Lumen symbol

The two letters lm, the SI unit symbol for the lumen.

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Meanings

lm is the SI symbol for the lumen, the derived unit of luminous flux. Lamp packaging states lumens because they measure light output, where watts only measured power drawn.

Officially standardisedVerified fact
Denotes the lumen, the SI derived unit of luminous flux.
Contexts: Mathematics
Common modern usageVerified fact
Replaced the watt as the headline figure on lamp packaging once efficient lamps broke the old relationship between power drawn and light produced — a 10 W LED and a 60 W incandescent bulb can both give about 800 lm.
Contexts: Electrical equipment

Visual description

Two lower-case Latin letters: an l drawn as a single vertical stroke with an ascender, followed by an m of two arches sharing a left-hand downstroke.

Symmetry: none
Orientation: upright

Used alongside: Lux symbol
Total light emitted, and how much of it lands on a surface.

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

lm is the SI symbol for the lumen, the derived unit of luminous flux. Lamp packaging states lumens because they measure light output, where watts only measured power drawn.

Officially standardisedVerified fact

Denotes the lumen, the SI derived unit of luminous flux.

Contexts: Mathematics

Common modern usageVerified fact

Replaced the watt as the headline figure on lamp packaging once efficient lamps broke the old relationship between power drawn and light produced — a 10 W LED and a 60 W incandescent bulb can both give about 800 lm.

Contexts: Electrical equipment

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

Meanings

Visual description

Related & commonly confused symbols

Meanings

Visual description

Related & commonly confused symbols