Planck constant symbol

An italic lower-case h, denoting the quantum of action.

Also known as: Quantum of action, Italic h

ScientificMathematicalStandard profile

Meanings

h relates a photon's energy to its frequency and sets the scale at which quantum effects matter. Its value has been exact since 2019, when the kilogram was redefined in terms of it — the last SI unit to depend on a physical object.

Common modern usageVerified fact
Denotes the Planck constant, relating a photon's energy to its frequency and setting the scale at which quantum behaviour becomes significant.
Contexts: Mathematics, Laboratory
Officially standardisedVerified fact
Has had an exact value since the 2019 SI redefinition, which used it to define the kilogram and so retired the last physical artefact any unit depended on.
Contexts: Mathematics, Laboratory
Common modern usageVerified fact
Appears far more often in its reduced form ℏ, the constant divided by two pi, which is a different character and a different value.
Contexts: Mathematics, Laboratory

Visual description

A lower-case Latin letter h set in italic type: a vertical ascender with an arch springing from its middle to a second descending stroke.

Symmetry: none
Orientation: upright

Part of the same system as: Speed of light symbol
SI defining constants.
Used alongside: Kilogram symbol
The kilogram is defined in terms of this constant.

Confidence

IdentityHigh confidence: High confidence because the identity is defined by a current official standard, but multiple independent sources agree.
Historical originModerate confidence: Moderate confidence because the historical origin is documented in encyclopedic references, but multiple independent sources agree, but early or supporting evidence is limited, but the regional or cultural match to this specific claim is not precise.
Current useHigh confidence: High confidence because the current use is defined by a current official standard, but multiple independent sources agree.
Visual matchHigh confidence: High confidence because the visual match is documented in encyclopedic references, but multiple independent sources agree.

Sources

The SI defining constants
Bureau International des Poids et Mesures · retrieved 2026-08-01
The seven constants whose exact values define the SI since 2019. Cited for the fact that a constant's value is fixed by definition rather than measured — a claim about the unit system, not about the notation.
CODATA Internationally Recommended Values of the Fundamental Physical Constants
NIST / CODATA Task Group on Fundamental Constants · retrieved 2026-08-01
Authoritative for the recommended value and uncertainty of a physical constant, and for whether that value is exact by definition or measured. It fixes values; it does not decide which letter denotes which quantity, and no record cites it for a symbol's meaning.
Planck constant
Wikipedia · retrieved 2026-08-01
Tertiary reference. Used for what a symbol denotes in ordinary scientific practice and for which fields reuse it, never for a claim that any body assigns the letter its meaning.
Physical constant
Wikipedia · retrieved 2026-08-01
Tertiary reference. Used for what a symbol denotes in ordinary scientific practice and for which fields reuse it, never for a claim that any body assigns the letter its meaning.

Last reviewed 2026-07-31

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Physical constants and quantity symbols

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Meanings

Common modern usageVerified fact

Denotes the Planck constant, relating a photon's energy to its frequency and setting the scale at which quantum behaviour becomes significant.

Contexts: Mathematics, Laboratory

Officially standardisedVerified fact

Has had an exact value since the 2019 SI redefinition, which used it to define the kilogram and so retired the last physical artefact any unit depended on.

Contexts: Mathematics, Laboratory

Common modern usageVerified fact

Appears far more often in its reduced form ℏ, the constant divided by two pi, which is a different character and a different value.

Contexts: Mathematics, Laboratory

Confidence

IdentityHigh confidence: High confidence because the identity is defined by a current official standard, but multiple independent sources agree.
Historical originModerate confidence: Moderate confidence because the historical origin is documented in encyclopedic references, but multiple independent sources agree, but early or supporting evidence is limited, but the regional or cultural match to this specific claim is not precise.
Current useHigh confidence: High confidence because the current use is defined by a current official standard, but multiple independent sources agree.
Visual matchHigh confidence: High confidence because the visual match is documented in encyclopedic references, but multiple independent sources agree.

Sources

The SI defining constants
Bureau International des Poids et Mesures · retrieved 2026-08-01
The seven constants whose exact values define the SI since 2019. Cited for the fact that a constant's value is fixed by definition rather than measured — a claim about the unit system, not about the notation.
CODATA Internationally Recommended Values of the Fundamental Physical Constants
NIST / CODATA Task Group on Fundamental Constants · retrieved 2026-08-01
Authoritative for the recommended value and uncertainty of a physical constant, and for whether that value is exact by definition or measured. It fixes values; it does not decide which letter denotes which quantity, and no record cites it for a symbol's meaning.
Planck constant
Wikipedia · retrieved 2026-08-01
Tertiary reference. Used for what a symbol denotes in ordinary scientific practice and for which fields reuse it, never for a claim that any body assigns the letter its meaning.
Physical constant
Wikipedia · retrieved 2026-08-01
Tertiary reference. Used for what a symbol denotes in ordinary scientific practice and for which fields reuse it, never for a claim that any body assigns the letter its meaning.

Last reviewed 2026-07-31

Meanings

Visual description

Related & commonly confused symbols

Meanings

Visual description

Related & commonly confused symbols