Ampere symbol

A capital A, the SI unit symbol for the ampere.

Also known as: Amp

ScientificEngineering & technicalStandard profile

Meanings

The capital letter A is the SI symbol for the ampere, the base unit of electric current. It is capitalised because the unit honours a person — the rule that governs every eponymous unit symbol.

Officially standardisedVerified fact
Denotes the ampere, the SI base unit of electric current.
Contexts: Mathematics
Officially standardisedVerified fact
Takes a capital first letter under the SI rule for units named after a person, while the unit name itself stays lower case: 13 A of current, one ampere.
Contexts: Mathematics
Common modern usageVerified fact
Is printed on plugs, fuses and circuit breakers to state the current a device draws or a protective device will carry, which is where most readers meet it.
Contexts: Electrical equipment

Visual description

The capital Latin letter A: two diagonal strokes meeting at a point above, joined partway down by a horizontal crossbar.

Symmetry: vertical
Orientation: upright

Used alongside: Ohm sign
Current, potential difference and resistance are the three quantities Ohm's law relates.
Used alongside: Volt symbol
Current and potential difference are read together on any electrical rating plate.

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 A is the SI symbol for the ampere, the base unit of electric current. It is capitalised because the unit honours a person — the rule that governs every eponymous unit symbol.

Officially standardisedVerified fact

Denotes the ampere, the SI base unit of electric current.

Contexts: Mathematics

Officially standardisedVerified fact

Takes a capital first letter under the SI rule for units named after a person, while the unit name itself stays lower case: 13 A of current, one ampere.

Contexts: Mathematics

Common modern usageVerified fact

Is printed on plugs, fuses and circuit breakers to state the current a device draws or a protective device will carry, which is where most readers meet it.

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