A constant with dimensions
The Planck constant, written h, is a fundamental constant of quantum physics and one of the seven defining constants of the International System of Units. Its SI value is exactly 6.62607015 × 10⁻³⁴ joule seconds. The unit J s is equivalent to kg m² s⁻¹: it has the dimensions of action, not energy alone. The word “exactly” is significant. In the modern SI this number is fixed by definition, rather than being a rounded experimental estimate with an uncertainty attached.
Photon energy
For a photon of frequency ν, energy is given by E = hν. Multiplying h in joule seconds by frequency in hertz produces an energy in joules. Consequently, doubling a photon's frequency doubles its energy; increasing the number of photons at a fixed frequency changes the total energy without changing the energy of each photon. The relation does not say that h itself is a universal minimum packet of energy, since h has different dimensions and photon energy depends on frequency. Planck introduced the constant in his work on thermal radiation. Its use in photon physics and in present-day metrology connects that historical problem with a much broader range of measurements.
The reduced constant
The reduced Planck constant is ħ = h/(2π), pronounced “h-bar.” Its value is approximately 1.054571817 × 10⁻³⁴ J s; the displayed decimal is truncated, while the defining relation is exact. If angular frequency is ω = 2πν, the same photon-energy relation can be written E = ħω. These are not different physical predictions. They express the same relationship using frequency in cycles per second or angular frequency in radians per second. Substituting ħ for h without also adjusting the frequency convention produces an erroneous factor of 2π. Explicit units help prevent that mistake.
From a measured constant to a mass definition
Since 20 May 2019, the kilogram has no longer been defined by the mass of the international prototype artifact. Fixing h, together with the definitions of the metre and second, establishes the unit of mass through the dimensional relation J s = kg m² s⁻¹. Previously, the artifact's mass was exact by definition and experiments determined h; the revised system fixes h and measures the artifact's mass instead. This reverses the metrological role of the two quantities without changing the underlying physics.
Exact definition, imperfect realization
A fixed constant does not make a weighing experiment free of uncertainty. Instruments must realize the unit, and their observations, calibration and environmental corrections remain imperfect. Kibble balances provide one route by linking mechanical and electrical measurements; their historical use to measure h could be reversed after its numerical value was fixed. The distinction is between a definition and its practical realization. The constant supplies a common reference, while an actual result still requires an uncertainty statement.