An order of magnitude is a factor of ten. For example, two numbers are said to differ by "three orders of magnitude" if they differ by three factors of ten, that is one is approximately 1000 times larger than other.
The order of magnitude of a number is, intuitively speaking, the number of powers of 10 contained in the number or a close approximation to it. More precisely, the order of magnitude of a number can be defined in terms of the logarithm of the number to the base of 10, usually as the integer part of the logarithm. Thus the order of magnitude of 400 is 2. The order of magnitude of a number may also be defined as the exponent of the power of 10 when the number is represented using scientific notation.
An order of magnitude estimate of a variable whose precise value is unknown is an estimate rounded in some way to the nearest power of 10. For example, an accurate order of magnitude estimate for the human population of the Earth in the year 2000 is 10 billion. An order of magnitude estimate is sometimes also called a zeroth order approximation.
One way of categorising things in the physical world is by their size. The pages below contain lists of items that are of the same order of magnitude in time, length, area, volume, mass, energy or temperature. This is useful for getting an intuitive sense of the comparative size of things and the overall scale of the universe. SI units are used together with SI prefixes: these were devised with orders of magnitude in mind. Each individual page also gives other units; see also conversion of units.
1 Orders of magnitude of various quantities
Table of contents 

Orders of magnitude of various quantities
In the following table the different quantities are lined up so that the following are in the same row:
 length and the approximate time taken by light to cross that length
 area of a square and the length of one side
 volume of a cube and the area of one face
 mass of some water and its volume at 4 degrees Celsius or 277.16 K
0.01 meV
0.1 meV10 meV
100 meV100 K
1000 K10 eV
100 eV100,000 K
10^{6} K10^{32}kg
10^{31} kg10^{4} eV
10^{5} eV10^{29} kg
10^{28} kg10 MeV
100 MeV10^{29} kg
10^{28} kg10 MeV
100 MeV10^{19}m^{2}10^{26} kg
10^{25} kg10 GeV
100 GeV10^{17}m^{2}10^{23} kg
10^{22} kg10 TeV
100 TeV10^{15}m^{2}10^{20} kg
10^{19} kg0.001 J
0.01 J10^{13}m^{2}10^{20}m^{3}
10^{19}m^{3}10^{17} kg
10^{16} kg1 J
10 J10^{11}m^{2}10^{17}m^{3}
10^{16}m^{3}10^{14} kg
10^{13} kg1000 J
10000 J10^{9}m^{2}10^{14}m^{3}
10^{13}m^{3}10^{11} kg
10^{10} kg0.001 kWh
0.01 kWh10^{7}m^{2}10^{11}m^{3}
10^{10}m^{3}10^{8} kg
10^{7} kg1 kWh
10 kWh10^{5}m^{2}10^{8}m^{3}
10^{7}m^{3}10^{5} kg
10^{4} kg1000 kWh
10000 kWh10 cm^{2}10 ml
100 ml 10 g
100 g1 GWh
10 GWh0.1 m^{2}10 l
100 l 10 kg
100 kg1000 GWh
10000 GWh10 m^{2}10 m^{3}
100 m^{3}10 t
100 t10^{6} GWh
10^{7} GWh1,000 m^{2}10,000 m^{3}
10^{5}m^{3}10^{7} kg
10^{8} kg10^{9} GWh
10 ha 10^{7}m^{3}
10^{8}m^{3}10^{10} kg
10^{11} kg10 km^{2}10 km^{3}
100 km^{3}10^{13} kg
10^{14} kg10^{9}m^{2}10^{16} kg
10^{17} kg10^{11}m^{2}10^{19} kg
10^{20} kg10^{13}m^{2}10^{22} kg
10^{23} kg10^{15}m^{2}10^{17}m^{2}10^{19}m^{2}10^{21}m^{2}10^{41}m^{2}
 Each time shown is linked to that time. However, the time taken for light to cross the corresponding length is 3 times the time shown.
Units used in the table
The table uses units and prefixes that are commonly recognized: Time:

 femtosecond (fs)
 nanosecond (ns)
 microsecond (μs)
 millisecond (ms)
 second (s)
 hour (h)
 day (d)
 year (yr)
 Length:

 attometre (am)
 femtometre (fm)
 picometre (pm)
 nanometre (nm)
 micrometre (µm)
 millimetre (mm)
 centimetre (cm)
 metre (m)
 kilometre (km)
 astronomical unit (AU)
 light year (LY)
 Area:

 square metre (m^{2})
 hectare (ha)
 square kilometre (km^{2})
 Mass:
 Volume:

 millilitre (ml)
 litre (l)
 cubic metre (m^{3})
 Energy:
 Temperature:

 nanokelvin (nK)
 microkelvin (µK)
 millikelvin (mK)
 kelvin (K)
See also
 Orders of approximation
 Orders of magnitude (time)
 Orders of magnitude (length)
 Orders of magnitude (area)
 Orders of magnitude (volume)
 Orders of magnitude (mass)
 Orders of magnitude (energy)
 Orders of magnitude (power)
 Orders of magnitude (temperature)
 Orders of magnitude (numbers)
 Orders of magnitude (U.S. money)
 Orders of magnitude (data)
 Powers of Ten
 SI base units
 SI derived units
 Complete list of SI prefixes
 Timeline of the Big Bang
 Timeline of the Universe
External links
 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.
 Orders of magnitude  Distance