How Many? A Dictionary of Units of Measurement
© Russ Rowlett and the University of North Carolina at Chapel Hill

Table of Contents
About the Dictionary
Using the Dictionary


e [1]
a symbol for the electric charge on one electron. Since the charges on other particles in atomic physics are whole-number multiples of this charge, the symbol e is often used as a unit of measure. 1 e is equal to approximately 1.602 176 487 x 10-19 coulomb, or 160.217 648 7 zeptocoulombs (zC).
e [2]
a mathematical unit used as the base of "natural" logarithms and exponentials. The real number e is irrational, which means that its decimal expansion is infinite and non-repeating. To 25 significant digits e equals 2.718 281 828 459 045 235 360 287. Of the many properties of this number, the most important is that the rate of change in the function ex is equal to the value of the function itself: an example of the behavior we call "exponential growth." As a result, the larger the value of this function is, the faster the function grows. The Swiss mathematician Leonard Euler (1707-1783) introduced the symbol e, probably because it is the first letter of the word "exponential." Other mathematicians continued to use the letter in his honor. It is sometimes called the Euler number.
earth-rate unit (eru)
a unit of angular velocity equal to 15° per hour (one revolution per day), the rate at which the earth rotates on its axis. This unit is used to measure the drift rates of gyroscopes and various pointing devices in aerospace engineering.
abbreviation for European Brewing Commission, frequently used for a unit of turbidity equal to 4.08 NTU or FNU.
a unit of telecommunications traffic density equal to 2 call minutes (120 call seconds) per hour, or 1/30 erlang (see below). EBHC stands for "equated busy-hour call."
eclipse year
a unit of time used in astronomy in the prediction of eclipses. The plane of the moon's orbit around the earth and the plane of the earth's orbit around the sun intersect in a line called the line of nodes. Eclipses of the sun or moon can only occur at times when the moon and sun both lie very close to this line. However, the line of nodes rotates slowly, so comparable appearances of the sun on the line do not fall exactly a year apart. The actual interval between these crossings, the eclipse year, has a length of 346.62 days.
a unit of electric dipole moment used in physics. The moment of an electric dipole is the product of an electric charge and the distance by which the charge is displaced from the center of charge. The ecm is the product of the charge e on an electron and a distance of 1 centimeter (cm). In SI units, 1 ecm = 1.602 178 x 10-21 coulomb meter (C·m).
EDR (sq ft)
an abbreviation for Equivalent Direct Radiation, a unit used in the design of hot water heating systems. The measurement is traditionally stated in square feet, representing the area of the radiator surface, but it is actually interpreted as a unit of power,with 1 EDR ft2 equal to 240 Btu per hour.
an abbreviation for energy efficiency rating, a U.S. measure of the efficiency of an air conditioner. The EER is computed as the cooling capacity of the unit (in Btu per hour) divided by the electric power consumed (in watts) at a temperature of 95 °F (35 °C) and under specified test conditions. Typical values are in the range 8-12. The EER is equal to 0.293 071 times the COP under the specified conditions. See also SEER, and energy factor (EF, below).
a symbol for energy factor (see below) or for the enhanced Fujita scale for tornado intensity.
effective dose (ED)
a measure used in pharmacology to express the percentage of a population that receives the desired benefit from a dose of the substance being studied. The measurement is usually given as a subscript. For example, the ED50 dose is the amount of the substance that benefits 50% of the test population.
means 1/8, of course; in meteorology the eighth is the unit for measuring the proportion of the sky covered by clouds. See okta.
einstein, Einstein unit (E)
a unit of light energy concentration sometimes used in physical chemistry. One einstein (or Einstein unit) is the energy per mole of photons carried by a beam of monochromatic light. Suppose a beam of light (in vacuum) has frequency v (in hertz) and wavelength l (in meters). Then one einstein is equal to (3.990 313 x 10-10)v or (0.119 627)/l joules per mole (J/mol). The unit is named for the physicist Albert Einstein (1879-1955), who explained how light carries energy in a famous 1905 paper.
the Dutch ell, a traditional unit of length equal to about 68-70 centimeters (roughly 27 inches). See the discussion of the ell, below.
electric horsepower
a unit of power, equal to exactly 746 watts (550.221 foot pounds per second), used in the electric industry. This is slightly larger than the ordinary or mechanical horsepower of exactly 550 foot pounds per second.
electron (me)
the mass of the electron, often used as a unit of mass in particle physics. An electron has a mass of about 9.109 382 x 10-31 kilogram, 9.109 382 x 10-28 gram, or 0.510 9989 million electronvolts (MeV, see next two entries).
electronvolt (eV) [1]
a unit of work or energy used in physics. One electronvolt is the work required to move an electron through a potential difference of one volt. The size of the electronvolt must be determined experimentally; the currently accepted value (1998) is 160.217 646 2 x 10-21 joule, or 1.602 176 462 x 10-12 erg. This unit is accepted for use with SI units. The official spelling is electronvolt (one word) rather than electron volt.
