Massiühiku

Massiühik (mass unit) is a standard by which the mass of an object is measured. In physics, mass is a scalar quantity representing the amount of matter in an object and is distinct from weight, which depends on gravity. A mass unit provides a consistent way to compare masses across different experiments and applications.

The SI base unit for mass is the kilogram (kg). Since 2019, the kilogram is defined by fixing the numerical value of the Planck constant h to exactly 6.62607015×10^-34 J·s, with the unit realized through devices such as the Kibble balance or the silicon-sphere method. Prior to this redefinition, the kilogram was defined by a physical artifact, the International Prototype of the Kilogram (IPK), kept in Sevres, France. The gram (g) is a derived unit equal to one thousandth of a kilogram, and the tonne (t) is 1000 kilograms.

In addition to SI units, several non-SI mass units have been used historically or in specific contexts. The pound (lb) and ounce (oz) are common in some countries, while the stone (st) is used in the UK for human body mass. In scientific disciplines, the atomic mass unit (amu or dalton) equals 1/12 the mass of a carbon-12 atom and is approximately 1.66053906660×10^-27 kilograms. The slug is an imperial unit of mass used in some US contexts, with 1 slug roughly 14.5939 kilograms.

Estonia and most other countries use the SI system, so the primary mass unit in education and industry is the kilogram, with gram and metric tonnes as practical submultiples.