# Constants

This page lists various constants used in physics and some useful conversions.

## Physical constants

The number in brackets represents the uncertainty in the constant, so that has an uncertainty of .

Name Symbol Value Meaning Used in Exact
Planck's constant  It relates the frequency of a photon to its energy Quantum mechanics Inexact
Speed of light  This is the speed of light in a vacuum Most of physics, such as optics and relativity Exact
Permittivity of free space  Related to the strength of an electric field produced by a charge Electromagnetism Exact
Permeability of free space  Related to the strength of magnetic fields produced by a current Electromagnetism Exact
Elementary charge  The magnitude of the electric charge on an electron Determines the strength of electric and magnetic fields produced by protons, electrons etc. Inexact
Avogadro's number  The number of constituents in 1 mole of something Chemistry, statistical mechanics, thermodynamics Inexact
Ideal gas constant  Relates the pressure, volume, temperature and number of moles of an ideal gas Thermodynamics Inexact
Faraday Constant  The electric charge on one mole of electrons (Avogadro's number times elementary charge) Chemistry, physics Inexact
Gravitational constant  Constant related to the strength of Newtonian gravity Newtonian gravity Inexact
Boltzmann constant  Thermodynamics Inexact
Reduced Planck's constant  Quantum mechanics It is Planck's constant divided by . Inexact
Wien displacement constant  Relates the temperature of a black body to the peak in its spectrum Inexact
Standard acceleration due to gravity  This is the acceleration of any object near to the earth's surface when gravity is the only force acting (i.e., no air resistance). Mechanics Exact
Proton mass  Rest mass of a proton Inexact
Neutron mass  Rest mass of a neutron Inexact
Electron mass  Rest mass of an electron Inexact

## Useful Conversions

### Length

1 metre = 39.37 inches 1 metre = 3.281 feet 1 kilometre = 0.6213 miles

1 mile = 5280 feet 1 mile = 1760 yards

### Weight

1 kilogram = 35.27 ounce 1 kilogram = 2.205 pounds

### Pressure

1 atm = 101.1 kPa 1 atm = 760 torr 1 atm = 760 mm Hg

### Other

Atomic mass unit: 1.00 amu = 1.67 x 10−27 kg

1.000 calorie = 4.184 Joules

Standard Temperature and Pressure (STP): temperature = 273 K and pressure = 1.00 atm

Relative mass and charge of subatomic particles found in atoms.

Atomic particles
Relative Mass Relative Charge
Proton 1 +1
Neutron 1 0
Electron 1/1837 -1

## Prefixes

Prefixes in front of a unit indicate that that unit should be multiplied by a multiple of ten, e.g. Giga is 109, so 1 GPa = 109 Pa.

Name Symbol Power
Pico p 10−12
nano n 10−9
micro μ 10−6
milli m 10−3
centi c 10−2
deci d 10−1
deca da 101
hecto h 102
kilo k 103
mega M 106
giga G 109
tera T 1012