Kilo prefix symbol

A lower-case k, the SI prefix for a factor of one thousand.

ScientificMathematicalEveryday & digitalStandard profile

Meanings

k is the SI prefix kilo, a factor of one thousand. It is lower case by definition, which is what distinguishes kilo from the capital K of the kelvin.

Officially standardisedVerified fact
Denotes the SI prefix kilo, a factor of one thousand.
Contexts: Mathematics
Officially standardisedVerified fact
Is lower case, unlike the prefixes for a million and above. The capital K is the unit symbol for the kelvin, so kW and KW are not the same thing.
Contexts: Mathematics
Common modern usageVerified fact
Is frequently written as a capital in computing contexts, where KB has historically also meant 1024 bytes rather than 1000 — a usage the SI does not recognise and for which the separate binary prefix kibi (Ki) exists.
Contexts: Software

Visual description

The lower-case Latin letter k: a vertical stroke with an ascender, met at mid-height by two short diagonals, one rising and one falling.

Symmetry: none
Orientation: upright

Commonly confused with: Boltzmann constant symbol
A third claimant to the letter k, alongside the kelvin.
Commonly confused with: Kelvin symbol
Lower-case k is the prefix kilo; capital K is the kelvin. The case is the whole distinction.

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.
Metric prefix
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

k is the SI prefix kilo, a factor of one thousand. It is lower case by definition, which is what distinguishes kilo from the capital K of the kelvin.

Officially standardisedVerified fact

Denotes the SI prefix kilo, a factor of one thousand.

Contexts: Mathematics

Officially standardisedVerified fact

Is lower case, unlike the prefixes for a million and above. The capital K is the unit symbol for the kelvin, so kW and KW are not the same thing.

Contexts: Mathematics

Common modern usageVerified fact

Is frequently written as a capital in computing contexts, where KB has historically also meant 1024 bytes rather than 1000 — a usage the SI does not recognise and for which the separate binary prefix kibi (Ki) exists.

Contexts: Software

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.
Metric prefix
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

Meanings

Visual description

Related & commonly confused symbols

Meanings

Visual description

Related & commonly confused symbols