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# Physics > Physical Constants

The table below shows a variety of physical constants that you may come across in school science.

Quantity Symbol SI unit value Alternate unit value
Astronomical unit AU 1.4959787 x 1011 metres 1.4959787 x 108 kilometres
Atomic mass unit amu 1.67377 x 10-27 kilogram 1.67377 x 10-24 gram
Avogadro Constant N, L 6.022169 x 1023 6.022169 x 1023
Bohr magneton µB 9.274 x 10-24 joule per tesla 9.274 x 10-28 joule per gauss
Bohr radius αo 5.29177 x 10-11 metre 5.29177 x 10-9 centimetre
Boltzmann's constant k 1.3807 x 10-23 joule per Kelvin 1.3807 x 10-16 erg per Kelvin
Characteristic impedance of free space Z o 377 ohms
Compton wavelength of electron λce 2.4263 x 10-12 metre 2.4263 x 10-10 centimetre
Compton wavelength of proton λcp 1.3214 x 10-15 metre 1.3214 x 10-13 centimetre
Compton wavelength of neutron λcn 1.3196 x 10-15 metre 1.3196 x 10-13 centimetre
Earth's mass m earth 5.974 x 1024 kilograms 5.974 x 1027 grams
Earth's mean orbital speed v earth 30 kilometres per second 18.5 miles per second
Earth's mean radius r earth 6.371 x 10-6 metres 6371 kilometres
Electron radius re 2.81794 x 10-15 metre 2.81794 x 10-13 centimetre
electron rest mass me 9.10956 x 10-31 kilogram 9.10956 x 10-28 gram
Elementary charge e 1.60 x 10-19 coulomb
Euler's constant γ 0.577216 0.577216
Faraday constant F 9.65 x 10-4 coulombs per mole
Fine structure constant α 0.007297 0.007297
First radiation constant c1 4.9926 x 10-24 joule-metre 4.9926 x 10-15 erg-centimetre
Gas constant R 8.3145 joules per kelvin per mole 8.3145 x 107 ergs per kelvin per mole
Gravitational acceleration at Earth's surface g 9.8067 metres per second squared 980.67 centimetres per second squared
Gravitational constant G 6.67 x 10-11 newton metre squared per kilogram squared
Hartree energy E h 4.3597 x 10-18 joule 4.3597 x 10-11 erg
Moon's mass mmoon 7.348 x 1022 kilograms 7.348 x 1025 grams
Moon's mean radius rmoon 1.738 x 106 metres 1738 kilometres
Natural logarithm base e 2.718282 2.718282
Neutron rest mass mn 1.67492 x 10-27 kilogram 1.67492 x 10-24 gram
Nuclear magneton µn 5.051 x 10-27 joule per tesla 5.051 x 10-31 joule per gauss
Permeability of free space µo 1.257 x 10-6 henry per metre 1.257 microhenrys per metre
Permittivity of free space εo 8.85 x 10-12 farad per metre 8.85 picofarads per metre
pi π 3.14159 3.14159
Planck's constant h 6.626176 x 10-34 joule-second 6.626176 x 10-27 erg-second
Proton radius rp 2 x 10-16 metre * 2 x 10-14 centimetre *
Proton rest mass mp 1.67261 x 10-27 kilogram 1.67261 x 10-24 gram
Rydberg constant R or RH 1.097 x 107 per metre 1.097 x 105 per centimetre
Second radiation constant c2 0.014388 metre-kelvin 1.4388 centimetre-kelvin
Speed of light c 2.99792 x 108 metres per second 2.99792 x 1010 centimetres per second
Speed of sound in air vsound 331.4 metres per second 1087 feet per second
Stefan-Boltzmann constant α 5.67 x 10-8 watt per metre squared per kelvin to the fourth
Sun's mass msun 1.989 x 1030 kilograms 1.989 x 1033 grams
Sun's mean radius rsun 6.970 x 108 metres 6.970 x 105 kilometres
Wien's constant αw 2.898 x 10-3 metre-kelvin 0.2898 centimetre-kelvin