Fundamental physical constants are the fixed numbers that appear throughout the laws of physics, from the speed of light in relativity to the Planck constant in quantum mechanics. This reference collects more than thirty of them with their CODATA 2018 values, units, and an exact-or-measured flag.
How it works
Filter the table by name, symbol, or unit to find any constant. The exact column distinguishes two kinds of constant:
- Exact constants were assigned defined values when the SI was redefined in 2019. The speed of light, Planck constant, elementary charge, Boltzmann constant, and Avogadro number now have zero uncertainty — they define the base units rather than being measured against them.
- Measured constants such as the gravitational constant
G, the electron mass, and the fine-structure constant are determined by experiment and carry an uncertainty in their final digits.
Because the defining constants are exact, many derived quantities that depend
only on them (for example the Faraday constant F = NA · e) are also exact.
The 2019 SI redefinition — why so many constants became exact
Before 2019, a kilogram was defined by a physical lump of platinum-iridium alloy kept in a vault near Paris. The problem: you had to compare every mass measurement against that single object, and the object itself drifted slightly. The 2019 redefinition eliminated all physical artifacts by anchoring each SI base unit to a fixed numerical value of a named constant:
| Constant | Fixed value | Defines |
|---|---|---|
Speed of light c | 299 792 458 m/s | metre |
Planck constant h | 6.626 070 15 × 10⁻³⁴ J·s | kilogram |
Elementary charge e | 1.602 176 634 × 10⁻¹⁹ C | ampere |
Boltzmann constant k | 1.380 649 × 10⁻²³ J/K | kelvin |
Avogadro number NA | 6.022 140 76 × 10²³ mol⁻¹ | mole |
Once these five constants were fixed by definition, the units they define became
exact too — no physical artifact needed. A lab in Tokyo and one in São Paulo now
realise the kilogram independently, both tracing back to the same fixed value of h.
Notes and examples
The contrast in precision is striking. The Planck constant is exact to its full
written value, while the gravitational constant G is known to only about four
significant figures because gravity is too weak to measure cleanly in the lab.
Experiments to pin down G more precisely are ongoing — shielding gravitational
effects from electrical and mechanical noise is fiendishly difficult.
The fine-structure constant is dimensionless — roughly 1/137 — so it carries no units and is identical in every measurement system. It sets the strength of the electromagnetic interaction between charged particles. Because it is dimensionless, any future physics that changed its value would alter observable atomic spectra in a way experiments would catch — physicists use it as a canary for new physics.
For frontier-precision work, always cross-check against the most recent CODATA release, since measured values shift slightly with each adjustment cycle as experimental techniques improve.
When you might use this reference
- Writing physics code: paste exact values of
c,h, orerather than hardcoding approximate numbers and wondering later why your simulator drifts. - Unit conversion: spot which constants link different unit systems — for example the Stefan-Boltzmann constant connects temperature to radiated power.
- Homework and exam prep: confirm symbol, unit, and full numerical value for a constant you are working with before writing it in a derivation.
- Dimensional analysis: checking units on a derived expression is easier when you can quickly look up the SI units of each constant involved.