---
tags:
  - science/physics
  - constants
  - reference
created: 2026-05-16
breadcrumb: "Physics > Constants"
---

# Physical constants

> **Mechanism:** universal values that anchor the equations of physics.

## Universal

- **Speed of light in vacuum** — c = 299,792,458 m/s  (exact, defines the metre)
- **Gravitational constant** — G ≈ 6.674 × 10⁻¹¹ N·m²/kg²
- **Planck constant** — h ≈ 6.626 × 10⁻³⁴ J·s
- **Reduced Planck constant** — ℏ = h/2π ≈ 1.055 × 10⁻³⁴ J·s
- **Fine-structure constant** — α ≈ 1/137.036

## Electromagnetism

- **Elementary charge** — e ≈ 1.602 × 10⁻¹⁹ C
- **Vacuum permittivity** — ε₀ ≈ 8.854 × 10⁻¹² F/m
- **Vacuum permeability** — µ₀ ≈ 1.257 × 10⁻⁶ N/A²
- **Coulomb constant** — k_e = 1/(4πε₀) ≈ 8.988 × 10⁹ N·m²/C²
- **Impedance of free space** — Z₀ = µ₀c ≈ 376.73 Ω

## Thermal & quantum

- **Boltzmann constant** — k_B ≈ 1.381 × 10⁻²³ J/K
- **Avogadro number** — N_A ≈ 6.022 × 10²³ /mol
- **Ideal-gas constant** — R = N_A k_B ≈ 8.314 J/(mol·K)
- **Stefan–Boltzmann constant** — σ ≈ 5.670 × 10⁻⁸ W/(m²·K⁴)
- **Wien displacement constant** — b ≈ 2.898 × 10⁻³ m·K
- **Rydberg constant** — R_∞ ≈ 1.097 × 10⁷ /m
- **Bohr radius** — a₀ ≈ 5.292 × 10⁻¹¹ m

## Particle masses

- **Electron mass** — m_e ≈ 9.109 × 10⁻³¹ kg  ≈ 0.511 MeV/c²
- **Proton mass** — m_p ≈ 1.673 × 10⁻²⁷ kg  ≈ 938.3 MeV/c²
- **Neutron mass** — m_n ≈ 1.675 × 10⁻²⁷ kg  ≈ 939.6 MeV/c²
- **Atomic mass unit** — u ≈ 1.661 × 10⁻²⁷ kg

## Astronomical

- **Astronomical unit** — AU = 1.496 × 10¹¹ m
- **Light-year** — ly ≈ 9.461 × 10¹⁵ m
- **Parsec** — pc ≈ 3.086 × 10¹⁶ m ≈ 3.262 ly
- **Solar mass** — M_☉ ≈ 1.989 × 10³⁰ kg
- **Solar luminosity** — L_☉ ≈ 3.828 × 10²⁶ W
- **Earth mass** — M_⊕ ≈ 5.972 × 10²⁴ kg
- **Earth radius (equatorial)** — R_⊕ ≈ 6.378 × 10⁶ m
- **Standard gravity** — g₀ = 9.806 65 m/s²  (exact, by definition)
- **Hubble constant (current best)** — H₀ ≈ 67–73 km/s/Mpc  (tension between methods)

## Related

- [[famous-physics-equations]]
- [[maxwells-equations-overview]]
- [[thermodynamics]]
- [[relativity]]