F

face cord
a unit of volume for firewood. See rick.
faden
an alternate name for the klafter, a German unit of distance.
faggot
a traditional unit of volume for firewood. A faggot was 3 feet in length and 2 feet in circumference. There are about 134 faggots in a cord.
Fahrenheit
see degree Fahrenheit.
fall [1]
a traditional unit of distance equal to 6 ells. The fall was used in land measurement somewhat like the rod. Measurements in rods were often made with an actual wood pole, while measurements in falls were often made with a rope 6 ells long. The distance falling under the rope was called a fall. The fall was used mostly in Scotland, where its traditional length was 6 Scots ells or about 18.6 English feet (5.67 meters). The Scots furlong was equal to 40 falls (226.8 meters) rather than 40 rods, and the Scots mile was equal to 320 falls (5952 English feet, 1.127 English mile or 1814.2 meters). After the unification of Scotland and England the fall was reinterpreted to equal 6 English ells (22.5 feet or 6.858 meters).
fall [2]
a traditional unit of area equal to one square fall [1]. In the traditional Scots system of measurement, a fall of land equals about 346 square feet or 32.15 square meters. A traditional Scots acre was equal to 160 falls or about 6150 square English yards (1.27 English acres or 0.514 hectare). In the English system a fall of land is 506.25 square feet, 56.25 square yards, or about 47.03 square meters.
fanega [1]
a Spanish unit of volume, mostly for dry goods. The word is derived from an Arabic word faniqa for a large sack. The fanega equals 12 almudes or 48 cuartillos; this is about 55.50 liters or 1.960 cubic feet (1.575 U.S. bushels). Very similar units were used in Portugal and in most of the Latin American countries. In Chile and Argentina, however, a much larger fanega, roughly 2.5 U.S. bushels, was customary.
fanega [2] or fanegada
a traditional unit of land area in Spain and in some Latin American countries. The unit originated as the amount of land which could be planted with a fanega [1] of seed. It varied considerably from one area to another. In 1801 it was standardized in Spain as the area of a square 96 varas (80.2 meters) on a side; this comes to 0.643 hectare (1.59 acres). After the introduction of the metric system it became customary to refer to an area 80 meters square as a fanega. This unit is not legal but is still used informally in some parts of Spain. The Central American manzana is a counterpart of this traditional Spanish unit.
farad (F)
the SI unit of electric capacitance. Very early in the study of electricity scientists discovered that a pair of conductors separated by an insulator can store a much larger charge than an isolated conductor can store. The better the insulator, the larger the charge that the conductors can hold. This property of a circuit is called capacitance, and it is measured in farads. One farad is defined as the ability to store one coulomb of charge per volt of potential difference between the two conductors. This is a natural definition, but the unit it defines is very large. In practical circuits, capacitance is often measured in microfarads, nanofarads, or sometimes even in picofarads (10-12 farad, or trillionths of a farad). The unit is named for the British physicist Michael Faraday (1791-1867), who was known for his work in electricity and electrochemistry.
faraday (Fd)
a unit of electric charge. In a process called electrolysis, chemists separate the components of a dissolved chemical compound by passing an electric current through the compound. The components are deposited at the electrodes, where the current enters or leaves the solution. The British electrochemist and physicist Michael Faraday (1791-1867) determined that the same amount of charge is needed to deposit one mole of any element or ion of valence one (meaning that each molecule of the ion has either one too many or one too few electrons). This amount of charge, equal to about 96.4853 kilocoulombs or 26.8015 ampere hours, became known as Faraday's constant. Later, it was adopted as a convenient unit for measuring the charges used in electrolysis. One faraday is equal to the product of Avogadro's number (see mole) and the charge (1 e) on a single electron.
farsakh, farasang
Arabic and Persian names, respectively, for the unit classically known as the parasang.
farthing
an old English word for 1/4, later used as the name of a coin equal in value to 1/4 penny.
