 Order of magnitude

An order of magnitude is the class of scale or magnitude of any amount, where each class contains values of a fixed ratio to the class preceding it. In its most common usage, the amount being scaled is 10 and the scale is the (base 10) exponent being applied to this amount (therefore, to be an order of magnitude greater is to be 10 times as large). Such differences in order of magnitude can be measured on the logarithmic scale in "decades" (i.e. factors of ten).
It is common among scientists and technologists to say that a parameter whose value is not accurately known, or is within a range, is "of the order of" some value. For example, standby electrical power used in a household is not accurately known and varies between households, but is typically of the order of a few tens of watts.
Contents
Use
Orders of magnitude are generally used to make very approximate comparisons, but reflect deceptively large differences. If two numbers differ by one order of magnitude, one is about ten times larger than the other. If they differ by two orders of magnitude, they differ by a factor of about 100. Two numbers of the same order of magnitude have roughly the same scale: the larger value is less than ten times the smaller value. This is the reasoning behind significant figures: the amount rounded by is usually a few orders of magnitude less than the total, and therefore insignificant.
The order of magnitude of a number is, intuitively speaking, the number of powers of 10 contained in the number. More precisely, the order of magnitude of a number can be defined in terms of the common logarithm, usually as the integer part of the logarithm, obtained by truncation. For example, the number 4,000,000 has a logarithm (in base 10) of 6.602; its order of magnitude is 6. When truncating, a number of this order of magnitude is between 10^{6} and 10^{7}. In a similar example, with the phrase "He had a sevenfigure income", the order of magnitude is the number of figures minus one, so it is very easily determined without a calculator to be 6. An order of magnitude is an approximate position on a logarithmic scale.
An orderofmagnitude estimate of a variable whose precise value is unknown is an estimate rounded to the nearest power of ten. For example, an orderofmagnitude estimate for a variable between about 3 billion and 30 billion (such as the human population of the Earth) is 10 billion. To round a number to its nearest order of magnitude, one rounds its logarithm to the nearest integer. Thus 4,000,000, which has a logarithm (in base 10) of 6.602, has 7 as its nearest order of magnitude, because "nearest" implies rounding rather than truncation. For a number written in scientific notation, this logarithmic rounding scale requires rounding up to the next power of ten when the multiplier is greater than the square root of ten (about 3.162). For example, the nearest order of magnitude for 1.7 × 10^{8} is 8, whereas the nearest order of magnitude for 3.7 × 10^{8} is 9. An orderofmagnitude estimate is sometimes also called a zeroth order approximation.
An orderofmagnitude difference between two values is a factor of 10. For example, the mass of the planet Saturn is 95 times than of Earth, so Saturn is two orders of magnitude more massive than Earth. Orderofmagnitude differences are called decades when measured on a logarithmic scale.
In words
(long scale)In words
(short scale)Prefix Symbol Decimal Power
of tenOrder of
magnitudequadrillionth septillionth yocto y 0.000.000.000.000.000.000.000.001 10^{−24} −24 trilliardth sextillionth zepto z 0.000.000.000.000.000.000.001 10^{−21} −21 trillionth quintillionth atto a 0.000.000.000.000.000.001 10^{−18} −18 billiardth quadrillionth femto f 0.000.000.000.000.001 10^{−15} −15 billionth trillionth pico p 0.000.000.000.001 10^{−12} −12 milliardth billionth nano n 0.000.000.001 10^{−9} −9 millionth millionth micro µ 0.000.001 10^{−6} −6 thousandth thousandth milli m 0.001 10^{−3} −3 hundredth hundredth centi c 0.01 10^{−2} −2 tenth tenth deci d 0.1 10^{−1} −1 one one – – 1 10^{0} 0 ten ten deca da 10 10^{1} 1 hundred hundred hecto h 100 10^{2} 2 thousand thousand kilo k 1,000 10^{3} 3 million million mega M 1,000,000 10^{6} 6 milliard billion giga G 1,000,000,000 10^{9} 9 billion trillion tera T 1,000,000,000,000 10^{12} 12 billiard quadrillion peta P 1,000,000,000,000,000 10^{15} 15 trillion quintillion exa E 1,000,000,000,000,000,000 10^{18} 18 trilliard sextillion zetta Z 1,000,000,000,000,000,000,000 10^{21} 21 quadrillion septillion yotta Y 1,000,000,000,000,000,000,000,000 10^{24} 24 Nondecimal orders of magnitude
See also: Logarithmic scaleOther orders of magnitude may be calculated using bases other than 10. The ancient Greeks ranked the nighttime brightness of celestial bodies by 6 levels in which each level was the fifth root of one hundred (about 2.512) as bright as the nearest weaker level of brightness, so that the brightest level is 5 orders of magnitude brighter than the weakest, which can also be stated as a factor of 100 times brighter.
The different decimal numeral systems of the world use a larger base to better envision the size of the number, and have created names for the powers of this larger base. The table shows what number the order of magnitude aim at for base 10 and for base 1,000,000. It can be seen that the order of magnitude is included in the number name in this example, because bi means 2 and tri means 3 (these make sense in the long scale only), and the suffix illion tells that the base is 1,000,000. But the number names billion, trillion themselves (here with other meaning than in the first chapter) are not names of the orders of magnitudes, they are names of "magnitudes", that is the numbers 1,000,000,000,000 etc.
order of magnitude is log_{10} of is log_{1000000} of short scale long scale 1 10 1,000,000 million million 2 100 1,000,000,000,000 trillion billion 3 1000 1,000,000,000,000,000,000 quintillion trillion SI units in the table at right are used together with SI prefixes, which were devised with mainly base 1000 magnitudes in mind. The IEC standard prefixes with base 1024 was invented for use in context of electronic technology.
