About this reference
Every value on this page is drawn from the 2022 CODATA (Committee on Data of the International Science Council) recommended values, published by NIST in Reviews of Modern Physics (2025) and NIST Special Publication 959 (May 2024). CODATA issues a new self-consistent set of recommended constants roughly every four years, based on a least-squares adjustment of all available experimental and theoretical data through the end of the prior year.
Common mistakeNot every entry on this page carries measurement uncertainty. Since the 2019 redefinition of the SI base units, seven constants — including the speed of light, the Planck constant, and the elementary charge — are exact by definition, with zero uncertainty. Constants like the gravitational constant G and the fine-structure constant α remain experimentally measured and carry genuine uncertainty.
Universal Constants
| Constant | Symbol | Value | Unit | Uncertainty |
|---|
| Speed of light in vacuum | c | 299,792,458 | m s⁻¹ | 0 (exact) |
| Planck constant | h | 6.626 070 15 × 10⁻³⁴ | J Hz⁻¹ | 0 (exact) |
| Reduced Planck constant | ħ (h-bar) | 1.054 571 817 × 10⁻³⁴ | J s | 0 (exact, derived) |
| Newtonian constant of gravitation | G | 6.674 30 × 10⁻¹¹ | m³ kg⁻¹ s⁻² | ±0.000 15 × 10⁻¹¹ |
One sentence that captures itG is the least precisely known fundamental constant in physics — its relative uncertainty (~2.2 × 10⁻⁵) is roughly a million times larger than that of the speed of light.
Electromagnetic Constants
| Constant | Symbol | Value | Unit | Uncertainty |
|---|
| Elementary charge | e | 1.602 176 634 × 10⁻¹⁹ | C | 0 (exact) |
| Vacuum electric permittivity | ε₀ | 8.854 187 8188 × 10⁻¹² | F m⁻¹ | ±0.000 000 0014 × 10⁻¹² |
| Vacuum magnetic permeability | μ₀ | 1.256 637 061 27 × 10⁻⁶ | N A⁻² | ±0.000 000 000 20 × 10⁻⁶ |
| Fine-structure constant | α | 7.297 352 5643 × 10⁻³ | dimensionless | ±0.000 000 0011 × 10⁻³ |
Common mistakeε₀ and μ₀ were exact by definition before 2019. After the SI redefinition tied the ampere to a fixed elementary charge instead of a fixed μ₀, both quantities became experimentally measured, with the fine-structure constant α = e²/(4πε₀ħc) now carrying the primary uncertainty that propagates into both.
Atomic & Particle Constants
| Constant | Symbol | Value | Unit | Uncertainty |
|---|
| Electron mass | mₑ | 9.109 383 7139 × 10⁻³¹ | kg | ±0.000 000 0028 × 10⁻³¹ |
| Proton mass | mₔ | 1.672 621 925 95 × 10⁻²⁷ | kg | ±0.000 000 000 52 × 10⁻²⁷ |
| Neutron mass | mₙ | 1.674 927 500 56 × 10⁻²⁷ | kg | ±0.000 000 000 85 × 10⁻²⁷ |
| Proton-electron mass ratio | mₔ/mₑ | 1836.152 673 426 | dimensionless | ±0.000 000 032 |
| Rydberg constant | R∞ | 10,973,731.568 157 | m⁻¹ | ±0.000 012 |
| Bohr radius | a₀ | 5.291 772 105 44 × 10⁻¹¹ | m | ±0.000 000 000 82 × 10⁻¹¹ |
Physicochemical Constants
| Constant | Symbol | Value | Unit | Uncertainty |
|---|
| Boltzmann constant | kₑ | 1.380 649 × 10⁻²³ | J K⁻¹ | 0 (exact) |
| Avogadro constant | Nₐ | 6.022 140 76 × 10²³ | mol⁻¹ | 0 (exact) |
| Molar gas constant | R | 8.314 462 618 | J mol⁻¹ K⁻¹ | 0 (exact, derived) |
| Faraday constant | F | 96,485.332 12 | C mol⁻¹ | 0 (exact, derived) |
| Stefan-Boltzmann constant | σ | 5.670 374 419 × 10⁻⁸ | W m⁻² K⁻⁴ | 0 (exact, derived) |
| Electron volt | eV | 1.602 176 634 × 10⁻¹⁹ | J | 0 (exact, derived) |
Derived Quantum-Electrical Constants
| Constant | Symbol | Value | Unit | Uncertainty |
|---|
| Magnetic flux quantum | Φ₀ | 2.067 833 848 × 10⁻¹⁵ | Wb | 0 (exact, derived) |
| von Klitzing constant | R_K | 25,812.807 45 | Ω | 0 (exact, derived) |
| Josephson constant | K_J | 483,597.848 4 × 10⁹ | Hz V⁻¹ | 0 (exact, derived) |
These three constants underpin the quantum electrical standards: the Josephson effect (voltage standard, K_J = 2e/h) and the quantum Hall effect (resistance standard, R_K = h/e²) allow national metrology institutes to realise the volt and the ohm from fundamental constants alone, without a physical artefact.
Unit systems comparison
| System | Basis | Where it's used |
|---|
| SI | Seven base units (m, kg, s, A, K, mol, cd), all now defined via fixed constants | Universal standard for science and engineering since 2019 |
| CGS (Gaussian) | Centimetre, gram, second; electromagnetic quantities expressed without ε₀, μ₀ explicitly | Some theoretical physics and older astrophysics literature |
| Natural units | Set c = ħ = kₑ = 1 | Particle physics and quantum field theory, where SI units add unnecessary conversion factors |
Sources
- Mohr, P.J., Newell, D.B., Taylor, B.N., Tiesinga, E. (2025). "CODATA Recommended Values of the Fundamental Physical Constants: 2022." Reviews of Modern Physics, 97, 025002.
- NIST (2024). "2022 CODATA Recommended Values of the Fundamental Constants of Physics and Chemistry." NIST SP 959.
- NIST Physical Measurement Laboratory. physics.nist.gov/cuu/Constants — live database, version 9.0.
Last verified: July 2026 · Source: NIST CODATA 2022 · Confidence: high · Next review trigger: next CODATA release