Universal & Atomic constants

55 values, each with its units, its uncertainty, and where it came from.

Astronomical (106)Chemistry (45)Electromagnetic (31)Material Properties (69)Mathematical (16)Thermodynamic (37)Universal & Atomic (47)

All 351 constants →

A B C D E F H I M N P Q R S T W

Absolute zero exact

T = 0\ \mathrm{K} = 0\ \text{K}

KThe zero of the thermodynamic temperature scale: 0 K, equal to -273.15 °C and -459.67 °F exactly, both figures fixed by definition.

Alpha particle mass measured

m_{\alpha} = 6.644657345 \times 10^{-27}\ \text{kg}

kgThe mass of the helium-4 nucleus, 6.644 657 345 × 10⁻²⁷ kg — 4.001 506 u, 3727.379 MeV/c², and the most tightly bound light nucleus.

Atomic mass constant (unified atomic mass unit) measured

m_{\mathrm{u}} = 1.66053906892 \times 10^{-27}\ \text{kg}

kgOne twelfth of the mass of a free carbon-12 atom at rest, 1.660 539 069 × 10⁻²⁷ kg — the dalton used in every mass spectrum.

Atomic mass constant energy equivalent measured

m_{\mathrm{u}} c^{2} = 1.49241808768 \times 10^{-10}\ \text{J}

JThe rest energy of one dalton, 1.492 418 088 × 10⁻¹⁰ J or 931.494 MeV — the conversion factor behind every mass-defect calculation.

Avogadro constant exact

N_{\mathrm{A}} = 6.02214076 \times 10^{23}\ \text{mol}^{-1}

mol⁻¹Exactly 6.022 140 76 × 10²³ entities per mole — the fixed number that has defined the mole since the 2019 SI revision.

Barn (nuclear cross-section unit) exact

\mathrm{b} = 1 \times 10^{-28}\ \text{m}^{2}

m²Exactly 10⁻²⁸ m², or 100 fm²: the unit every nuclear cross section is quoted in, and roughly the geometric area of a uranium nucleus.

Bohr radius measured

a_{0} = 5.29177210544 \times 10^{-11}\ \text{m}

mThe most probable electron-proton distance in ground-state hydrogen, 5.291 772 105 × 10⁻¹¹ m — the natural size of an atom.

Boltzmann constant exact

k_{\mathrm{B}} = 1.380649 \times 10^{-23}\ \text{J/K}

J/KThe energy per kelvin carried by a single particle's degree of freedom, fixed at exactly 1.380649e-23 J/K to define the kelvin.

Classical electron radius measured

r_{\mathrm{e}} = 2.8179403205 \times 10^{-15}\ \text{m}

mr_e = α²a₀ = 2.817 940 321 × 10⁻¹⁵ m — the radius a classical sphere of charge e would need to have rest energy m_e c².

Compton wavelength of the electron measured

\lambda_{\mathrm{C}} = 2.42631023538 \times 10^{-12}\ \text{m}

mλ_C = h/(m_e c) = 2.426 310 235 × 10⁻¹² m — the wavelength shift of a photon scattered through 90° by a free electron.

Deuteron mass measured

m_{\mathrm{d}} = 3.3435837768 \times 10^{-27}\ \text{kg}

kgThe mass of the deuteron, one proton bound to one neutron: 3.343 583 777 × 10⁻²⁷ kg, or 2.013 553 u and 1875.613 MeV/c².

Electron charge-to-mass quotient measured

-e/m_{\mathrm{e}} = -1.75882000838 \times 10^{11}\ \text{C/kg}

C/kgThe electron's charge divided by its mass, −1.758 820 008 × 10¹¹ C/kg — the quantity J. J. Thomson measured in 1897 to discover the electron.

Electron g-factor measured

g_{\mathrm{e}} = -2.00231930436092

dimensionlessThe electron's magnetic moment in Bohr magnetons, −2.002 319 304 360 92 — the most precisely tested prediction in all of science.

