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To help compare different orders of magnitude, the following lists describe various mass levels between 10−59 kg and 1052 kg.
The table below is based on the kilogram (kg), the base unit of mass in the International System of Units (SI). The kilogram is the only standard unit to include an SI prefix (kilo-) as part of its name. The gram (10−3 kg) is an SI derived unit of mass. However, the names of all SI mass units are based on gram, rather than on kilogram; thus 103 kg is a megagram (106 g), not a “kilokilogram”.
The tonne (t) is a SI-compatible unit of mass equal to a megagram, or 103 kg. The unit is in common use for masses above about 103 kg and is often used with SI prefixes. For example, a gigagram or 109 g is 103 tonne, commonly called a kilotonne.
Other units of mass are also in use. Historical units include the stone, the pound, the carat, and the grain.
For subatomic particles, physicists use the mass equivalent to the energy represented by an electronvolt (eV). At the atomic level, chemists use the mass of one-twelfth of a carbon-12 atom (the dalton). Astronomers use the mass of the sun (M☉).
Unlike other physical quantities, mass-energy does not have an a priori expected minimal quantity, as is the case with time or length, or an observed basic quantum as in the case of electric charge. Planck’s law allows for the existence of photons with arbitrarily low energies. Consequently, there can only ever be an experimental lower bound on the mass of a supposedly massless particle; in the case of the photon, this confirmed lower bound is of the order of 3×10−27 eV = 10−62 kg.