View source for Order of magnitude

{{Short description|Scale of numbers with a fixed ratio}}
{{other uses}}
In a [[ratio scale]] based on [[powers of ten]], the '''order of magnitude''' is a measure of the nearness of two figures. Two numbers are "within an order of magnitude" of each other if their ratio is between 1/10 and 10. In other words, the two numbers are within about a factor of 10 of each other.<ref>{{cite web|title=Order of Magnitude|url=http://mathworld.wolfram.com/OrderofMagnitude.html|website=Wolfram MathWorld|access-date=3 August 2024|quote=Two quantities A and B which are within about a factor of 10 of each other are then said to be "of the same order of magnitude," written A∼B. }}</ref>

For example, 1 and 1.02 are within an order of magnitude. So are 1 and 2, 1 and 9, or 1 and 0.2. However, 1 and 15 are not within an order of magnitude, since their ratio is 15/1 = 15 > 10. The reciprocal ratio, 1/15, is less than 0.1, so the same result is obtained.

[[Difference (mathematics)|Differences]] in order of magnitude can be measured on a base-10 [[logarithmic scale]] in "[[Decade (log scale)|decades]]" (i.e., factors of ten).<ref>{{cite web
 |url          = http://public.wsu.edu/~brians/errors/orders.html
 |url-status   = dead
 |archive-url  = https://web.archive.org/web/20180822170542/https://brians.wsu.edu/2016/05/19/orders-of-magnitude/
 |archive-date = 22 August 2018
 |title        = Orders of Magnitude
 |last         = Brians
 |first        = Paus
 |access-date  = 9 May 2013
}}</ref> For example, there is one order of magnitude between 2 and 20, and two orders of magnitude between 2 and 200.  Each division or multiplication by 10 is called an order of magnitude.<ref>
{{cite web
 |url = https://www.bbc.co.uk/bitesize/guides/zsnssbk/revision/1
 |title = Orders of magnitude
 |publisher = British Broadcasting Corporation
 |access-date = 8 August 2024
}}</ref> 
This phrasing helps quickly express the difference in scale between 2 and 2,000,000: they differ by 6 orders of magnitude.

Examples of numbers of different magnitudes can be found at [[Orders of magnitude (numbers)]].

Below are examples of different methods of partitioning the real numbers into specific "orders of magnitude" for various purposes. There is not one single accepted way of doing this, and different partitions may be easier to compute but less useful for approximation, or better for approximation but more difficult to compute.

==Calculating the order of magnitude==
Generally, the order of magnitude of a number is the smallest power of 10 used to represent that number.<ref>{{cite web|title=Order of Magnitude|url=http://mathworld.wolfram.com/OrderofMagnitude.html|website=Wolfram MathWorld|access-date=3 January 2017|quote=Physicists and engineers use the phrase "order of magnitude" to refer to the smallest power of ten needed to represent a quantity.}}</ref> To work out the order of magnitude of a number <math>n</math>, the number is first expressed in the following form:
:<math>n =a\times10^b</math>
where <math>\frac{1}{\sqrt{10}}\leq a<\sqrt{10}</math>, or approximately <math>0.316\lesssim a \lesssim 3.16</math>.<ref>{{cite book |author=Arihant Experts |chapter=Physical World and Measurement |title=Indian Air Force Airmen X&Y Group Online Test Complete Study Package |date=10 March 2020 |publisher=Arihant Publications India limited |isbn=978-93-241-9317-9 |at=Section 2, p. 5 |url=https://www.google.ca/books/edition/Indian_Air_Force_X_Y_Group_Technical_Non/GBvzDwAAQBAJ?hl=en&gbpv=1&dq=%22order+of+magnitude%22+3.16&pg=RA1-PA5&printsec=frontcover |language=en |quote=The magnitude of a physical quantity, whose order of magnitude is to be determined, is written in the form ''N'' x 10, where ''N'' is a number between 1 and 10 and ''x'' is a positive or negative integer. If N is equal to or smaller than {{sqrt|1x10}} = 3.16, then the order of magnitude of the quantity is 10<sup>''x''</sup>. But, if ''N'' is greater than 3.16, then the order of magnitude of the quantity is 10<sup>''x''+1</sup>.}}</ref> Then, <math>b</math> represents the order of magnitude of the number. The order of magnitude can be any [[integer]]. The table below enumerates the order of magnitude of some numbers using this definition:
{| class="wikitable"
|-
! Number <math>N</math>!! Expression in <math>N =a\times10^b</math>!! Order of magnitude <math>b</math>
|-
| 0.2 || 2 × 10<sup>−1</sup> ||−1 
|-
| 1 || 1 × 10<sup>0</sup> ||0 
|-
| 5 || 0.5 × 10<sup>1</sup> ||1
|-
| 6 || 0.6 × 10<sup>1</sup> ||1
|-
| 31 || 3.1 × 10<sup>1</sup> || 1
|-
| 32 || 0.32 × 10<sup>2</sup> || 2
|-
| 999 || 0.999 × 10<sup>3</sup>|| 3
|-
| 1000 || 1 × 10<sup>3</sup>|| 3
|}
The [[geometric mean]] of <math>10^{b-1/2}</math> and <math>10^{b+1/2}</math> is <math>10^b</math>, meaning that a value of exactly <math>10^b</math> (i.e., <math>a=1</math>) represents a geometric ''halfway point'' within the range of possible values of <math>a</math>.

