As of Version 9.0, physical constant functionality is built into the Wolfram Language

Physical Constants Package

 

ReferenceReference

AccelerationDueToGravity  ▪  AgeOfUniverse  ▪  EarthMass  ▪  EarthRadius  ▪  GravitationalConstant  ▪  HubbleConstant  ▪  IcePoint  ▪  SpeedOfSound  ▪  WeakMixingAngle

Universal Constants

PlanckConstant  ▪  PlanckConstantReduced  ▪  PlanckMass  ▪  SpeedOfLight  ▪  VacuumPermeability  ▪  VacuumPermittivity

Atomic Constants

BohrRadius  ▪  FineStructureConstant  ▪  RydbergConstant

Electron Constants

ClassicalElectronRadius  ▪  ElectronComptonWavelength  ▪  ElectronGFactor  ▪  ElectronMagneticMoment  ▪  ElectronMass  ▪  FaradayConstant  ▪  ThomsonCrossSection

Proton Constants

ProtonComptonWavelength  ▪  ProtonMagneticMoment  ▪  ProtonMass

Neutron Constants

NeutronComptonWavelength  ▪  NeutronMagneticMoment  ▪  NeutronMass

Muon Constants

MuonGFactor  ▪  MuonMagneticMoment  ▪  MuonMass

Deuteron Constants

DeuteronMagneticMoment  ▪  DeuteronMass

Electromagnetic Constants

ElectronCharge  ▪  MagneticFluxQuantum  ▪  QuantizedHallConductance

Chemical Constants

AvogadroConstant  ▪  MolarVolume

Physicochemical Constants

BoltzmannConstant  ▪  MolarGasConstant  ▪  SackurTetrodeConstant  ▪  StefanConstant

Astronomical Constants

CosmicBackgroundTemperature  ▪  GalacticUnit  ▪  SolarConstant  ▪  SolarLuminosity  ▪  SolarRadius  ▪  SolarSchwarzschildRadius