Electron mass measured

m_{\mathrm{e}} = 9.1093837139 \times 10^{-31}\ \text{kg}

kgThe rest mass of the electron, 9.109 383 714 × 10⁻³¹ kg — equivalently 5.485 799 091 × 10⁻⁴ u or 0.510 999 MeV/c².

Electron mass energy equivalent measured

m_{\mathrm{e}} c^{2} = 8.187105788 \times 10^{-14}\ \text{J}

JThe electron's rest energy, 8.187 105 788 × 10⁻¹⁴ J or 510.999 keV — the photon energy of every positron annihilation.

Electronvolt (in joules) exact

\mathrm{eV} = 1.602176634 \times 10^{-19}\ \text{J}

JThe energy one electron gains crossing one volt: exactly 1.602 176 634 × 10⁻¹⁹ J, the working currency of atomic physics.

Elementary charge exact

e = 1.602176634 \times 10^{-19}\ \text{C}

CThe charge of a proton, exactly 1.602 176 634 × 10⁻¹⁹ C — the quantum of free charge and the SI definition of the ampere.

Fermi coupling constant measured

G_{\mathrm{F}}/(\hbar c)^{3} = 0.000011663787\ \text{GeV}^{-2}

GeV⁻²The strength of the weak interaction at low energy, 1.166 378 7 × 10⁻⁵ GeV⁻² — measured from the muon's 2.2 microsecond lifetime.

Fine-structure constant measured

\alpha = 0.0072973525643

dimensionlessThe dimensionless strength of the electromagnetic interaction, 0.007 297 352 564 — roughly 1/137, and pure number with no units at all.

First radiation constant exact

c_1 = 3.741771852 \times 10^{-16}\ \text{W}{\cdot}\text{m}^{2}

W·m²c₁ = 2πhc² = 3.741771852e-16 W·m², the numerator of Planck's law in its spectral exitance form and the scale of all blackbody emission.

Hartree energy measured

E_{\mathrm{h}} = 4.359744722206 \times 10^{-18}\ \text{J}

JThe atomic unit of energy, 4.359 744 722 × 10⁻¹⁸ J or 27.211 386 eV — twice the Rydberg and the currency of quantum chemistry.

Inverse fine-structure constant measured

\alpha^{-1} = 137.035999177

dimensionlessThe reciprocal of the fine-structure constant, 137.035 999 177 — famously near 137, and definitively not equal to it.

Molar gas constant exact

R = 8.31446261815324\ \text{J/(mol}{\cdot}\text{K)}

J/(mol·K)The universal gas constant, R = k·N_A = 8.314462618 J/(mol·K), exact since 2019 and the R in PV = nRT and in every entropy table.

Muon mass measured

m_{\mu} = 1.883531627 \times 10^{-28}\ \text{kg}

kgThe rest mass of the muon, 1.883 531 627 × 10⁻²⁸ kg or 105.658 MeV/c² — 207 electrons in one unstable package.

Muon-electron mass ratio measured

m_{\mu}/m_{\mathrm{e}} = 206.7682827

dimensionlessThe muon is 206.768 times heavier than the electron — the same particle in every respect except mass, and nobody knows why.

Neutron mass measured

m_{\mathrm{n}} = 1.67492750056 \times 10^{-27}\ \text{kg}

kgThe rest mass of the neutron, 1.674 927 501 × 10⁻²⁷ kg — 1.008 665 u or 939.565 MeV/c², just heavier than the proton.

Neutron mass energy equivalent measured

m_{\mathrm{n}} c^{2} = 1.50534976514 \times 10^{-10}\ \text{J}

JThe neutron's rest energy, 1.505 349 765 × 10⁻¹⁰ J or 939.565 MeV — exceeding the proton's by the 1.293 MeV that drives beta decay.

