Title: unified atomic mass unit
Long Title: IUPAC Gold Book - unified atomic mass unit
DOI: 10.1351/goldbook.U06554
Status: current
Index: unit

Definition
Non-SI unit of mass (equal to the atomic mass constant), defined as one twelfth of the mass of a carbon-12 atom in its ground state and used to express masses of atomic particles, \(\rm{u} \approx \pu{1.6605402(10)E-27 kg}\).

Related Terms
- atomic mass constant: https://goldbook.iupac.org//terms/view/A00497
- ground state: https://goldbook.iupac.org//terms/view/G02704

Sources
- Green Book, 2nd ed., p. 75 (https://goldbook.iupac.org/files/pdf/green_book_2ed.pdf)
- PAC, 1996, 68, 957. 'Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)' on page 997 (https://doi.org/10.1351/pac199668040957)

Other Outputs
- html: https://goldbook.iupac.org/terms/view/U06554/html
- json: https://goldbook.iupac.org/terms/view/U06554/json
- xml: https://goldbook.iupac.org/terms/view/U06554/xml

Citation: Citation: 'unified atomic mass unit' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. 10.1351/goldbook.U06554
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Accessed: 2026-04-20T14:10:46+00:00