farthingdale
an older name for the rood [2], a unit of land area equal to 1/4 acre (0.1012 hectare). "Farthingdale" means a fourth part.
fathom (fth or fath)
a traditional unit of distance equal to 2 yards or 6 feet (approximately 1.829 meters). The word comes from the Old English fæthm, meaning "outstretched arms", because a fathom is the distance between a man's outstretched fingertips. This is a generic unit that has been used in many cultures; other versions include the Spanish braza, the French toise, the German klafter, the Danish favn (6.18 feet or 1.88 meters), the Swedish famn (5.84 feet or 1.78 meters) and the Japanese ken.
fatt
a traditional unit of volume for grain, generally equal to 9 bushels or 1/4 chalder. This would be about 317 liters based on the traditional grain bushel now used in the U.S. or about 327 liters based on the British imperial bushel.
fbm
a symbol sometimes used for the board foot. The "bm" stands for "board measure."
feddan
an Egyptian unit of land area, formerly used throughout the Middle East and North Africa. The feddan equals about 0.42 hectare or 1.038 acre.
femto- (f-)
a metric prefix standing for 10-15 (one quadrillionth). The prefix was coined from femten, which means fifteen in Danish and Norwegian. See Prefix
fermi (fm or f)
a metric unit of distance used in atomic physics. One fermi equals 10-15 meter, or 1 femtometer. The unit is named for Enrico Fermi (1901-1954), the Italian-American physicist who built the first nuclear reactor. There was a need for this unit before the metric prefixes for very small quantities were defined; now distances formerly measured in fermis are stated in femtometers instead.
FEU
a unit of cargo capacity, especially for container ships. These ships carry cargo in standard metal boxes, called containers, that can be transferred easily to trains or trucks. FEU is an abbreviation for "forty-foot equivalent unit." One FEU represents the cargo capacity of a standard container 40 feet long, 8 feet wide, and (usually) a little over 8 feet high. One FEU equals roughly 25 register tons (see ton [3]) or 72 cubic meters.
fifth [1]
a traditional U.S. unit of liquid volume equal to 1/5 gallon. The fifth contains exactly 46.2 cubic inches, or about 757.1 milliliters. This unit is an American version of the traditional bottle.
fifth [2]
a unit used in music to describe the ratio in frequency between notes. Two notes differ by a fifth if the higher note has frequency exactly 3/2 times the frequency of the lower one. On the standard 12-tone scale, the perfect fifth is very closely approximated as 7 half steps, corresponding to a frequency ratio of 27/12 = 1.4983. See Pythagorean Hammers.
fineness (fine)
a unit of proportion equal to 1/1000. This unit is used to express the purity of alloys of gold or other precious metals. A statement that a gold bar is "999 fine" means that the bar contains at most 0.1% other metals. See also per mill.
finger [1]
a traditional unit of distance equal to 2 nails or 4.5 inches (11.43 centimeters). This unit represents the length of the middle finger, from the tip to the joint where the finger is attached to the hand.
finger [2]
an informal name for the digit, a unit of distance equal to 3/4 inch (19.05 mm).
Finsen unit (FU)
a unit formerly used in medicine to measure the intensity of the ultraviolet light used in various medical treatments. Ultraviolet radiation of the standard wavelength 296.7 nanometers has intensity 1 Finsen unit if its energy density is 105watts per square meter. The unit honors the Danish physician Niels Finsen (1860-1904), who received the Nobel prize for medicine in 1903 for his research on the use of light in the treatment of various diseases.
firkin (fir) [1]
a traditional unit of volume equal to 1/4 barrel. Since barrels are of various sizes, the capacity of a firkin varies. Based on the standard U.S. barrel of 31.5 gallons, a firkin would equal 7.875 gallons, 1.05 cubic feet, or about 29.81 liters. Traditional British barrels and firkins are larger; in the imperial system a firkin holds 1.445 cubic feet or 40.91 liters. The unit is of Dutch origin, and its name is based on the Dutch word veer for four.