Neutron-proton mass ratio measured

m_{\mathrm{n}}/m_{\mathrm{p}} = 1.00137841946

dimensionlessThe neutron is heavier than the proton by just 0.1378 % — the 1.293 MeV difference that makes free neutrons decay and stars burn.

Newtonian constant of gravitation measured

G = 6.6743 \times 10^{-11}\ \text{m}^{3}\text{/(kg}{\cdot}\text{s}^{2}\text{)}

m³/(kg·s²)The coupling strength of gravity, 6.674 30 × 10⁻¹¹ m³/(kg·s²) — the worst-measured constant in all of physics.

Nuclear radius constant

r_{0} = 1.2 \times 10^{-15}\ \text{m}

mThe empirical coefficient in R = r₀A^(1/3), about 1.2 × 10⁻¹⁵ m — the constant that says nuclear matter has a fixed density.

Planck constant exact

h = 6.62607015 \times 10^{-34}\ \text{J}{\cdot}\text{s}

J·sThe quantum of action, exactly 6.626 070 15 × 10⁻³⁴ J·s — the constant that has defined the kilogram since 2019.

Planck energy measured

E_{\mathrm{P}} = 1,956,081,637\ \text{J}

Jm_P c² = 1.956 × 10⁹ J, or 1.221 × 10¹⁹ GeV — the energy scale of quantum gravity, and about the kinetic energy of a car on the motorway.

Planck length measured

\ell_{\mathrm{P}} = 1.616255 \times 10^{-35}\ \text{m}

m√(ħG/c³) = 1.616 255 × 10⁻³⁵ m — the length scale where quantum mechanics and gravity must both apply, and neither alone works.

Planck mass measured

m_{\mathrm{P}} = 2.176434 \times 10^{-8}\ \text{kg}

kg√(ħc/G) = 2.176 434 × 10⁻⁸ kg — about 22 micrograms, the only Planck unit on a human scale, and the mass where gravity meets quantum.

Planck temperature measured

T_{\mathrm{P}} = 1.416784 \times 10^{32}\ \text{K}

K√(ħc⁵/G)/k = 1.416 784 × 10³² K — the temperature at which thermal photons carry the Planck energy and gravity becomes quantum.

Planck time measured

t_{\mathrm{P}} = 5.391247 \times 10^{-44}\ \text{s}

s√(ħG/c⁵) = 5.391 247 × 10⁻⁴⁴ s — the time light takes to cross a Planck length, and the earliest instant physics can describe.

Proton charge-to-mass quotient measured

e/m_{\mathrm{p}} = 95,788,331.43\ \text{C/kg}

C/kg9.578 833 143 × 10⁷ C/kg — the proton's charge-to-mass ratio, smaller than the electron's by the full factor of 1836.

Proton mass measured

m_{\mathrm{p}} = 1.67262192595 \times 10^{-27}\ \text{kg}

kgThe rest mass of the proton, 1.672 621 926 × 10⁻²⁷ kg — 1.007 276 u, 938.272 MeV/c², and 1836 times the electron.

Proton mass energy equivalent measured

m_{\mathrm{p}} c^{2} = 1.50327761802 \times 10^{-10}\ \text{J}

JThe proton's rest energy, 1.503 277 618 × 10⁻¹⁰ J or 938.272 MeV — the yardstick for accelerator and nuclear energies.

Proton rms charge radius measured

r_{\mathrm{p}} = 8.4075 \times 10^{-16}\ \text{m}

mThe root-mean-square radius of the proton's charge distribution, 8.4075 × 10⁻¹⁶ m — 0.841 femtometres, and recently controversial.

Proton-electron mass ratio measured

m_{\mathrm{p}}/m_{\mathrm{e}} = 1,836.152673426

dimensionlessThe proton outweighs the electron by 1836.152 673 4 — a pure number, known to 17 parts per trillion, that shapes all of chemistry.

