Decibel symbol

The two letters dB, marking a ratio on a logarithmic scale.

Engineering & technicalEveryday & digitalScientificStandard profile

Meanings

dB is the decibel, a logarithmic ratio rather than a unit of a quantity. A decibel figure means nothing until the reference it is measured against is stated, which is why sound levels are properly written dB(A) or dB SPL.

Common modern usageVerified fact
Expresses a ratio between two values of a power-like quantity on a logarithmic scale, one tenth of a bel.
Contexts: Mathematics
Common modern usageVerified fact
Is not an absolute measure. A figure in decibels is meaningless without its reference level, which is why sound pressure is written dB SPL and frequency-weighted noise measurements dB(A).
Contexts: Mathematics
Officially standardisedVerified fact
Is written with a lower-case d for the deci prefix and a capital B for the bel, which is named after Alexander Graham Bell.
Contexts: Mathematics

Visual description

A lower-case Latin d — a round bowl with a vertical ascender on its right — followed by a capital B, a vertical stem carrying two stacked bowls on its right.

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

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.
Decibel
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.

Last reviewed 2026-08-01

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Units of measurement and their symbols

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Meanings

Common modern usageVerified fact

Expresses a ratio between two values of a power-like quantity on a logarithmic scale, one tenth of a bel.

Contexts: Mathematics

Common modern usageVerified fact

Is not an absolute measure. A figure in decibels is meaningless without its reference level, which is why sound pressure is written dB SPL and frequency-weighted noise measurements dB(A).

Contexts: Mathematics

Officially standardisedVerified fact

Is written with a lower-case d for the deci prefix and a capital B for the bel, which is named after Alexander Graham Bell.

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

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.
Decibel
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.

Last reviewed 2026-08-01