This page lists useful physical and astronomical constants, in SI units and cgs units.
| Symbol | SI value | cgs value | Name |
| G | 6.673 x 10-11 N m2 kg-2 | 6.673 x 10-8 cm3 g-1 s-2 | Gravitational constant |
| c | 2.99792458 x 108 m s-1 | 2.99792458 x 1010 cm s-1 | Speed of light |
| μo (mu zero) | 4π x 10-7 N A-2 | Permeability of free space | |
| εo (epsilon zero) | 8.85418782 x 10-12 m-3 kg-1 s4A2 | Permittivity of free space | |
| e | 1.602176462 x 10-19 C | 4.803 x 10-10 esu | Elementary charge |
| h | 6.62606876 x 10-34 J s | 6.62606876 x 10-27 erg s | Planck's constant |
| k | 1.3806503 x 10-23 J K-1 | 1.3806503 x 10-16 erg K-1 | Boltzmann's constant |
| σ (sigma) | 5.670400 x 10-8 W m-2 K-4 | 5.670400 x 10-5 erg cm-2 s-1 K-4 | Stefan-Boltzmann constant |
| a=4σ/c (=4 sigma/c) | 7.565767 x 10-16 J m-3 K-4 | 7.565767 x 10-15 erg cm-3 K-4 | Radiation constant |
| me | 9.10938188 x 10-31 kg | 9.10938188 x 10-28 g | Electron mass |
| mp | 1.67262158 x 10-27 kg | 1.67262158 x 10-24 g | Proton mass |
| mn | 1.67492716 x 10-27 kg | 1.67492716 x 10-24 g | Neutron mass |
| mH | 1.673532499 x 10-27 kg | 1.673532499 x 10-24 g | Hydrogen mass |
| u | 1.66053873 x 10-27 kg | 1.66053873 x 10-24 g | Atomic mass unit |
| AU | 1.4959787066 x 1011 m | 1.4959787066 x 1013 cm | Astronomical unit |
| pc | 3.0856776 x 1016 m | 3.0856776 x 1018 cm | Parsec |
| ly | 9.460730472 x 1015 m | 9.460730472 x 1017 cm | Light year |
| Msun | 1.9891 x 1030 kg | 1.9891 x 1033 g | Solar mass |
| Rsun | 6.95508 x 108 m | 6.95508 x 1010 cm | Solar radius |
| Lsun | 3.839 x 1026 W | 3.839 x 1033 erg s-1 | Solar luminosity |
| Mearth | 5.9736 x 1024 kg | 5.9736 x 1027 g | Earth mass |
| Rearth | 6.378136 x 106 m | 6.378136 x 108 cm | Earth radius |
| y | 3.155692519 x 107 s | 3.155692519 x 107 s | Tropical year |
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1 eV = 1.602176462 x 10-19 J = 1.602176462 x 10-12 erg 1 Å = 10-10 m = 10-8 cm 1 arc second = 1/60 arc minute = 1/3600 degrees 1 erg = 1 g cm2 s-2 1 J = 1 kg m2 s-2 |