firkin (fir) [2]
a traditional unit of weight for butter and soap, equal to 4 stone or 56 pounds (about 25.40 kilograms).
fiscal year
a unit of one year used for budgeting or accounting. A fiscal year has the same length as an ordinary year, but its starting date may be different. In the U.S., for example, the federal government's fiscal year starts on October 1. Generally, fiscal year n ends in ordinary year n, so the U.S. federal fiscal year 2001 began October 1, 2000, and ends September 30, 2001.
flagon
a traditional unit of liquid volume, generally equal to the wine (or U.S. liquid) gallon (about 3.785 liters). A flagon is a large, narrow-necked pitcher or bottle.
flask
a commercial unit of weight formerly used to measure liquid mercury (or quicksilver, as it was called). The flask was equal to 76.5 pounds avoirdupois, which is equivalent to almost exactly 34.7 kilograms.
flock
an old English unit of quantity equal to 2 score or 40.
floor
another name for a story as a unit of height for buildings. In Britain, the floor above the ground floor is usually called the first floor, so a three-floor building has four stories. In the U.S., the numbering of floors is most often the same as the numbering of stories.
flops
a unit of computing power equal to one floating point operation per second. In computer science, there is a distinction between fixed point numbers (which have a fixed number of decimal places) and floating point numbers (which are stored with as many digits as the computer's design allows). A floating point operation is an addition or subtraction of two floating point numbers. As of the mid-1990's, the power of supercomputers was being measured in teraflops (Tflops): trillions of floating point operations per second.
fluid dram or fluidram (fl dr)
a unit of volume in the traditional apothecary system, equal to 1/8 fluid ounce. This unit is usually called the fluid dram or fluidram to avoid confusion with the weight dram. The U. S. fluid dram contains about 0.225 586 cubic inches or 3.696 691 milliliters. In the British imperial system, the fluid dram is about 0.216 734 cubic inches or 3.551 633 milliliters.
fluid ounce (fl oz)
a traditional unit of liquid volume, called the fluid ounce to avoid confusion with the weight ounce. In the U. S. customary system there are 16 fluid ounces in a pint, so each fluid ounce represents 1.804 687 cubic inches or 29.573 531 milliliters. In the British imperial system there are 20 fluid ounces in an imperial pint, so each fluid ounce represents about 1.733 871 cubic inches or 28.413 063 milliliters. A fluid ounce of water weighs just a bit more than one ounce avoirdupois. See Volume.
fluid scruple
a traditional British unit of liquid volume equal to 1/3 fluidram or about 1.1839 milliliters.
flux unit (fu)
a former name for the jansky, a unit used in astronomy to measure the strength of a radio signal received from an object in the sky.
fod
the traditional Danish foot, equal to about 12.365 inches or 31.41 centimeters.
folio
in traditional legal practice, a unit of quantity for the words in a legal document. In the U.S., a folio is 100 words; in Britain it is 72 or 90 words, depending on the type of document. In the days before mechanical copying, clerks were paid by the folio to copy legal documents. The word is from the Latin folium, leaf, and originally referred to a page; apparently the unit once represented the number of words on a page.
food calorie
the large or kilogram Calorie; see calorie [2].
