices just short of exorbitant.

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.

Distance. The basic unit of distance is the foot (0.3048 m), which is divided into twelve inches (2.54 cm). Five centimeters is very close to two inches. When more precision than that is required, fractions of an inch are used (Generally power of 2 based (1/2, 1/4, 1/8)), except in certain engineering disciplines which use decimal inches. Measurements in feet and inches are commonly abbreviated using one apostrophe for feet and two apostrophes for inches - six-foot-four, for example, becomes 6' 4. Three feet make up a yard (0.9144 m). On land, 5,280 feet make up a mile (~1.609 km), whereas a nautical mile is ~6,076 feet (1852 m, or one minute of latitude, hence its use in the SI.). 4,840 square yards, or 43,560 square feet, constitute an acre (~4047 square meters, or just over two fifths of a hectare).

Since 1959, the foot has been internationally defined as exactly 30.48 cm. For historical compatibility, though, real estate maps are still drawn in the older "survey feet", which are 1200⁄3937, or about .3048006 m long.

Two smaller units, "points" and "picas", are used in graphic design and typesetting, and refer to one twelfth and one seventy-second of an inch. Outside those fields, one is liable to run into "points" in the context of font sizes (though the usage of point measurements for font sizes on a computer screen is mostly traditional and will not usually reflect the actual physical size of a font—technically, 12 pt font should render in the same physical size regardless of the physical size of the screen and screen resolution).

One notable exception to all of this is the US automotive industry. They standardized on metric fasteners in The Eighties, and because of this, it's common to see both fractional-inch and metric sized 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 mm for decades.

Mass. The basic unit of mass is the pound, defined as 453.59237 grams, which is further divided into 16 ounces (~28.35 g). The pound can be further divided into 7,000 grains (~64.8 mg), though this unit is seldom used except in certain specialized fields, mostly archery and firearms. Two thousand pounds make a short ton (~907.18 kg). A unit almost unheard of in America, but which is still used in ordinary conversation in the UK, is the "stone", which equals 14 pounds (~6.35 kg); likewise, the imperial (or long) ton in the UK, insofar as it's still used, is defined as 2,240 pounds (~1016 kg - note the near similarity to the metric tonne of 1,000 kg). Those who deal with precious metals use instead a unit called the troy pound, defined as 5760 grains (~373.2 g). The troy pound consists of 12, rather than 16, troy ounces (~31.10 g). When one refers to both units, the standard ounce and pound are sometimes called the ounce and pound avoirdupois to avoid confusion. Pound is often abbreviated lb from the Roman equivalent libra (which is also the source of the £ symbol).

Volume. The basic unit of liquid volume is the gallon, defined as 231 cubic inches (~3.79 L). This is divided into four quarts (around 946.35 mL), each of which are divided into two pints (~473.18 mL), divided in turn into two cups (~236.59 mL), each divided into eight fluid ounces (~29.57 mL), not to be confused with the avoirdupois ounce or troy ounce discussed above. (Note, however, that a fluid ounce of a water-based liquid weighs close enough to one ounce avoirdupois for government work; hence the American saying "A pint's a pound the world around," which turns a blind eye to every British part of the world.) For historical reasons, a few dry goods (such as strawberries and cherry tomatoes) are sold using the dry volume system, where one quart is 1.101220942715 L (again, exactly); crops are typically sold by the bushel, which is 8 dry gallons, or peck, which is 2 — so Peter Piper picked 2 dry gallons of pickled peppers. The Commonwealth, on the other hand, defines a gallon, both wet and dry, as exactly 4.54609 L, with all other units adjusted accordingly. Forty-two US gallons equal a petroleum barrel (~158.99 L), the standard international unit in which oil is sold, but other liquids sold in "barrels" typically use a 31.5 gallon barrel.

Spaces you don't expect to fill with liquid (car trunks, apartments, warehouses, etc.) are expressed in cubic feet—one cubic foot is about 28,300 cubic centimeters.

Cooks use three additional units: the teaspoon (5 mL), the tablespoon (15 mL), and the kitchen cup (240 mL), slightly more (or less in the Commonwealth) than the fluid cup above. Three teaspoons make a tablespoon, two tablespoons make about a fluid ounce, sixteen tablespoons make a cup, and two cups make a pint.

Ingredients in recipes are usually measured by volume, not by weight. (The exceptions are meat and produce, which are bought by the pound.) Most American households do not possess a kitchen scale. Because of this, kitchen scales are generally priced only for the upscale or gourmet market, and can cost significantly more than an accurate measuring cup (e.g. 50 cents for a cup vs. $30 or more 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 1970s, 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 the early 1970s, when the energy crisis lead to the downsizing of engines and it became preferable to advertisers to describe the size in liters rather than in cubic inches. but since some cubic inch-based engine 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 Chevy V8s are still closely associated with 350 c.i, although the company hasn't made one (or at least a 5.7l) in years.

