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Dalton ○◂｜Definition｜1st｜20260604233205-00-⌔

Dalton (unit) - Wikipedia

Dalton (unit)

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The dalton (symbol: Da), or unified atomic mass unit (symbol: u), is a unit of mass defined as ⁠ 1/12 ⁠ of the mass of an unbound neutral atom of carbon-12 in its nuclear and electronic ground state and at rest.123 The word “unified” emphasizes that the definition was jointly accepted by physics (IUPAP) and chemistry (IUPAC) organizations.4 The atomic mass constant, denoted m, is an atomic-scale reference mass, defined identically, but it is not a unit of mass. Expressed in terms of m ( C), the atomic mass of carbon-12: m = m ( C)/12 = 1 Da.5

The value serves as a conversion factor of mass from daltons to kilograms, which can easily be converted to grams and other metric units of mass. The 2019 revision of the SI redefined the kilogram by fixing the value of the Planck constant (h), improving the precision of the atomic mass constant expressed in SI units by anchoring it to fixed physical constants. Although the dalton remains defined via carbon-12, the revision enhances traceability and accuracy in atomic mass measurements.

The dalton’s numerical value in terms of the fixed- h kilogram is an experimentally determined quantity that, along with its inherent uncertainty, is updated periodically. The 2022 CODATA recommended value of the atomic mass constant expressed in the SI base unit kilogram is:

m = 1.660 539 068 92 (52) × 10 kg.6

The previous 2018 CODATA value was used in the traditional definition of the Avogadro number to obtain the value g/Da = 6.022 140 7620 (18) × 10, which was then rounded to 9 significant figures (thus guaranteeing continuity, to the precision indicated) and used to define it at exactly that value for the 2019 redefinition of the mole.

The mole is a unit of amount of substance used in chemistry and physics, such that the mass of one mole of a substance expressed in grams (i.e., the molar mass in g/mol or kg/kmol) is numerically equal to the average mass of an elementary entity of the substance (atom, molecule, or formula unit) expressed in daltons. For example, the average mass of one molecule of water is about 18.0153 Da, and the mass of one mole of water is about 18.0153 g. A protein whose molecule has an average mass of 64 kDa would have a molar mass of 64 kg/mol. However, while this equality can be assumed for practical purposes, it is only approximate, because of the 2019 redefinition of the mole.71

Printed 2026-06-28.

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Footnotes

  1. Bureau International des Poids et Mesures (2019): The International System of Units (SI), 9th edition, English version, page 146. Available at the BIPM website. ↩ ↩2

  2. IUPAC, Compendium of Chemical Terminology, 5th ed. (the “Gold Book”) (2025). Online version: (2006–) “dalton”. doi: 10.1351/goldbook.D01514 ↩

  3. International Standard ISO 80000-10:2019 – Quantities and units – Part 10: Atomic and nuclear physics, International Organization for Standardization, 2019 ↩

  4. Holden, Norman E. (2004). “Atomic Weights and the International Committee—A Historical Review”. Chemistry International. 26 (1): 4–7. ↩

  5. Taylor, Barry N. (2009). “Molar mass and related quantities in the new SI”. Metrologia. 46 (3): L16–L19. doi:10.1088/0026-1394/46/3/L01. S2CID 115540416. ↩

  6. “2022 CODATA Value: atomic mass constant”. The NIST Reference on Constants, Units, and Uncertainty. NIST. May 2024. Retrieved 2024-05-18. ↩

  7. Berg, Jeremy M.; Tymoczko, John L.; Stryer, Lubert (2007). “2”. Biochemistry (6th ed.). Macmillan. p. 35. ISBN 978-0-7167-8724-2. ↩

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