Some use a simpler definition where <math>0.5\leq a< 5</math>.<ref>{{Cite web |last=Shaalaa.com |title=Answer the following question. Describe what is meant by order of magnitude. - Physics {{!}} Shaalaa.com |url=https://www.shaalaa.com/question-bank-solutions/answer-the-following-question-describe-what-is-meant-by-order-of-magnitude-significant-figures_171864 |access-date=2023-06-04 |website=www.shaalaa.com |language=en-IN}}</ref> This definition has the effect of lowering the values of <math>b</math> slightly:
{| class="wikitable"
|-
! Number <math>N</math>!! Expression in <math>N =a\times10^b</math>!! Order of magnitude <math>b</math>
|-
| 0.2 || 2 × 10<sup>−1</sup> ||−1 
|-
| 1 || 1 × 10<sup>0</sup> ||0 
|-
| 5 || 0.5 × 10<sup>1</sup>||1
|-
| 6 || 0.6 × 10<sup>1</sup> ||1
|-
| 31 || 3.1 × 10<sup>1</sup> || 1
|-
| 32 || 3.2 × 10<sup>1</sup> || 1
|-
| 999 || 0.999 × 10<sup>3</sup>|| 3
|-
| 1000 || 1 × 10<sup>3</sup>|| 3
|}

==Uses==
Orders of magnitude are used to make approximate comparisons.  If numbers differ by one order of magnitude, one number is about 10 times larger than the other.  If values 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. The growing amounts of Internet data have led to addition of new [[SI prefix]]es over time, most recently in 2022.<ref>{{cite journal
 |url        = https://www.nature.com/articles/d41586-022-03747-9
 |title      = How many yottabytes in a quettabyte? Extreme numbers get new names
 |last       = Gibney 
 |first      = Elizabeth
 |journal = Nature
 |year = 2022
 |doi = 10.1038/d41586-022-03747-9
 |pmid = 36400954
 |s2cid = 253671538
 |access-date = 20 November 2022
|url-access = subscription
 }}</ref>

{| class="wikitable"
! In words
! Prefix (symbol)
! Decimal
! [[Exponent#Powers of ten|Power]]<br />of&nbsp;ten
! Order&nbsp;of<br />magnitude
|-
|nonillionth
|quecto- (q)
| align=right | {{val|0.000000000000000000000000000001}}
| 10<sup>−30</sup>
| −30
|-
|octillionth
|ronto- (r)
| align=right | {{val|0.000000000000000000000000001}}
| 10<sup>−27</sup>
| −27
|-
| septillionth
| yocto- (y)
| align=right | {{val|0.000000000000000000000001}}
| 10<sup>−24</sup>
| −24
|-
| sextillionth
| zepto- (z)
| align=right | {{val|0.000000000000000000001}}
| 10<sup>−21</sup>
| −21
|-
| quintillionth
| atto- (a)
| align=right | {{val|0.000000000000000001}}
| 10<sup>−18</sup>
| −18
|-
| quadrillionth
| femto- (f)
| align=right | {{val|0.000000000000001}}
| 10<sup>−15</sup>
| −15
|-
| trillionth
| pico- (p)
| align=right | {{val|0.000000000001}}
| 10<sup>−12</sup>
| −12
|-
| billionth
| nano- (n)
| align=right | {{val|0.000000001}}
| 10<sup>−9</sup>
| −9
|-
| millionth
| [[micro-]] ([[Mu (letter)|μ]])
| align=right | {{val|0.000001}}
| 10<sup>−6</sup>
| −6
|-
| thousandth
| [[Milli-|milli]]- (m)
| align=right | 0.001
| 10<sup>−3</sup>
| −3
|-
| hundredth
| centi- (c)
| align=right | 0.01
| 10<sup>−2</sup>
| −2
|-
| tenth
| [[Deci-|deci]]- (d)
| align=right | 0.1
| 10<sup>−1</sup>
| −1
|-
| one
| &nbsp;
| align=right | 1
| 10<sup>0</sup>
| 0
|-
| ten
| [[deca-]] (da)
| align=right | 10
| 10<sup>1</sup>
| 1
|-
| hundred
| [[Hecto-|hecto]]- (h)
| align=right | 100
| 10<sup>2</sup>
| 2
|-
| thousand
| kilo- (k)
| align=right | {{val|1000|fmt=none}}
| 10<sup>3</sup>
| 3
|-
| million
| mega- (M)
| align=right | {{val|1000000}}
| 10<sup>6</sup>
| 6
|-
| billion
| [[Giga-|giga]]- (G)
| align=right | {{val|1000000000}}
| 10<sup>9</sup>
| 9
|-
| trillion
| tera- (T)
| align=right | {{val|1000000000000}}
| 10<sup>12</sup>
| 12
|-
| quadrillion
| peta- (P)
| align=right | {{val|1000000000000000}}
| 10<sup>15</sup>
| 15
|-
| quintillion
| exa- (E)
| align=right | {{val|1000000000000000000}}
| 10<sup>18</sup>
| 18
|-
| sextillion
| zetta- (Z)
| align=right | {{val|1000000000000000000000}}
| 10<sup>21</sup>
| 21
|-
| septillion
| yotta- (Y)
| align=right | {{val|1000000000000000000000000}}
| 10<sup>24</sup>
| 24
|-
| octillion
| ronna- (R)
| align=right | {{val|1000000000000000000000000000}}
| 10<sup>27</sup>
| 27
|-
| nonillion
| quetta- (Q)
| align=right | {{val|1000000000000000000000000000000}}
| 10<sup>30</sup>
| 30
|-
! In words
! Prefix (symbol)
! Decimal
! [[Exponent#Powers of ten|Power]]<br />of&nbsp;ten
! Order&nbsp;of<br />magnitude
|}

