CODATA is the Committee on Data for Science and Technology, which periodically publishes internationally recommended values of the fundamental physical constants. The values here follow the CODATA 2018 adjustment, the basis of the 2019 SI redefinition.

Which constants are exact?

Constants like the speed of light, Planck constant, elementary charge, Boltzmann constant, and Avogadro number are now exact by definition, with zero uncertainty. They were fixed to defined values when the SI was redefined in 2019, which in turn defines the base units.

Why does the gravitational constant have more uncertainty?

Big G is notoriously hard to measure because gravity is extremely weak and cannot be shielded. It is known to only about four significant figures, far less precisely than electromagnetic or atomic constants, and its value is still actively refined.

What is the fine-structure constant?

The fine-structure constant, about 1/137, is a dimensionless number that sets the strength of the electromagnetic interaction. Because it has no units, it is the same in every system of measurement and is considered one of the most fundamental numbers in physics.

Are these values reliable for calculations?

Yes. These are the internationally accepted reference values used in physics and engineering. For the highest-precision work consult the latest CODATA release directly, since measured constants are refined as experimental techniques improve.

Physical Constants Reference

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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.

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. The fine-structure constant is dimensionless — roughly 1/137 — so it carries no units and is identical in every measurement system, which is why physicists treat it as an especially deep quantity. For frontier-precision work, always cross-check against the most recent CODATA release, since measured values shift slightly with each adjustment.