Physics Constants Reference

Fundamental physical constants — CODATA 2018 values and common derived quantities.

Reference Reference Updated Apr 19, 2026

Source: CODATA 2018. Constants marked (exact) were redefined by SI 2019 to have no uncertainty.

Constant	Symbol	Value	Unit
Speed of light in vacuum	c	299 792 458	m/s (exact)
Planck constant	h	6.626 070 15 × 10⁻³⁴	J·s (exact)
Reduced Planck constant	ℏ	1.054 571 817 × 10⁻³⁴	J·s
Gravitational constant	G	6.674 30 × 10⁻¹¹	N·m²/kg²
Vacuum permittivity	ε₀	8.854 187 8128 × 10⁻¹²	F/m
Vacuum permeability	μ₀	1.256 637 062 12 × 10⁻⁶	N/A²

Constant	Symbol	Value	Unit
Elementary charge	e	1.602 176 634 × 10⁻¹⁹	C (exact)
Fine-structure constant	α	7.297 352 5693 × 10⁻³	—
Bohr radius	a₀	5.291 772 109 03 × 10⁻¹¹	m
Classical electron radius	rₑ	2.817 940 3227 × 10⁻¹⁵	m

Constant	Symbol	Value	Unit
Boltzmann constant	k_B	1.380 649 × 10⁻²³	J/K (exact)
Avogadro number	N_A	6.022 140 76 × 10²³	mol⁻¹ (exact)
Gas constant	R	8.314 462 618	J/(mol·K)
Stefan–Boltzmann constant	σ	5.670 374 419 × 10⁻⁸	W/(m²·K⁴)

Last updated: April 19, 2026