electronvolt (eV) [2]
a unit of mass used in particle physics. Mass and energy are related by Einstein's famous equation, E = mc2. The constant c is the speed of light, 299.79 x 106 m/sec. An energy of 1 electronvolt is therefore equivalent to a mass of about 1.782 662 x 10-33 gram, or about 1.073 544 x 10-9 atomic mass unit. This is such a small unit that most particle masses are measured in MeV or GeV.
a traditional unit of length used primarily for measuring cloth. In the English system, one ell equals 20 nails, 45 inches, or 1.25 yards (exactly 1.143 meters). The word comes from the Latin ulna, which originally meant the elbow and is now the name of the bone on the outside of the forearm. The history of the unit is not clear. Some authorities believe the ell was originally a double forearm length, that is, 2 cubits or 36 inches, the same length as a yard. The ell and the yard do seem to be identified in some medieval documents, with ulna being used for both, and in Scotland the ell was equal to 37 Scots inches or 37.2 English inches (94.5 centimeters), only slightly longer than the yard. (This Scottish length might also reflect an old practice of cloth merchants in giving an extra inch with each yard, to allow for any irregular cutting at the ends of the piece.) However, the English cloth ell is definitely longer than the yard; it seems to be the distance from the shoulder to the fingers of the opposite hand. This reflects a practice of cloth merchants of holding the cloth at the shoulder with one hand and pulling the piece through with the opposite hand. This cloth ell was used with a similar length in France, where it was called the aune. The Dutch el and German elle are a little more than half the English ell; they may represent "arm's-length" units like the Italian braccio, the Russian sadzhen, and the Turkish pik.
a traditional unit of distance in German speaking countries. The elle varied considerably, but it was always shorter than the English ell or French aune. A typical value in northern Germany was exactly 2 fuss (German feet), which would be close to 24 inches or 60 centimeters. In the south, the elle was usually longer, about 2.5 fuss. In Vienna, the elle was eventually standardized at 30.68 inches (77.93 centimeters). Although the German word Elle is often translated "yard" in English, this is not a very good equivalent.
a printer's unit of relative distance. One em is the height of the type size (in points) being used. If 12 point type is being set, then one em is 12 points, and so on. See point [2].
emu [1]
an abbreviation for "electromagnetic unit." See ab-.
emu [2]
a CGS unit of magnetic dipole moment equal to 4pi micro-oersteds (1.256 637 x 10-5 Oe). In SI units, one emu equals 0.001 A·m2.
emu/cm3 or emu/cc
a CGS unit of magnetization. In SI units, one emu/cm3 can be interpreted either as 4pi/10 milliteslas (1.256 637 mT) as a unit of magnetic polarization or excess magnetic induction, or as 1000 amperes per meter as a unit of magnetic dipole moment per unit volume.
a printer's unit of relative distance, equal to 1/2 em. If 12 point type is being set, then one en is 6 points, and so on.
a traditional French unit of distance corresponding to the English cable. The encablure was equal to 120 brasses or 600 pieds; this is about 194.88 meters or 639.37 English feet. Most navies, including the French, now use a metric cable equal to exactly 200 meters; in French this metric unit is sometimes called the encablure nouvelle (new cable).
energy factor (EF)
a measure of the energy efficiency of an appliance. In the U.S., the Department of Energy has defined energy factors for a variety of appliances. For dishwashers, the energy factor is the number of cycles per kilowatt hour of power input. For clothes washers, it is the capacity of the washer in cubic feet divided by the number of kilowatt hours of power input per washing cycle. For clothes dryers, it is the number of pounds of clothes dried per kilowatt hour of power consumed.
engineer's chain
see chain.
Engler degree
a unit of kinematic viscosity given by readings on an Engler viscometer. The reading is the time (in seconds) required for 200 milliliters of the liquid being tested to flow through the device. The conversion of Engler degrees to absolute units requires an appropriate table, but for liquids having a viscosity of 100 centistokes or more the Engler degree is roughly equal to 7.6 centistokes.
enhanced Fujita scale (EF)
the revised Fujita scale for estimating the strength of tornados, implemented by the U.S. National Weather Service in 2007.
a unit of quantity equal to 9, coined from the Greek word for nine, ennea.
enzyme unit (U or EU)
a unit used by biochemists to measure the activity of enzymes, which are proteins produced by living cells to cause or facilitate necessary chemical reactions within the cell. One enzyme unit is the quantity of enzyme needed to cause a reaction to process 1 micromole of substance per minute under specified conditions. Thus, one enzyme unit has a catalytic activity of 1/60 microkatal (µkt) or 16.667 nanokatals (nkt).
alternate spelling for aeon, a unit of time equal to 1 billion years.
