Metre symbol

A lower-case m, the SI unit symbol for the metre.

Also known as: Meter symbol

Engineering & technicalMathematicalScientificStandard profile

Meanings

The lower-case letter m is the SI symbol for the metre, the base unit of length. Its case carries meaning: a capital M is the prefix mega, so Mm and mm differ by a factor of a billion.

Officially standardisedVerified fact
Denotes the metre, the SI base unit of length.
Contexts: Mathematics
Officially standardisedVerified fact
Is written as a lower-case m in upright roman type, with no full stop after it and no plural form: 5 m, never 5 ms or 5 m.
Contexts: Mathematics
Common modern usageVerified fact
Changes meaning entirely with case, because the capital M is the SI prefix mega. A megametre (Mm) is a million metres and a millimetre (mm) is a thousandth of one, so the two differ by a factor of a billion.
Contexts: Mathematics

Visual description

The lower-case Latin letter m: two arches sharing a left-hand downstroke, sitting on the writing line with no ascender or descender.

Symmetry: none
Orientation: upright

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

Part of

Units of measurement and their symbols

Help improve this page

Meanings

The lower-case letter m is the SI symbol for the metre, the base unit of length. Its case carries meaning: a capital M is the prefix mega, so Mm and mm differ by a factor of a billion.

Officially standardisedVerified fact

Denotes the metre, the SI base unit of length.

Contexts: Mathematics

Officially standardisedVerified fact

Is written as a lower-case m in upright roman type, with no full stop after it and no plural form: 5 m, never 5 ms or 5 m.

Contexts: Mathematics

Common modern usageVerified fact

Changes meaning entirely with case, because the capital M is the SI prefix mega. A megametre (Mm) is a million metres and a millimetre (mm) is a thousandth of one, so the two differ by a factor of a billion.

Contexts: Mathematics

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