The ancient apparent magnitudes for the brightness of stars uses the base and is reversed. The modernized version has however turned into a logarithmic scale with noninteger values.
Extremely large numbers
For extremely large numbers, a generalized order of magnitude can be based on their double logarithm or superlogarithm. Rounding these downward to an integer gives categories between very "round numbers", rounding them to the nearest integer and applying the inverse function gives the "nearest" round number.
The double logarithm yields the categories:
 ..., 1.0023–1.023, 1.023–1.26, 1.26–10, 10–10^{10}, 10^{10}–10^{100}, 10^{100}–10^{1000}, ...
(the first two mentioned, and the extension to the left, may not be very useful, they merely demonstrate how the sequence mathematically continues to the left).
The superlogarithm yields the categories:
 , or
 negative numbers, 0–1, 1–10, 10–1e10, 1e10–10^{1e10}, 10^{1e10}–^{4}10, ^{4}10–^{5}10, etc. (see tetration)
The "midpoints" which determine which round number is nearer are in the first case:
 1.076, 2.071, 1453, 4.20e31, 1.69e316,...
and, depending on the interpolation method, in the second case
 −.301, .5, 3.162, 1453, 1e1453, , ,... (see notation of extremely large numbers)
For extremely small numbers (in the sense of close to zero) neither method is suitable directly, but of course the generalized order of magnitude of the reciprocal can be considered.
Similar to the logarithmic scale one can have a double logarithmic scale (example provided here) and superlogarithmic scale. The intervals above all have the same length on them, with the "midpoints" actually midway. More generally, a point midway between two points corresponds to the generalised fmean with f(x) the corresponding function log log x or slog x. In the case of log log x, this mean of two numbers (e.g. 2 and 16 giving 4) does not depend on the base of the logarithm, just like in the case of log x (geometric mean, 2 and 8 giving 4), but unlike in the case of log log log x (4 and 65536 giving 16 if the base is 2, but, otherwise).
See also
 Orders of approximation
 Big O notation
 Decibel
 Number sense
 Names of large numbers
 Names of small numbers
Further reading
 Asimov, Isaac The Measure of the Universe (1988)
External links
 Cosmos – an Illustrated Dimensional Journey from microcosmos to macrocosmos – from Digital Nature Agency
 Powers of 10, a graphic animated illustration that starts with a view of the Milky Way at 10^{23} meters and ends with subatomic particles at 10^{−16} meters.
 What is Order of Magnitude?
Orders of magnitude Quantity acceleration · angular velocity · area · charge · computing · currency · data · density · energy · entropy · force · frequency · length · luminous flux · magnetic field · mass · numbers · power · pressure · radiation · resistance · specific energy density · specific heat capacity · speed · temperature · time · voltage · volumeSee also Categories: Orders of magnitude
 Elementary mathematics
Wikimedia Foundation. 2010.
Look at other dictionaries:
order of magnitude — plural orders of magnitude n 1.) if something is an order of magnitude greater or smaller than something else, it is ten times greater or smaller in size or amount 2.) the scale of the size of something ▪ That was a problem but this crisis is of… … Dictionary of contemporary English
order of magnitude — noun count TECHNICAL a level in a system for measuring things, in which each level is ten times higher than the one before: Processing speeds exceed those of five years ago by several orders of magnitude. a. also used in a non technical way to… … Usage of the words and phrases in modern English
order of magnitude — ► NOUN 1) a class in a system of classification determined by size, typically in powers of ten. 2) size or quantity … English terms dictionary
order of magnitude — noun 1. a degree in a continuum of size or quantity (Freq. 5) it was on the order of a mile an explosion of a low order of magnitude • Syn: ↑order • Derivationally related forms: ↑order (for: ↑ … Useful english dictionary
order of magnitude — UK / US noun [countable] Word forms order of magnitude : singular order of magnitude plural orders of magnitude a) a level in a system for measuring things, in which each level is ten times higher than the one before Processing speeds exceed… … English dictionary
order of magnitude — a logarithmic unit used to compare the sizes of quantities. Two quantities differ by one order of magnitude if one is 10 times the other, by two orders of magnitude if one is 10·10 = 100 times the other, and so on. Thus a difference of n… … Dictionary of units of measurement
order of magnitude — dydžio eilė statusas T sritis fizika atitikmenys: angl. order of magnitude vok. Größenordnung, f rus. порядок величины, m pranc. ordre de grandeur, m … Fizikos terminų žodynas
order of magnitude — order of size … English contemporary dictionary
order of magnitude — noun The class of scale or magnitude of any amount, where each class contains values of a fixed ratio (most often 10) to the class preceding it. For example, something that is 2 orders of magnitude larger is 100 times larger, something that is 3… … Wiktionary
order of magnitude — Date: 1875 a range of magnitude extending from some value to ten times that value … New Collegiate Dictionary