Eotvos unit (E)
a unit used in geophysics to measure the change in the acceleration of gravity with horizontal distance. One Eotvos unit equals 10-9 Gal per centimeter or 10-4 Gal per kilometer. In proper SI terms, the Eotvos unit equals 10-9 per second squared (s-2). The unit honors the Hungarian physicist Roland von Eötvös (1848-1919).
a ancient unit of volume for grains and dry commodities, used in the Bible. The ephah is believed to equal about 40.32 liters or 1.4239 cubic feet. This is equivalent to about 1.144 U.S. bushel.
epoch [1]
a measure of time used in astronomy. In an epoch system, times are specified as years and fractions of years (such as epoch 1998.5). To set a starting point for the system, a specific epoch time must be fixed as a particular clock time of a particular date. In 1984, the International Astronomical Union agreed that epoch times should be fixed by requiring that epoch 2000.0 equals 12 hours Universal Time of the day 2000 January 1. Other epoch conventions were used in the past. See also Julian epoch. The name comes from a Greek word epoche meaning a stopping point or fixed point.
epoch [2]
a unit of time equal to 19 years, used in predictions of the tides. In this use, an epoch is another name for a Metonic cycle. All possible alignments of the sun and moon occur in this 19-year cycle, so tidal heights and other tidal phenomena are averaged over this period.
equivalent or equivalent weight (Eq)
a unit of relative amount of substance used in chemistry. One equivalent weight of an element, compound, or ion is the weight in grams of that substance which would react with or replace one gram of hydrogen. Since one gram of hydrogen is very nearly equal to one mole and since hydrogen has one electron free to react with other substances, 1 Eq of a substance is effectively equal to one mole divided by the valence of the substance (the number of electrons the substance would engage in participating in the reaction). In practice, this is a large unit and measurements are more likely to be in milliequivalents (mEq or meq).
the unit of work or energy in the CGS system, equal to the work done by a force of one dyne acting through a distance of one centimeter. Equivalently, one erg is the kinetic energy of a mass of 2 grams moving at a velocity of 1 cm/sec. This is equal to 0.1 microjoule, or about 7.375 x 10-8 foot-pound. The name of the unit is from the Greek word ergon, work. There is no symbol; the word is spelled out in full. Adding prefixes is a problem: 1000 ergs is usually called a kiloerg, but one million ergs is a megalerg, the "l" being added to ease pronunciation.
erlang (E)
a measure of telecommunications traffic density. The erlang is a dimensionless "unit" representing a traffic density of one call-second per second (or one call-hour per hour, etc.). The erlang is sometimes divided into 36 unit calls or 30 EBHC. Also called the traffic unit (TU), the erlang honors A. K. Erlang (1878-1929), a Danish mathematician who studied the mathematics of telephone networks.
a traditional unit of distance in Spain and Portugal. The estadio, like the stade and the English furlong, is equal to 1/8 mile. The Spanish estadio is equal to 1/8 milla or 625 pies; this is about 571 feet or 174 meters. The Portuguese unit is 1/8 milha, which is much longer: about 856 feet or 261 meters.
an abbreviation for "electrostatic unit." See stat-.
ett- or etto- (h-)
Italian spelling for the metric prefix hecto- (100). The hectare, for example, is ettaro in Italian. The International System allows national variations in spelling of the names of units, but not in the symbols used for them; thus the symbol for etto- is h-.
etto (hg)
an informal Italian name for the hectogram, a unit of mass equal to 100 grams or about 3.5274 ounces.
a symbol sometimes used for the enzyme unit (see above).
exa- (E-)
a metric prefix denoting 1018 (one quintillion in U.S. nomenclature). The Latin and Greek prefix ex- means "out of," and is often used to indicate "a long way," as in the words "expanse" or "extreme." In addition, the prefix suggests the Greek hexa, meaning 6, this being the prefix for 10006.
exajoule (EJ)
a metric unit of energy. One exajoule equals 947.817 (U.S.) trillion Btu, 277.7778 petawatt hours, or about 9480 megatherms. The unit is often used in discussing global energy production, which is measured in hundreds of exajoules per year.
exameter (Em)
a metric unit of distance equal to 1015 kilometers. This is equivalent to about 621.371 trillion miles, 105.7 light years, or 32.408 parsecs. One exameter is approximately the distance from the earth to the Hyades star cluster in Taurus.
exbi- (Ei-)
a binary prefix meaning 260 = 1 152 921 504 606 846 976. This prefix, adopted by the International Electrotechnical Commission in 1998, replaces exa- for binary applications in computer science. The prefix is a contraction of "exabinary."
exposure value (EV)
a unit used in photography to describe relative exposure. EV 0 is assigned to a specific combination of exposure time and lens aperture, such as 1 second at f/1. The difference between two exposure values is equal to the number of stops separating the two exposure settings. Different combinations of exposure time and lens aperture can have the same exposure value; the unit was invented to simplify the relationship. Regardless of camera settings, EV 6 is one stop "faster" than EV 5, that is, an EV 6 setting records an image with half as much light as an EV 5 setting.
a term often used with a Snellen fraction in phrases such as "20/20 eyesight."


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Checked and updated December 9, 2008