Candela symbol

The two letters cd, the SI unit symbol for the candela.

ScientificMathematicalEngineering & technicalStandard profile

Meanings

cd is the SI symbol for the candela, the base unit of luminous intensity — the only base unit whose definition is weighted to the sensitivity of the human eye.

Officially standardisedVerified fact
Denotes the candela, the SI base unit of luminous intensity.
Contexts: Mathematics
Common modern usageVerified fact
Measures perceived brightness rather than raw radiant power: its definition fixes the luminous efficacy of radiation at 540 × 10¹² hertz, close to the eye's peak daylight sensitivity, so the same wattage of red or blue light yields fewer candelas.
Contexts: Mathematics

Visual description

Two lower-case Latin letters side by side: a c forming an open curve, followed by a d with a round bowl on the left and a vertical ascender on its right.

Symmetry: none
Orientation: upright

Used alongside: Lumen symbol
Luminous intensity in one direction, and total light output in every direction.

Confidence

IdentityHigh confidence: High confidence because the identity is defined by a current official standard.
Historical originModerate confidence: Moderate confidence because the historical origin is documented in encyclopedic references, 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.
Visual matchHigh confidence: High confidence because the visual match is documented in encyclopedic references.

Sources

SI base units
Bureau International des Poids et Mesures · retrieved 2026-07-31
The seven base units and their symbols as published by the defining body. A summary page: the SI Brochure remains the full instrument.
The International System of Units (SI), 9th edition
Bureau International des Poids et Mesures · retrieved 2026-07-31
The defining document for SI unit names and symbols, including the capitalisation and spacing rules. It defines units; it says nothing about how a symbol is encoded in a character set, and nothing about non-SI units except where it lists those accepted for use with the SI.

Last reviewed 2026-08-01

Part of

Units of measurement and their symbols

Help improve this page

Meanings

cd is the SI symbol for the candela, the base unit of luminous intensity — the only base unit whose definition is weighted to the sensitivity of the human eye.

Officially standardisedVerified fact

Denotes the candela, the SI base unit of luminous intensity.

Contexts: Mathematics

Common modern usageVerified fact

Measures perceived brightness rather than raw radiant power: its definition fixes the luminous efficacy of radiation at 540 × 10¹² hertz, close to the eye's peak daylight sensitivity, so the same wattage of red or blue light yields fewer candelas.

Contexts: Mathematics

Confidence

IdentityHigh confidence: High confidence because the identity is defined by a current official standard.
Historical originModerate confidence: Moderate confidence because the historical origin is documented in encyclopedic references, 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.
Visual matchHigh confidence: High confidence because the visual match is documented in encyclopedic references.

Sources

SI base units
Bureau International des Poids et Mesures · retrieved 2026-07-31
The seven base units and their symbols as published by the defining body. A summary page: the SI Brochure remains the full instrument.
The International System of Units (SI), 9th edition
Bureau International des Poids et Mesures · retrieved 2026-07-31
The defining document for SI unit names and symbols, including the capitalisation and spacing rules. It defines units; it says nothing about how a symbol is encoded in a character set, and nothing about non-SI units except where it lists those accepted for use with the SI.

Last reviewed 2026-08-01

Meanings

Visual description

Related & commonly confused symbols

Meanings

Visual description

Related & commonly confused symbols