===Calculating the order of magnitude by truncation===
The order of magnitude of a number is, intuitively speaking, the number of digits in it which are above the ones place.  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 {{val|4000000}} 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<sup>6</sup> and 10<sup>7</sup>. In a similar example, with the phrase "seven-figure 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]].

===Order-of-magnitude estimate===
An order-of-magnitude estimate of a variable, whose precise value is unknown, is an estimate [[Rounding|rounded]] to the nearest power of ten.  For example, an order-of-magnitude estimate for a variable between about 3 billion and 30 billion (such as the human [[population]] of the [[Earth]]) is 10 [[1000000000 (number)|billion]]. To round a number to its nearest order of magnitude, one rounds its logarithm to the nearest integer.  Thus {{val|4000000}}, 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).<ref>{{cite book |author=Arihant Experts |chapter=Physical World and Measurement |title=Indian Air Force Airmen X&Y Group Online Test Complete Study Package |date=10 March 2020 |publisher=Arihant Publications India limited |isbn=978-93-241-9317-9 |at=Section 2, p. 5 |url=https://www.google.ca/books/edition/Indian_Air_Force_X_Y_Group_Technical_Non/GBvzDwAAQBAJ?hl=en&gbpv=1&dq=%22order+of+magnitude%22+3.16&pg=RA1-PA5&printsec=frontcover |language=en |quote=The magnitude of a physical quantity, whose order of magnitude is to be determined, is written in the form ''N'' x 10, where ''N'' is a number between 1 and 10 and ''x'' is a positive or negative integer. If N is equal to or smaller than {{sqrt|1x10}} = 3.16, then the order of magnitude of the quantity is 10<sup>''x''</sup>. But, if ''N'' is greater than 3.16, then the order of magnitude of the quantity is 10<sup>''x''+1</sup>.}}</ref> For example, the nearest order of magnitude for {{val|1.7|e=8}} is 8, whereas the nearest order of magnitude for {{val|3.7|e=8}} is 9. An order-of-magnitude estimate is sometimes also called a [[zeroth order approximation]].

==Non-decimal orders of magnitude==
{{See also|Logarithmic scale}}
An order of magnitude is an approximation of the [[logarithm]] of a value relative to some contextually understood reference value, usually 10, interpreted as the [[Radix|base]] of the logarithm and the representative of values of magnitude one. [[Logarithmic distribution]]s are common in nature and considering the order of magnitude of values sampled from such a distribution can be more intuitive. When the reference value is 10, the order of magnitude can be understood as the number of digits minus one in the base-10 representation of the value. Similarly, if the reference value is one of some powers of 2 since computers store data in a [[Binary_number|binary]] format, the magnitude can be understood in terms of the amount of computer memory needed to store that value.

===Irrational orders of magnitude===
Other orders of magnitude may be calculated using bases other than integers. In the field of [[astronomy]], the nighttime brightnesses of celestial bodies are ranked by [[Magnitude (astronomy)|"magnitudes"]] in which each increasing level is brighter by a [[factor (arithmetic)|factor]] of <math>\sqrt[5]{100} \approx 2.512</math> greater than the previous level. Thus, a level being 5 magnitudes brighter than another indicates that it is a factor of <math>(\sqrt[5]{100})^5 = 100</math> times brighter: that is, two base 10 orders of magnitude. 

