To help compare different orders of magnitude, the following lists describe various mass levels between 10<sup>−67</sup> kilograms (kg) and 10<sup>52</sup> kg. The least massive thing listed here is a graviton, and the most massive thing is the observable universe. Typically, an object having greater mass will also have greater weight (see mass versus weight), especially if the objects are subject to the same gravitational field strength.
The table above is based on the kilogram, 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<sup>âÂÂ3</sup> 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 10<sup>3</sup> kg is a megagram (10<sup>6</sup> g), not a *kilokilogram.
The tonne (t) is an SI-compatible unit of mass equal to a megagram (Mg), or 10<sup>3</sup> kg. The unit is in common use for masses above about 10<sup>3</sup> kg and is often used with SI prefixes. For example, a gigagram (Gg) or 10<sup>9</sup> g is 10<sup>3</sup> tonnes, 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 of 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 ().
Unlike other physical quantities, massâÂÂenergy does not have an a priori expected minimal quantity, 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 upper bound on the mass of a supposedly massless particle; in the case of the photon, this confirmed upper bound is of the order of = .