# 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[1]
Speed of light   This is the speed of light in a vacuum Most of physics, such as optics and relativity Exact[2]
Permittivity of free space   Related to the strength of an electric field produced by a charge Electromagnetism Exact[3]
Permeability of free space   Related to the strength of [[magnetic field]s produced by a current Electromagnetism Exact[4]
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[5]
Avogadro's number   The number of constituents in 1 mole of something Chemistry, statistical mechanics, thermodynamics Inexact[6]
Ideal gas constant   Relates the pressure, volume, temperature and number of moles of an ideal gas Thermodynamics Inexact[7]
Faraday Constant   The electric charge on one mole of electrons (Avogadro's number times elementary charge) Chemistry, physics Inexact[8]
Gravitational constant   Constant related to the strength of Newtonian gravity Newtonian gravity Inexact[9]
Boltzmann constant   Thermodynamics Inexact[10]
Reduced Planck's constant   Quantum mechanics It is Planck's constant divided by . Inexact[11]
Wien displacement constant   Relates the temperature of a black body to the peak in its spectrum Inexact[12]
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 (ie, no air resistance). Mechanics Exact[13]
Proton mass   Rest mass of a proton Inexact[14]
Neutron mass   Rest mass of a neutron Inexact[15]
Electron mass   Rest mass of an electron Inexact[16]

## 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