The speed of light in vacuum is exactly 299,792,458 m/s, and standard gravity g₀ is 9.80665 m/s² by definition.
| Constant | Symbol | Value | Unit | Status |
|---|---|---|---|---|
| Speed of light in vacuum | c | 299,792,458 | m/s | Exact |
| Standard acceleration of gravity | g₀ | 9.80665 | m/s² | Exact |
| Newtonian constant of gravitation | G | 6.67430 × 10⁻¹¹ | m³/(kg·s²) | Measured |
| Planck constant | h | 6.62607015 × 10⁻³⁴ | J·s | Exact |
| Reduced Planck constant | ħ | 1.054571817 × 10⁻³⁴ | J·s | Exact |
| Boltzmann constant | k_B | 1.380649 × 10⁻²³ | J/K | Exact |
| Avogadro constant | N_A | 6.02214076 × 10²³ | mol⁻¹ | Exact |
| Molar gas constant | R | 8.314462618 | J/(mol·K) | Exact |
| Elementary charge | e | 1.602176634 × 10⁻¹⁹ | C | Exact |
| Electron mass | mₑ | 9.1093837139 × 10⁻³¹ | kg | Measured |
| Proton mass | mₚ | 1.67262192595 × 10⁻²⁷ | kg | Measured |
| Neutron mass | mₙ | 1.67492750056 × 10⁻²⁷ | kg | Measured |
| Atomic mass constant | mᵤ | 1.66053906892 × 10⁻²⁷ | kg | Measured |
| Vacuum electric permittivity | ε₀ | 8.8541878188 × 10⁻¹² | F/m | Measured |
| Vacuum magnetic permeability | μ₀ | 1.25663706127 × 10⁻⁶ | N/A² | Measured |
| Faraday constant | F | 96,485.33212 | C/mol | Exact |
| Stefan–Boltzmann constant | σ | 5.670374419 × 10⁻⁸ | W/(m²·K⁴) | Exact |
| Fine-structure constant | α | 7.2973525643 × 10⁻³ | — | Measured |
| Rydberg constant | R∞ | 10,973,731.568157 | m⁻¹ | Measured |
| Bohr radius | a₀ | 5.29177210544 × 10⁻¹¹ | m | Measured |
| Electron volt | eV | 1.602176634 × 10⁻¹⁹ | J | Exact |
| Standard atmosphere | atm | 101,325 | Pa | Exact |
| Molar volume of an ideal gas (273.15 K, 101.325 kPa) | Vₘ | 22.41396954 × 10⁻³ | m³/mol | Exact |
How it is computed
Values are the CODATA 2022 recommended values published by NIST. Since the 2019 SI redefinition, c, h, e, k_B and N_A are exact and so are the constants derived from them; the others are measured and carry an uncertainty in their last digits.
FAQ
Why are some constants exact?+
Since 2019 the SI units are defined by fixing the values of c, h, e, k_B and N_A, so those numbers have no uncertainty.
Is g₀ the gravity where I live?+
No. g₀ = 9.80665 m/s² is a conventional standard value; actual gravity varies from about 9.78 m/s² at the equator to 9.83 m/s² at the poles.