Gram prefixes and the kilogram exception
Mass prefixes attach to gram even though kilogram is the SI base unit.
Key knowledge
The kilogram is the SI base unit of mass and already contains the historical prefix kilo. Decimal multiples and submultiples of mass are named by attaching one prefix to gram, not by stacking a new prefix on kilogram: 1 mg=10⁻⁶ kg and 1 μg=10⁻⁹ kg.
Values and equations
1 g=10⁻³ kg; 1 mg=10⁻³ g=10⁻⁶ kg; 1 μg=10⁻⁶ g=10⁻⁹ kg
- g,kg,mg,μg
- Gram, kilogram, milligram and microgram unit symbols mass units
Conditions and scope
- Use prescribed symbol case: mg is milligram and Mg is megagram.
- The gram is 10⁻³ kg; this convention does not make gram the SI base unit.
Sources · 3
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
- The International System of Units, 9th edition, version 4.01 (2026)Bureau International des Poids et Mesures (BIPM)Reference guide
- Metric (SI) PrefixesNational Institute of Standards and TechnologyReference guide
- O Sistema Internacional de Unidades (SI) · 2ª edição luso-brasileiraInmetro and Instituto Português da QualidadeReference guide · PDF
Evidence and review
Prefixed mass units are formed from gram; exact decimal factors relate kilogram, gram and microgram.
2026-10-10 · Independent Chemistry, Biology and Earth/Cosmos editor (AI editorial review). Source checks and independent AI editorial review of compact reference claims, equations, units, conditions and conceptual mappings. This is not human specialist certification, laboratory accreditation, whole-book verification, or a claim that all possible foundational knowledge is present.
Foundational review
2026-10-10 · Independent AI editorial inspection of this new compact reference entry, its equations, applicability, sources and knowledge connections.
Sources from other places
Agreement within this scope
Mass multiples use prefixes attached to gram, despite kilogram being the base unit.
Case-sensitive prefixes distinguish mg, µg and Mg.
Inmetro and Instituto Português da Qualidade
- O Sistema Internacional de Unidades (SI) · 2ª edição luso-brasileira Brazil and PortugalPortugueseShared standard
3 paragraph after Table7, printed p20 / PDF p36
- O Sistema Internacional de Unidades (SI) · 2ª edição luso-brasileira
Entry details
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- foundation-measurement-mass-prefixes
- Revision
- foundations-audit.1
- Structured reference record
- Foundational core completion