foot (ft or ')
a traditional unit of distance. Almost every culture has used the human foot as a unit of measurement. In England the Roman foot (29.6 centimeters) was replaced in the eighth century by the much larger Saxon foot (33.5 centimeters) and then in the thirteenth century by the modern foot (1/3 yard or about 30.5 centimeters). The length of the foot was standardized by standard yardsticks kept in England as early as the medieval period. Late in the nineteenth century, after both Britain and the U.S. signed the Treaty of the Meter, these standards were set aside in favor of the new metric standards. In the U.S., the Metric Act of 1866 defined the foot to equal exactly 1200/3937 meter, or about 30.480 060 96 centimeters. This unit, still used for geodetic surveying in the United States, is now called the survey foot. In 1959, the U.S. agreed with Britain and the other English-speaking countries in defining the foot to equal exactly 30.48 centimeters. This metric unit, called the international foot, is equal to 0.999 998 survey foot. The foot is divided into 12 inches, 3 hands, 4 palms, or 16 digits. See Length
football field
a common informal unit of distance in the United States. Americans aren't quite agreed as to whether the unit is exactly 100 yards (91.44 meters), the distance between the goal lines on an American football field, or 120 yards (109.728 meters), the distance including the two end zones. (Canadian football fields are 10 yards longer.) The football field is also used sometimes as an informal unit of area; including the end zones, an American football field represents an area of 1.3223 acre or about 0.535 hectare.
foot candle (fc)
a traditional unit of illumination, defined as the illumination received by a surface at a distance of one foot from a source of intensity one international candle. The "international candle" was the predecessor of the candela as the standard unit of light intensity. Illumination is now measured in lux; one foot candle equals 10.764 lux.
foot lambert (ftL)
a unit of luminance. The only difference between "luminance" and "illumination" is the source of the light: if it comes from the surface we speak of "luminance," and if it comes from outside we speak of "illumination." The foot lambert describes the luminance of a surface that emits or reflects one lumen per square foot; it is the luminance of a perfectly reflecting surface under an illumination of one foot candle. Like the foot candle, one foot lambert equals 10.764 lux.
foot of head (ft hd)
a traditional unit of water pressure used in plumbing and hydraulics. "Head" is short for "headwaters"; it refers to the depth of the water upstream from the point at which the pressure is measured. One foot of head is equivalent to a pressure of 0.433 lb/in2, 2.989 kilo pascals (kPa), 29.89 millibars (mb), or 0.882 inches of mercury (in Hg).
foot per minute (ft/min or fpm)
a traditional unit of velocity or flow rate. One foot per minute equals exactly 30.48 cm/min, 5.08 mm/s, or 0.018 288 miles per hour.
foot per second (ft/s or ft/sec or fps)
a traditional unit of velocity. One foot per second equals exactly 15/22 mile per hour, or exactly 1.097 28 kilometers per hour.
foot pound (ft-lb)
a traditional unit of work, equal to the work done by a force of one pound acting through a distance of one foot. This is equivalent to approximately 1.355818 joule, 1.285 07 x 10-3Btu, or 0.323832 (small) calorie.
foot poundal
a unit of work, equal to the work done by a force of one poundal acting through a distance of one foot. One foot poundal is equivalent to about 0.031 081 foot pound, or 0.042 140 joule.
foot ton (ft-tn)
a traditional unit of work, equal to the work done by a force of one ton acting through a distance of one foot. In the U.S. system, a foot ton equals 2000 foot pounds (2.7116 kilojoules or 2.570 Btu); in the British Imperial system a foot ton equals 2240 foot pounds (3.0370 kilojoules or 2.8786 Btu).
force
a measurement of wind velocity based on the Beaufort scale.
fortnight
a traditional English unit of time equal to 2 weeks or 14 days. The word is a contraction of "fourteen nights."
fot
the traditional Swedish foot, also called the Stockholm foot, equal to 11.689 inches or 29.69 centimeters. The fot was divided into 2 kvarter or into 12 tum.
fother
a traditional English unit of weight, used primarily for lead. The fother was always a little smaller than a ton; in one typical version still being used in the nineteenth century it was equal to 19.5 hundredweight or 2184 pounds. The fother was divided into 30 fotmals.
fourth
a unit used in music to describe the ratio in frequency between notes. Two notes differ by a fourth if the higher note has frequency exactly 4/3 times the frequency of the lower one. On the standard 12-tone scale, the perfect fourth is very closely approximated as 5 half steps, corresponding to a frequency ratio of 25/12 = 1.3348. See Musical Scales
fpm, fps
common abbreviations for foot per minute (ft/min) and foot per second (ft/s or ft/sec), traditional units of flow (see above).