Temperature. The standard unit of temperature in the United States is the degree Fahrenheit (See below for Celsius equivalent). The boiling point of water at sea level is 212° and its freezing point is 32°. Fun fact: the distance between freezing and boiling is 180°; this was intended by Daniel Fahrenheit as it would make it easier to mark a rotary-gauge thermometer: 180 angular degrees could equal 180 temperature degrees.

The mean body temperature of a healthy adult is approximately 98°-100° (an oft-quoted figure of 98.6° is really just an overprecise conversion from 37° Celsius.) Zero degrees is an arbitrary point, based on the lowest temperature that could be measured at the time the scale was developed, the approximate freezing point of seawater. Temperatures less than 0° are colloquially spoken as "X below"; thus, -10° would be "ten below". The Fahrenheit and Celsius scales coincide at -40°, and absolute zero is exactly -459.67°. To derive a Celsius temperature from a Fahrenheit reading, subtract 32 and divide by 1.8. An advantage to the Fahrenheit scale is the simplicity to the average person: 0° is really cold for a person and 100° is really hot, but both are within a range of temperatures you may experience over the year (in Europe and North America at least). 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 around 671.64°.

American cookbooks specify temperatures exclusively in degrees Fahrenheit. Oven temperatures are always given in increments of 25 degrees, e.g. "Preheat oven to 425". The "gas mark" notation seen in British cookbooks is not used.

Dates and Time: America almost exclusively uses the 12 hour clock, which is divided into two intervals "A.M." (ante meridiem, before noon) and "P.M." (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" or "in the evening." The 24 hour clock has limited use in America, being used primarily in the military (thus the sobriquet "military time"). In the Army and Air Force, 18:00 would be called "eighteen hundred hours", whereas the Navy drops the "hours" and would simply say "eighteen hundred". To a lesser extent, 24-hour time is used in America by professionals in industries that operate around the clock, 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 20, 2010 would be written as 12/20/2010, or just 12/20/10. This has the quirk of mimicking the way the date is written and said in full in English (unless you follow the "20 December" format), but it goes back and forth instead of being strictly hierarchical.

Money: The standard unit of American money is called the dollar, presumably from the German word "Taler" which was used for various now obsolete currencies and is originally derived from "Joachimsthal", a town in Bohemia where coins were minted. It is subdivided into 100 individual units, cents. Each cent is in turn further divided into 10 mills, a unit of currency largely rendered hypothetical through inflation (except in the price of gasoline, although property-tax rates are often expressed in mills), of which there are 1000 in a dollar. Each cent is one one hundredth of a dollar, and comes in the form of a small coin made of copper wax bronze steel zinc with a copper coating.

Other coins are the nickel, five cents; the dime, ten cents; the quarter, 25 cents; the half-dollar (guess); and various editions of a one-dollar coin. The half-dollar lost favor after 1965, and the dollar coin never really caught on. 50-cent pieces don't even fit in most vending machines, though dollar coins are sometimes given as change in the newer ones. Those not accustomed to US currency sometimes find it counterintuitive that the dime is physically smaller than the penny, and all the common dollar coins are similar in size to the quarter. (Dollar coins were larger than the half-dollar prior to 1979, but the half-dollar coin is already at or near the upper limit of how large a coin a person wants to carry around.) The size disparities are the legacy of pre-1965 silver coinage - the dime, quarter, half-dollar, and dollar were made from a silver alloy, and the dime weighed (and still does) 1/10 that of the "silver dollar". The nickel was and is still made of a copper-nickel alloy, hence the name; most dollar coins minted these days are at least intended to look golden. On top of that, the dime does not say "10 cents" on it; it says "one dime", and you're expected to figure out that that comes from the French for "one tenth" - di(s)me.

It's worth noting that Canadian coins come in the same size, color, and denominations as Americans up to one dollar (they also have a $2-coin called a "toonie" that just looks bizarrely awesome). They're usually worth less and weigh less than American coins but are usually interchangeable to all but the most nitpicky cashiers and vending machines. In the mid-19th century, the U.S. experimented unsuccessfully with 20 and 2 cent coins, and had half-cent coins as well as mill tokens for the truly silly. As well, it was at one time common in multiple world currencies to physically slice dollar (or equivalent) coins into eight bits worth 12.5 cents each. This denomination has survived today primarily in colloquialism; a Shave and a Haircut was worth a quarter dollar, "two bits", as advertised by barbershop quartets, and pirates sought "pieces of eight", from the historical origin of dollar as a copy of the aforementioned Spanish Dollar, which bore a nominal or 8 reals. (This convention of dividing a dollar into 8 pieces also persisted in the Stock Market all the way through the 1980s.)

Paper money, called “bills” in the U.S., start at the dollar. The next denomination up is the little-used $2 bill, which can sometimes lead to confusion as the younger generation does not always recognize it as legal tender. Next are the $5 and $10 bills. The $20 bill is probably the most commonly used in basic transactions, being the standard bill for ATM bank withdrawals, and is also known as a “yuppie food stamp.” Next up are the $50 and $100 bills. They're easy to misplace, what with being a thin strip of cottony paper, so don't! Unlike some other countries, American bills are all the same size and until recently all the same color (Black on white on the front. Green on white on the back. Hence greenbacks) In recent years redesigns have made some bills slightly more colorful as part of anti-counterfeiting measures, though still nowhere as colorful as Euro (or Canadian) bills.

