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❪𝛿₂.❫ Units of Mass ○｜Definition｜1st｜20260604233205-00-⌔

Mass - Wikipedia#Units_of_mass

Units of mass

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The International System of Units (SI) unit of mass is the kilogram (kg). It is defined in terms of three constants, Planck constant, the speed of light, and the definition of the second (itself defined by fixing the caesium hyperfine frequency). The specific values of these constants were selected to closely approximate the older definition of the kilogram, the platinum–iridium International Prototype of the Kilogram.¹${}^{:127}$

Non-SI-units that continue to be widely used include:¹${}^{:140}$

• the tonne (t) (or “metric ton”), equal to 1000 kg
• the dalton (Da) or unified atomic mass (u), equal to 1/12 of the mass of a free carbon-12 atom, approximately 1.66 × 10${}^{-27}$ kg. The dalton is convenient for expressing the masses of atoms and molecules.

Other units of mass include:

• the electronvolt (eV), a unit of energy, used in high energy physics to express mass in units of GeV/c${}^2$ (1.78 × 10${}^{-27}$ kg) through mass–energy equivalence,²${}^{:4}$
• the pound (lb), a unit of mass (about 0.45 kg).³${}^{:C-20}$
• the Planck mass (2.176 434 (24) × 10${}^{-8}$ kg ‍⁴) a quantity derived from fundamental constants
• the solar mass (M${}_{☉}$), defined as the mass of the Sun, primarily used in astronomy to compare large masses such as stars or galaxies (≈ 1.99 × 10${}^{30}$ kg)⁵

Printed 2026-06-28.

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Footnotes

1. The International System of Units (PDF), V4.01 (9th ed.), International Bureau of Weights and Measures, June 2026, ISBN 978-92-822-2272-0 ↩ ↩²

2. Perkins, Donald H. (2011). Particle astrophysics. Oxford master series in physics Particle physics, astrophysics, and cosmology (2. ed., reprint ed.). Oxford: Oxford Univ. Press. ISBN 978-0-19-954546-9. ↩

3. Chavez Baucom, Isabel; Benham, Elizabeth J; Konijnenburg, Jan; Lee, G Diane; Lippa, Katrice A; McGuire, John T; Minnich, Loren B; Williams, Juana S (6 January 2026). Specifications, tolerances, and other technical requirements for weighing and measuring devices:: as adopted by the 109th National Conference on Weights and Measures (PDF) (Report). Gaithersburg, MD: National Institute of Standards and Technology (U.S.). doi:10.6028/nist.hb.44-2026. ↩

4. “2022 CODATA Value: Planck mass”. The NIST Reference on Constants, Units, and Uncertainty. NIST. May 2024. Retrieved 18 May 2024. ↩

5. Prša, Andrej; Harmanec, Petr; Torres, Guillermo; Mamajek, Eric; Asplund, Martin; Capitaine, Nicole; Christensen-Dalsgaard, Jørgen; Depagne, Éric; Haberreiter, Margit; Hekker, Saskia; Hilton, James; Kopp, Greg; Kostov, Veselin; Kurtz, Donald W.; Laskar, Jacques; Mason, Brian D.; Milone, Eugene F.; Montgomery, Michele; Richards, Mercedes; Schmutz, Werner; Schou, Jesper; Stewart, Susan G. (2016). “Nominal Values for Selected Solar and Planetary Quantities: IAU 2015 Resolution B3”. The Astronomical Journal. 152 (2): 41. arXiv:1605.09788. Bibcode:2016AJ…152…41P. doi:10.3847/0004-6256/152/2/41. ↩

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