This series of magnitudes forms a logarithmic scale with a base of <math>\sqrt[5]{100}</math>.

===Base 1,000,000 orders of magnitude===
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 {{val|1000000}}. It can be seen that the order of magnitude is included in the number name in this example, because bi- means 2, tri- means 3, etc. (these make sense in the long scale only), and the suffix -illion tells that the base is {{val|1000000}}. But the number names billion, trillion themselves (here with [[Long and short scales|other meaning]] than in the first chapter) are not names of the ''orders of'' magnitudes, they are names of "magnitudes", that is the ''numbers'' {{val|1000000000000}} etc.

{| class="wikitable"
! Order of magnitude !! Is [[Common logarithm|log<sub>10</sub>]] of !! Is log<sub>{{val|1000000}}</sub> of !! Short scale !! Long scale
|-
| 1 || align=right |   {{val|10}}          || align=right |             {{val|1000000}} || align=right |     million || align=right |  million
|-
| 2 || align=right |  {{val|100}}          || align=right |       {{val|1000000000000}} || align=right |    trillion || align=right |  billion
|-
| 3 || align=right | {{val|1000|fmt=none}} || align=right | {{val|1000000000000000000}} || align=right | quintillion || align=right | trillion
|-
| 4 || align=right | {{val|10000|fmt=none}} || align=right | (1 000 000)<sup>4</sup> || align=right | septillion || align=right | quadrillion
|-
| 5 || align=right | {{val|100000|fmt=none}} || align=right | (1 000 000)<sup>5</sup> || align=right | nonillion || align=right | quintillion
|}

[[SI]] units in the table at right are used together with [[SI prefix]]es, which were devised with mainly base 1000 magnitudes in mind. [[Binary prefix#IEC standard prefixes|The IEC standard prefixes]] with base 1024 were invented for use in electronic technology.

==See also==
* [[Big O notation]]
* [[Decibel]]
* [[Mathematical operators and symbols in Unicode]]
* [[Names of large numbers]]
* [[Names of small numbers]]
* [[Number sense]]
* [[Orders of magnitude (acceleration)]]
* [[Orders of magnitude (area)]]
* [[Orders of magnitude (bit rate)]]
* [[Orders of magnitude (current)]]
* [[Orders of magnitude (data)]]
* [[Orders of magnitude (energy)]]
* [[Orders of magnitude (force)]]
* [[Orders of magnitude (frequency)]]
* [[Orders of magnitude (illuminance)]]
* [[Orders of magnitude (length)]]
* [[Orders of magnitude (mass)]]
* [[Orders of magnitude (numbers)]]
* [[Orders of magnitude (power)]]
* [[Orders of magnitude (pressure)]]
* [[Orders of magnitude (radiation)]]
* [[Orders of magnitude (speed)]]
* [[Orders of magnitude (temperature)]]
* [[Orders of magnitude (time)]]
* [[Orders of magnitude (voltage)]]
* [[Orders of magnitude (volume)]]
* ''[[Powers of Ten (film)|Powers of Ten]]''
* [[Scientific notation]]
* [[CJK Compatibility|Unicode symbols for CJK Compatibility]] includes [[International System of Units|SI]] Unit symbols
* [[Valuation (algebra)]], an algebraic generalization of "order of magnitude"
* [[Scale (analytical tool)]]

== References ==
{{Reflist}}

==Further reading==
* [[Isaac Asimov|Asimov, Isaac]], ''The Measure of the Universe'' (1983).

==External links==
* [http://htwins.net/scale2/ The Scale of the Universe 2 ] Interactive tool from [[Planck length]] 10<sup>−35</sup> meters to universe size 10<sup>27</sup>
* [https://web.archive.org/web/20080412094332/http://www.shekpvar.net/~dna/Publications/Cosmos/cosmos.html Cosmos &ndash; an Illustrated Dimensional Journey from microcosmos to macrocosmos] &ndash; from Digital Nature Agency
* [http://micro.magnet.fsu.edu/primer/java/scienceopticsu/powersof10/index.html Powers of 10], a graphic animated illustration that starts with a view of the [[Milky Way]] at 10<sup>23</sup> meters and ends with [[subatomic particle]]s at 10<sup>−16</sup> meters.
* [http://www.vendian.org/envelope/TemporaryURL/what_is_oom.html What is Order of Magnitude?]

{{orders of magnitude wide}}
{{Authority control}}

[[Category:Orders of magnitude| ]]
[[Category:Elementary mathematics]]
[[Category:Logarithmic scales of measurement]]