There were also 3¢, 5¢, 10¢, 15¢, 25¢, 50¢, $500, $1,000, $5,000, and $10,000 bills in circulation at one time, but they are no longer used and essentially never seen outside of collections. $100,000 bills were used to settle accounts between banks, but were never released outside the banking system.

Through most of U.S. history, there were four types of bills issued by the US government - silver and gold certificates, United States Notes, and Federal Reserve Notes. The first two were specie-backed currency, meaning that their value was backed by an equivalent amount of the specific precious metal in their name. United States Notes, and Federal Reserve Notes before 1963, were redeemable in “lawful money,” which was never defined by Congress; the only difference between these two types of paper money was that one was issued by the U.S. Treasury and the other was issued by the Federal Reserve. Modern Federal Reserve Notes, which have been the only sort of paper money issued by the US Government since 1963, are fiat currency - their value is not backed by any hard resource, but by the "full faith and credit" of the U.S. Government.

Energy. A few units of energy are in common use in the United States. 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, or about 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/schoolchildren an intuitive idea of what energy and work are.

The second is the kilowatt-hour (3.6 MJ), mostly used by electric companies. Most Americans know what a watt is through electrical device ratings, so to make things more intuitive, power meters in the US use this composite unit (the amount of energy used by a 1000-watt appliance in 1 hour) instead of measuring things directly in joules.

The third customary unit of energy in the United States is the calorie (~4.184 J), used in heating, nutrition, and kinesiology. An odd duck of sorts, the calorie, based on the specific heat of water, was introduced as a metric unit in 1824, but was displaced within the System Internationale by the joule - but not before it found usage within the United States. The unit most Americans refer to when they say "calorie"—the nutrition one, see below—is actually the kilocalorie.

There is also the "British Thermal Unit", normally abbreviated 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 from about 1,054 to 1,060 J according to the other wiki. To make things even more confusing to outsiders, American Engineers measure cooling in tons (12,000 BTU/hr, equivalent to 1 ton of ice per day or ~3516.85 W) and heating in MBH (one thousand BTU/hr or ~293.07 W; the M is the Roman numeral for 1000, not the expected one million); both of those 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.

Power: The common unit of power in the United States is the horsepower. There are five different types of horsepower, just to be even more confusing. The most common is Mechanical horsepower (more often called brake horsepower, from the method used to derive it) which is used by auto manufacturers to tell the public that their car is more powerful and therefore makes you more of a man if you own it. 1 unit of mechanical horsepower is 550 foot-pounds per second (~745.7 W). There is also metric horsepower, electrical horsepower, boiler horsepower, and hydraulic horsepower, all of which can be safely ignored, with the sole exception of metric horsepower, which is sometimes used to rate power outputs of automobiles and other engines in Europe and always used for cars in Japan. 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 W --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.

Force: Measured in pounds, as stated above. Often expressed as "pounds of force", "lb-force", or "lbf" for disambiguation from the mass unit.

Torque: Torque, 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 "pounds-feet". Most TV characters, and many real-life "car guys" who aren't engineers, will express it in foot-pounds. They're not completely wrong, but also not right. Actual engineers, like everywhere else, will use real units.

Pressure: Air pressure is usually given in psi, or pounds per square inch (~6.894 kPa). Atmospheric pressure is usually given in atmospheres or millimeters of mercury (760 mmHg = 1 atm = 101.325 kPa). 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 labeling in the US: As mentioned above, the food industry was one of the few to embrace be legally required to use the metric system during the failed attempt at forced conversion in the US, so food packaging is, outside of 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 American units in different ways on packaging. Most of the time, either the American or the metric unit 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 as "Calories". Note the capitalization; that's important.) and the 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 - common sizes are 12, 16 or 20 fl. oz. 450 milliliters, 0.5 liter, 1 liter, 1.5 liters, 2 liters, or (less commonly) 3 liters. Soft drinks of less than 1 liter size 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 oz. Juices and other more expensive beverages may have a little bit removed to save money, leading to 11.5 ounce juice cans. (Incidentally, 12 fluid ounces comes out to ~354.88 ml, but beverage canners almost always round that up to exactly 355 ml.)

Beverages sold in cartons or polyurethane jugs (dairy products and some fruit juices) are sold by the gallon, half-gallon, or quart. Milk often comes in Quarts, Half Gallons and Gallons,

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 & Alcohol: Beer 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. 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 1/5 gallon (a "fifth"). These bottles, as well as the half-size 375 mL bottles, are still colloquially referred to as pints and fifths 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.

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).

Marijuana is usually purchased in weights measuring 1/8th 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 1/8 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 Marijuana, Cocaine, LSD, and Ecstacy 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, 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.)
• mi: Miles
  • Except for mph (miles per hour)
    • Not to be confused with the metric "m" for "meter".
• 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. A gas-guzzler even by American standards of the quote's era, then.