Science & Engineering

Physics Constants Reference

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

Updated Apr 19, 2026 2 min read

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

Universal

ConstantSymbolValueUnit
Speed of light in vacuumc299 792 458m/s (exact)
Planck constanth6.626 070 15 × 10⁻³⁴J·s (exact)
Reduced Planck constantℏ1.054 571 817 × 10⁻³⁴J·s
Gravitational constantG6.674 30 × 10⁻¹¹N·m²/kg²
Vacuum permittivityε₀8.854 187 8128 × 10⁻¹²F/m
Vacuum permeabilityμ₀1.256 637 062 12 × 10⁻⁶N/A²

Electromagnetic

ConstantSymbolValueUnit
Elementary chargee1.602 176 634 × 10⁻¹⁹C (exact)
Fine-structure constantα7.297 352 5693 × 10⁻³—
Bohr radiusa₀5.291 772 109 03 × 10⁻¹¹m
Classical electron radiusrₑ2.817 940 3227 × 10⁻¹⁵m

Thermodynamic

ConstantSymbolValueUnit
Boltzmann constantk_B1.380 649 × 10⁻²³J/K (exact)
Avogadro numberN_A6.022 140 76 × 10²³mol⁻¹ (exact)
Gas constantR8.314 462 618J/(mol·K)
Stefan–Boltzmann constantσ5.670 374 419 × 10⁻⁸W/(m²·K⁴)

Particle masses

ParticleValue (kg)Value (MeV/c²)
Electron9.109 383 7015 × 10⁻³¹0.510 998
Proton1.672 621 923 69 × 10⁻²⁷938.272
Neutron1.674 927 498 04 × 10⁻²⁷939.565
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