Kelvin symbol

A capital K, the SI unit symbol for the kelvin.

ScientificMathematicalEngineering & technicalStandard profile

Meanings

The capital letter K is the SI symbol for the kelvin, the base unit of thermodynamic temperature. Unlike Celsius and Fahrenheit it takes no degree sign: 300 K, never 300 °K.

Officially standardisedVerified fact
Denotes the kelvin, the SI base unit of thermodynamic temperature.
Contexts: Mathematics
Officially standardisedVerified fact
Takes no degree sign. The 13th General Conference on Weights and Measures replaced 'degree Kelvin' with 'kelvin' in 1967, so the correct written form is 300 K.
Contexts: Mathematics
Common modern usageVerified fact
Appears on lighting packaging as a colour temperature — 2700 K for a warm domestic bulb, 6500 K for daylight — which for most readers is the only place they meet it.
Contexts: Mathematics

Visual description

The capital Latin letter K: a vertical stem with two diagonal strokes meeting it at the middle, one rising to the upper right and one falling to the lower right.

Symmetry: none
Orientation: upright

Commonly confused with: Kilo prefix symbol
Capital K is the kelvin; lower-case k is the prefix kilo. The case is the whole distinction.
Commonly confused with: Degree Celsius symbol
Both measure temperature and one kelvin equals one degree Celsius in size, but the scales start in different places and only Celsius takes a degree sign.

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

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.
SI derived unit
Wikipedia · retrieved 2026-07-31
Tertiary reference. Used for origin narrative and for units the SI does not define, never for an official-definition claim.
Guide for the Use of the International System of Units (SI), NIST Special Publication 811
National Institute of Standards and Technology · retrieved 2026-07-31
A national metrology institute's guidance on applying the SI, including symbol formatting. Authoritative for usage rules in the United States; it interprets the SI rather than defining it.
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

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Units of measurement and their symbols

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Meanings

The capital letter K is the SI symbol for the kelvin, the base unit of thermodynamic temperature. Unlike Celsius and Fahrenheit it takes no degree sign: 300 K, never 300 °K.

Officially standardisedVerified fact

Denotes the kelvin, the SI base unit of thermodynamic temperature.

Contexts: Mathematics

Officially standardisedVerified fact

Takes no degree sign. The 13th General Conference on Weights and Measures replaced 'degree Kelvin' with 'kelvin' in 1967, so the correct written form is 300 K.

Contexts: Mathematics

Common modern usageVerified fact

Appears on lighting packaging as a colour temperature — 2700 K for a warm domestic bulb, 6500 K for daylight — which for most readers is the only place they meet it.

Contexts: Mathematics

Related & commonly confused symbols

Commonly confused with: Kilo prefix symbol
Capital K is the kelvin; lower-case k is the prefix kilo. The case is the whole distinction.
Commonly confused with: Degree Celsius symbol
Both measure temperature and one kelvin equals one degree Celsius in size, but the scales start in different places and only Celsius takes a degree sign.

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

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.
SI derived unit
Wikipedia · retrieved 2026-07-31
Tertiary reference. Used for origin narrative and for units the SI does not define, never for an official-definition claim.
Guide for the Use of the International System of Units (SI), NIST Special Publication 811
National Institute of Standards and Technology · retrieved 2026-07-31
A national metrology institute's guidance on applying the SI, including symbol formatting. Authoritative for usage rules in the United States; it interprets the SI rather than defining it.
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