Science & Engineering
Physics Constants Reference
Quick-reference table of 20 fundamental physics constants including speed of light, Planck constant, gravitational constant, and more.
Showing 20 of 20 constants
| Name ▲▼ | Symbol ▲▼ | Value ▲▼ | Unit ▲▼ |
|---|---|---|---|
| Speed of Light | c | 2.9979e+8 | m/s |
| Planck Constant | h | 6.6261e-34 | J·s |
| Reduced Planck Constant | ℏ | 1.0546e-34 | J·s |
| Gravitational Constant | G | 6.6743e-11 | N·m²/kg² |
| Elementary Charge | e | 1.6022e-19 | C |
| Electron Mass | mₑ | 9.1094e-31 | kg |
| Proton Mass | mₚ | 1.6726e-27 | kg |
| Neutron Mass | mₙ | 1.6749e-27 | kg |
| Avogadro Constant | Nₐ | 6.0221e+23 | mol⁻¹ |
| Boltzmann Constant | k | 1.3806e-23 | J/K |
| Gas Constant | R | 8.3145e+0 | J/(mol·K) |
| Stefan-Boltzmann Constant | σ | 5.6704e-8 | W/(m²·K⁴) |
| Permittivity of Free Space | ε₀ | 8.8542e-12 | F/m |
| Permeability of Free Space | μ₀ | 1.2566e-6 | H/m |
| Electron Volt | eV | 1.6022e-19 | J |
| Atomic Mass Unit | u | 1.6605e-27 | kg |
| Standard Gravity | g | 9.8066e+0 | m/s² |
| Faraday Constant | F | 9.6485e+4 | C/mol |
| Rydberg Constant | R∞ | 1.0974e+7 | m⁻¹ |
| Fine-Structure Constant | α | 7.2974e-3 | (dimensionless) |
Fundamental physical constants
| Constant | Symbol | Value |
|---|---|---|
| Speed of light (vacuum) | c | 2.998 × 10⁸ m/s |
| Planck's constant | h | 6.626 × 10⁻³⁴ J·s |
| Gravitational constant | G | 6.674 × 10⁻¹¹ N·m²/kg² |
| Elementary charge | e | 1.602 × 10⁻¹⁹ C |
| Avogadro's number | Nₐ | 6.022 × 10²³ mol⁻¹ |
| Boltzmann constant | kʙ | 1.381 × 10⁻²³ J/K |
2019 SI redefinition
On May 20, 2019 (World Metrology Day), the SI system was redefined so that seven fundamental constants are now exact by definition. The kilogram, for example, is now defined by fixing Planck's constant at exactly 6.62607015 × 10⁻³⁴ J·s.
Why these constants matter
Physical constants define the limits and fabric of the universe:
- Speed of light (c): sets the ultimate speed limit for information and matter; defines the relationship E = mc². Also defines the meter via the second.
- Planck's constant (h): the fundamental unit of quantum action. Sets the scale at which quantum effects dominate; the energy of a photon is E = hf.
- Gravitational constant (G): the weakest of the four fundamental forces; appears in Newton's F = Gm₁m₂/r² and Einstein's field equations.
- Boltzmann constant (kB): links temperature to energy at the microscopic level; appears in the ideal gas law as pV = NkBT.
Fine-structure constant
The fine-structure constant α ≈ 1/137.036 is a dimensionless number that characterizes the strength of the electromagnetic interaction. It has no units and would have the same value for any observer in any unit system - it's a pure number describing reality. Richard Feynman called it "a magic number that comes to us with no understanding by man." If its value differed by more than a few percent, stars could not form and complex chemistry (and life) would be impossible.