The speed of light, Planck constant, elementary charge and the rest — with their SI units and a clear mark for which are exact by definition and which carry an uncertainty.
Constants
18
Exact
11
Measured
7
c
299 792 458 m/s
The speed of light is fixed exactly at 299,792,458 m/s, as are the Planck constant and five others. 7 of the 18 constants here are measured and carry an uncertainty.
18 of 18 rows
| Symbol | Name | Value | Unit | Relative uncertainty | Group |
|---|---|---|---|---|---|
| c | Speed of light in vacuum | 2.99792458 × 10⁸ | m/s | exact | Exact (defines the SI) |
| h | Planck constant | 6.62607015 × 10⁻³⁴ | J s | exact | Exact (defines the SI) |
| e | Elementary charge | 1.602176634 × 10⁻¹⁹ | C | exact | Exact (defines the SI) |
| k | Boltzmann constant | 1.380649 × 10⁻²³ | J/K | exact | Exact (defines the SI) |
| N_A | Avogadro constant | 6.02214076 × 10²³ | 1/mol | exact | Exact (defines the SI) |
| Δν(Cs) | Caesium hyperfine transition frequency | 9.19263177 × 10⁹ | Hz | exact | Exact (defines the SI) |
| K_cd | Luminous efficacy at 540 THz | 6.83 × 10² | lm/W | exact | Exact (defines the SI) |
| R | Molar gas constant | 8.31446261815324 | J/(mol K) | exact | Exact (derived) |
| F | Faraday constant | 9.648533212331 × 10⁴ | C/mol | exact | Exact (derived) |
| σ | Stefan–Boltzmann constant | 5.670374419 × 10⁻⁸ | W/(m² K⁴) | exact | Exact (derived) |
| g_n | Standard gravity | 9.80665 | m/s² | exact | Exact (derived) |
| G | Newtonian constant of gravitation | 6.67430 × 10⁻¹¹ | m³/(kg s²) | 2.2 × 10⁻⁵ | Measured |
| 1/α | Inverse fine-structure constant | 1.37035999177 × 10² | — | 1.6 × 10⁻¹⁰ | Measured |
| m_e | Electron mass | 9.1093837139 × 10⁻³¹ | kg | 3.1 × 10⁻¹⁰ | Measured |
| m_p | Proton mass | 1.67262192595 × 10⁻²⁷ | kg | 3.1 × 10⁻¹⁰ | Measured |
| R∞ | Rydberg constant | 1.0973731568157 × 10⁷ | 1/m | 1.1 × 10⁻¹² | Measured |
| μ₀ | Vacuum magnetic permeability | 1.25663706127 × 10⁻⁶ | N/A² | 1.6 × 10⁻¹⁰ | Measured |
| ε₀ | Vacuum electric permittivity | 8.8541878188 × 10⁻¹² | F/m | 1.6 × 10⁻¹⁰ | Measured |
The “Relative uncertainty” column says how far the true value can deviate at most. Where it reads “exact”, the value is fixed by definition and will never change.
Since the 2019 SI revision, seven constants are definitions. The metre, kilogram, second, ampere, kelvin, mole and candela derive from them — not the other way round.
R, F and σ are exact too, because they follow entirely from the defining constants. Their long digit strings are results of a calculation, not measurements.
Hover the chart to read the precise values.
From the Planck constant at 10⁻³⁴ to the Avogadro constant at 10²³ lie 57 powers of ten. That is why physical constants are essentially always written in scientific notation.
Hover the chart to read the precise values.
Higher is better: the bar shows how many digits are secure. The gravitational constant G stands out — it is by far the least precisely known constant of nature.
Hover the chart to read the precise values.
The constants cluster at the extremes: very small values from quantum physics, very large ones from chemistry and wave physics.
Since the 2019 redefinition of the SI, seven constants have exact defining values; those are reproduced from the SI Brochure. The measured constants carry the CODATA recommended value together with its standard uncertainty, and both are shown rather than silently rounded away.
Last checked against the sources on August 6, 2026.
Because since 1983 it is no longer measured but fixed: c is exactly 299,792,458 m/s by definition. The metre has since been defined as the distance light travels in 1/299,792,458 of a second.
It means the numeric value is fixed by international agreement and carries no measurement uncertainty. 11 of the 18 constants in this table are exact in that sense.
Gravity is many orders of magnitude weaker than the other interactions and cannot be shielded. Every measurement therefore fights disturbances — G is still known to only about five digits.
All seven base units are now defined through fixed constants of nature. That retired the prototype kilogram, the last physical artefact standard — the kilogram now hangs on the Planck constant.
Because R is exactly the Boltzmann constant times the Avogadro constant. Both are fixed, so their product is exact too — the 15 digits are a multiplication, not a measurement.
No. 9.80665 m/s² is standard gravity, fixed by convention. Actual gravitational acceleration varies with latitude and altitude between roughly 9.78 and 9.83 m/s².
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