frail
a kind of basket, sometimes used as a commercial unit of weight for fruit. Depending on the item, a frail could equal anywhere from 32 to 75 pounds (15-34 kilograms). In the nineteenth century a frail of raisins was often taken to equal 50 pounds (22.68 kilograms).
f ratio (f/)
a measure of the light-gathering power of the lenses in cameras and telescopes. The f ratio, expressed as "f/4" or whatever, is the focal length of the lens (the distance from the lens to the point where light is focused) divided by the aperture (the diameter of the lens, which in the case of cameras may be reduced by the shutter mechanism). Thus "f/4," for example, indicates that the focal length is 4 times the aperture. In cameras the f ratio is proportional to the square root of the exposure time, so in comparison to an an f/4 lens an f/8 lens requires a four times ((8/4)2) longer exposure than than the f/4 lens. The focal length of a camera's lens cannot be changed, so the shutter is used to reduce the aperture when longer exposure times are needed in order to match the available light to the speed of the film. Because of this connection with exposure times, the f ratio is often said to express the "speed" of a lens.
franklin (Fr)
a CGS unit of electric charge equal to 3.3356 x 10-10coulombs or 1 stat coulomb. An electrostatic charge of one franklin exerts a force of one dyne on an equal charge at a distance of one centimeter. The unit honors Benjamin Franklin (1706-1790), one of the leaders of the American Revolution, who was also an early investigator of electricity. See Armonica
freight ton
originally a unit of volume equal to 40 cubic feet; see ton [5]. More recently, another name for the revenue ton.
French (Fg or Ch)
a unit of distance used for measuring the diameters of small tubes such as catheters, fiber optic bundles, etc. One French is equal to 1/3 millimeter (about 13.123 mils). The name and the symbol Ch refer to the Charrière gauge scale, which is often called the French scale.
frigorie
a unit of refrigeration formerly used in Europe, equal to 1 (kilogram) Calorie per hour (4.1868 kilojoulesper hour, or 3.9683 Btu per hour). The name was coined from the calorie by replacing the Latin calor, for heat, with frigor, for cold.
fuder
a traditional German unit of liquid volume. A "fuder" is a cartload. In most of the German states the traditional fuder held about 9 hectoliters (roughly 238 U.S. gallons), making the fuder about the same size as the British tun or the French wine tonneau. In the Mosel wine region of Germany, a fuder is now a metric unit equal to 10 hectoliters (1 cubic meter, or 264.17 U.S. gallons). In Austria, the traditional fuder was equal to 18.11 hectoliters (478.42 U.S. gallons), twice the size of the German unit. In Belgium today, the fuder (or foudre) is a metric unit equal to 30 hectoliters (792.52 U.S. gallons). See Volume.
full step
see step [2].
funt
a traditional Russian unit of weight or mass, corresponding the German pfund (which is pronounced "funt"). The funt is equal to about 0.9028 pound avoirdupois or 409.5 grams. The plural is funte.
furlong (fur)
a traditional unit of distance. Long before the Norman Conquest in 1066, Saxon farmers in England were measuring distance in rods and furlongs and areas in acres. The word "furlong", from the Old English fuhrlang, means "the length of a furrow." A furlong equals 40 rods, which is exactly 220 yards, 660 feet, or 1/8 mile. One furlong is exactly 201.168 meters, so a 200-meter dash covers a distance very close to a furlong. The length of horse races is often stated in furlongs. See Length
fuss
the German foot (see foot, above). The length of the fuss varied somewhat; the Viennese version was equal to 12.444 inches or 31.608 centimeters, while the Rheinfuss (Rhine foot), used in much of western and northern Germany, was equal to 12.357 inches or 31.387 centimeters. There's no change in the plural.