Watt symbol

A capital W, the SI unit symbol for the watt.

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Meanings

W is the SI symbol for the watt, the derived unit of power — one joule per second. It is the number on every appliance rating plate and light bulb.

Officially standardisedVerified fact
Denotes the watt, the SI derived unit of power.
Contexts: Mathematics
Common modern usageVerified fact
Is defined as one joule per second, so it measures the rate at which energy is transferred rather than a quantity of energy.
Contexts: Mathematics
Common modern usageVerified fact
Is printed on appliance rating plates and lamp packaging to state power consumption, and the kilowatt hour on an electricity bill is a quantity of energy rather than a power, which is a common source of confusion.
Contexts: Electrical equipment

Visual description

The capital Latin letter W: four diagonal strokes forming two adjacent V shapes that share a middle peak.

Symmetry: vertical
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.
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

W is the SI symbol for the watt, the derived unit of power — one joule per second. It is the number on every appliance rating plate and light bulb.

Officially standardisedVerified fact

Denotes the watt, the SI derived unit of power.

Contexts: Mathematics

Common modern usageVerified fact

Is defined as one joule per second, so it measures the rate at which energy is transferred rather than a quantity of energy.

Contexts: Mathematics

Common modern usageVerified fact

Is printed on appliance rating plates and lamp packaging to state power consumption, and the kilowatt hour on an electricity bill is a quantity of energy rather than a power, which is a common source of confusion.

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