The Planck constant
h = 6.62607015 × 10⁻³⁴ joule per hertz, exact by definition since the 2019 revision of the SI. It is the constant that ties a photon’s energy to its frequency, and since 2019 it is also what defines the kilogram.
Updated on October 7, 2026
The value
The reduced Planck constant, written ħ and equal to h/2π, is 1.054571817 × 10⁻³⁴ J s, also exact. Physics papers use ħ far more often than h.
h = 0 J Hz⁻¹
- Value
- 6.62607015 × 10⁻³⁴
- Unit
- J Hz⁻¹
- Standard uncertainty
- exact by definition (zero uncertainty)
Source: NIST — CODATA Internationally Recommended Values of the Fundamental Physical Constants (2022)
From a measurement to a definition
Planck introduced h in 1900 to make the maths of thermal radiation work, without believing energy really came in packets. Einstein took the packets seriously in 1905, and quantum mechanics followed. For a century h was measured, most precisely with the Kibble balance and by counting atoms in a silicon sphere.
In 2019 the SI stopped defining the kilogram with a metal cylinder in a vault near Paris and defined it from h instead. The constant was fixed at the best measured value and the kilogram now follows from it. That is why h has no uncertainty today: the uncertainty moved to the experiments that realise the kilogram.
Source: BIPM — Sistema Internacional de Unidades (SI), 9ª edição
How to read this page
How to read the numbers here
Every value on "The Planck constant" comes from the source named beside its section, never from an estimate of our own with no origin.
Where the data comes from
Each section of "The Planck constant" names its official source; the verification date sits at the top of the page.
What the number doesn’t tell you
Atomic weight, molar mass and standard uncertainty are measurement concepts: none of them say anything about a real sample’s purity, physical state or specific isotope.
More about “The Planck constant”
h among the constants this site tracks
Of the 9 physical constants this module tracks, 7 are exact by definition since the 2019 SI revision — and h is one of them. Its value has order of magnitude 10⁻34 in the unit shown on "The Planck constant".
The order of magnitude alone says a lot: a value around 10⁻34 needs scientific notation to avoid a row of zeros, which is exactly why physics favours that notation over loose decimals.
Words on this page
Standard uncertainty
The margin attached to a measured value, usually written in parentheses on the last digits. A constant that is "exact by definition" has zero standard uncertainty; a measured one, like the gravitational constant G, always carries some.
Standard definition by BIPM — Bureau International des Poids et Mesures (SI) · Standard uncertainty
Mole
The SI counting unit for atoms, molecules or other entities: one mole is exactly 6.02214076 × 10²³ of them, by the definition fixed in 2019. It is what ties a mass in grams to a number of particles.
Standard definition by BIPM — Bureau International des Poids et Mesures (SI) · Mole
Atomic weight
The average weight of an element’s atoms, weighted by how abundant each isotope is in nature, in atomic mass units. For 14 elements CIAAW publishes a range instead of a single value, because that abundance varies a little from sample to sample.
Standard definition by CIAAW — Commission on Isotopic Abundances and Atomic Weights · Atomic weight
Atomic number
The number of protons in an atom’s nucleus, written Z. It is this number, not the mass, that defines which element an atom is: every atom with 8 protons is oxygen, whether it has 8 or 18 neutrons.
Standard definition by IUPAC · Atomic number
Astronomical unit
The solar system’s reference distance, fixed at exactly 149,597,870,700 metres since 2012, close to the average Earth–Sun distance. It saves writing every planet’s orbit in kilometres with ten digits.
Standard definition by IAU — International Astronomical Union · Astronomical unit