Quantum of circulation measured

\frac{h}{2m_{\mathrm{e}}} = 0.00036369475467\ \text{m}^{2}\text{/s}

m²/sh/(2m_e) = 3.636 947 547 × 10⁻⁴ m²/s — the ratio of Planck's constant to mass that atom interferometers measure directly.

Reduced Compton wavelength of the electron measured

\bar{\lambda}_{\mathrm{C}} = 3.8615926744 \times 10^{-13}\ \text{m}

mħ/(m_e c) = 3.861 592 674 × 10⁻¹³ m — the Compton wavelength divided by 2π, and the natural length scale of the Dirac equation.

Reduced Planck constant (Dirac constant) exact

\hbar = 1.054571817 \times 10^{-34}\ \text{J}{\cdot}\text{s}

J·sPlanck's constant divided by 2π, 1.054 571 817 × 10⁻³⁴ J·s — the natural quantum of angular momentum and spin.

Rydberg constant measured

R_{\infty} = 10,973,731.568157\ \text{m}^{-1}

m⁻¹The wavenumber scale of atomic spectra, 10 973 731.568 157 m⁻¹ — the most precisely measured constant in all of physics.

Rydberg energy (hcR∞) measured

hcR_{\infty} = 2.179872361103 \times 10^{-18}\ \text{J}

JThe ionisation energy of ground-state hydrogen, 2.179 872 361 × 10⁻¹⁸ J or 13.605 693 eV — the natural unit of atomic energy.

Second radiation constant exact

c_2 = 0.01438776877\ \text{m}{\cdot}\text{K}

m·Kc₂ = hc/k = 1.438776877e-2 m·K, the constant in the exponent of Planck's law and the basis of radiation thermometry.

Speed of light in vacuum exact

c = 299,792,458\ \text{m/s}

m/sThe invariant speed of light in vacuum, exactly 299 792 458 m/s since 1983 — the definition that now fixes the length of the metre.

Speed of light squared (mass-energy conversion factor) exact

c^{2} = 8.987551787368176 \times 10^{16}\ \text{J/kg}

J/kgThe exchange rate between mass and energy in E = mc²: 8.988 × 10¹⁶ joules locked in every kilogram of rest mass.

Standard acceleration of gravity exact

g_{n} = 9.80665\ \text{m/s}^{2}

m/s²The conventional value of free-fall acceleration, exactly 9.806 65 m/s² — a defined reference, not a measurement of your local g.

Stefan-Boltzmann constant exact

\sigma = 5.670374419 \times 10^{-8}\ \text{W/(m}^{2}{\cdot}\text{K}^{4}\text{)}

W/(m²·K⁴)Radiant emittance of a blackbody per fourth power of temperature: 5.670374419e-8 W/(m²·K⁴), exact in the post-2019 SI.

Tau lepton mass measured

m_{\tau} = 3.16754 \times 10^{-27}\ \text{kg}

kgThe rest mass of the tau lepton, 3.167 54 × 10⁻²⁷ kg or 1776.86 MeV/c² — heavier than a proton, and the shortest-lived lepton.

Thomson cross section measured

\sigma_{\mathrm{e}} = 6.6524587051 \times 10^{-29}\ \text{m}^{2}

m²The low-energy scattering cross section of a photon on a free electron, 6.652 458 705 × 10⁻²⁹ m² — that is 0.665 barn.

Wien displacement law constant (frequency) exact

b' = 5.878925757 \times 10^{10}\ \text{Hz/K}

Hz/KFrequency form of Wien's law, b' = 5.878925757e10 Hz/K: the peak frequency of a blackbody is b'·T, not c divided by the peak wavelength.

Wien displacement law constant (wavelength) exact

b = 0.002897771955\ \text{m}{\cdot}\text{K}

m·KWien's constant b = 2.897771955e-3 m·K: divide by absolute temperature to get the wavelength where a blackbody's spectrum peaks.