The standard International System of Units (SI) unit of mass is the kilogram (kg). The kilogram is 1000 grams (g), first defined in 1795 as one cubic decimeter of water at the melting point of ice. Then in 1889, the kilogram was redefined as the mass of the international prototype kilogram, and as such is independent of the meter, or the properties of water. As of January 2013, there are several proposals for redefining the kilogram yet again, including a proposal for defining it in terms of the Planck constant.
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Units of Mass: gram (gm), microgram (µg), milligram (mg), centigram (cm), decigram (dg), dekagram (dag), hectogram (hg), kilogram (kg), megagram (Mg) or metric ton (t), grain, pennyweight (dwt), ounce (oz), pound (lb), dram (dr), short & long hundredweight (cwt), short & long ton, scruple (s), carat (c), gamma (y).
Other units are accepted for use in SI:
- the tonne (t) (or “metric ton”) is equal to 1000 kg.
- the electronvolt (eV) is a unit of energy, but because of the mass–energy equivalence it can easily be converted to a unit of mass, and is often used like one. In this context, the mass has units of eV/c2. The electronvolt is common in particle physics.
- the atomic mass unit (u) is 1/12 of the mass of a carbon-12 atom, approximately 1.66×10−27 kg.[note 2] The atomic mass unit is convenient for expressing the masses of atoms and molecules.
Outside the SI system, other units include:
- the slug (sl) is an Imperial unit of mass (about 14.6 kg) similar to the kilogram.
- the pound (lb) is a unit of both mass and force, used mainly in the United States (about 0.45 kg or 4.5 N). In scientific contexts where pound (force) and pound (mass) need to be distinguished, SI units are usually used instead.
- the Planck mass (mP) is the maximum mass of point particles (about 2.18×10−8 kg). It is used in particle physics.
- the solar mass (M☉) is defined as the mass of the sun. It is primarily used in astronomy to compare large masses such as stars or galaxies (≈1.99×1030 kg).
- the mass of a very small particle may be identified with its inverse Compton wavelength (1 cm−1 ≈ 3.52×10−41 kg).
- the mass of a very large star or black hole may be identified with its Schwarzschild radius (1 cm ≈ 6.73×1024 kg).