Physical constants (NIST, OEIS)  

Physical Units
 Energy
 In physics, energy is a scalar quantity
describing the amount of work that can be performed by a force. It is an
attribute of objects and systems. Because of its definition, the
SI
unit for energy is the same as the unit of work  the joule (J), named after
James Prescott Joule.
1 joule is equal to 1 newtonmetre or in terms of
SI
base units:
Other units of energy include:
 The erg (measured in cgs units): 1 erg = 1 g cm^{2} / s^{2}, which equals 1.0 x 10^{7}J.
 The electronvolt (sometimes electron volt) eV: this is a common unit for energy measurements on atomic scale^{[1]}, it is equal to the amount of energy gained by a single unbound electron when it accelerates through an electrostatic potential difference of one volt: 1 eV = 1.60217653(14) x 10^{19}J.
 In spectroscopy, people tend to use cm^{1} since energy is inversely proportional to wavelength .
 kWh: 1 kWh = 3.6 x 10^{6}J
 BOE/toe: the barrel/ton of oil equivalent, based on the approximate energy released by burning a barrel/tonne of crude oil. The BOE is defined as 6.1178632 x 10^{9}J, a toe is approximately 42 GJ.
 Pressure

(symbol: p) is the force per unit area applied to an object in a direction perpendicular to the surface.
The SI derived unit used for pressure, stress, Young's modulus and
tensile strength is the pascal (Pa), named after
Blaise Pascal.
As a measure of force per unit area it is equivalent to to one newton per
square meter or one joule per cubic metre:
1 Pa = 1 N / m^{2}.There are lots of different units used for pressure measurement:
The standard atmosphere (atm) is an international reference pressure defined as 101325 Pa (just to confuse me further, there is one more unit called the technical atmosphere (at), defined as 98.0665 kPa, ≈ 0.96784 standard atmospheres).
The torr, named after Evangelista Torricelli is defined as 1/760 of an atmosphere.
The bar is a nonSI unit, but accepted for use with the SI. It is defined as 1 bar = 100 kPa (kilopascals) = 10^{5} Pa.
The psi (poundforce per square inch, also lbf/in^{2} or lb_{f }/in^{2}) is a unit of pressure or of stress based on avoirdupois units. 1 Pa is ≈ 1.4504 x 10^{4} psi ^{[2]}.
Some handy conversions
 Convert wavelength (measured in the SI unit m) to energy (in SI unit
J or in eV):
Here's the story: our wave has an energy of . Furthermore, frequency and wavelength are linked via the speed of light . This leads to
where the values used are
c = speed of light in vacuum = 2.99792458 x 10^{10} cm/sec = 2.99792458 x 10^{8} m/sec
h = Planck's constant = 6.626 x 10^{34} J/secA wave in vacuum with a wavelength of
m has a frequency of 5.451e+14 1/s (or Hz) and an energy E = 3.612e19 J = 2.254e+0 eV  Convert pressure units (SI unit of pressure is Pa):
There is a whole zoo of pressure units for historical reasons. Here is a list (probably incomplete) to convert from one to anotherPa = 1.0e5 bar = 9.8692e6 atm = 7.5006e3 Torr.
bar = 1.0e+5 Pa = 9.8692e1 atm = 7.5006e+2 Torr.
Torr = 1.3332e+2 Pa = 1.3332e3 bar = 1.3158e3 atm.
Links on constants and units
 Fundamental physical constants, National Institute of Standards and Technology.
 Wikipedia: Conversion of units.
 A Dictionary of Units of Measurement by Russ Rowlett
 NIST Reference on Constants, Units, and Uncertainty. National Institute of Standards and Technology.
 GNU Units, a crossplatform program for conversion of units of quantities.
Notes:
^{1 } In a lecture from 1961, Richard Feynman apologized to his students for using electron volts to measure energy instead of some multiple of the joule (see wikipedia).
^{2 } The pound (lb) used is the international avoirdupois pound, defined to be exactly 0.45359237 kg. The poundforce (lbf) approximately equals the force exerted on a mass of one lb on the surface of earth. Using standard gravity g = 9.80665m/s^{2}, one can calculate the poundforce:
1 lbf = 0.45359237 kg 9.806650 m/s^{2} = 4.448221615 kg m/s^{2}
One inch (in) is exactly 2.54 cm, so a pascal can be written as
1 Pa = (0.0254 m/in)^{2} / 4.448221615 N/lbf
= 1.4504 x 10^{4} psi