What is the SI Unit of Mass?

SI unit of mass is the kilogram (kg)

The SI unit of mass is the kilogram (kg). The kilogram is one of the seven base units in the International System of Units (SI) and is defined by a specific physical object and a fundamental constant.

Mass is a fundamental property of matter that quantifies the amount of substance in an object. It is an intrinsic property, meaning it does not depend on the location of the object and is distinct from weight, which is the force exerted on an object due to gravity.

As of May 2019, the kilogram is defined using a fundamental constant of nature known as the Planck constant (ℎ). The definition is based on the Planck constant, the speed of light (c), and the frequency of a particular atomic transition. This definition replaces the previous definition, which was based on a physical prototype mass called the International Prototype of the Kilogram (IPK).

The current definition is expressed as:

ℎ = 6.62607015×10⁻³⁴ J⋅s

The kilogram is then defined using this constant and other constants:

1 kg = ​ h/(6.62607015×10⁻³⁴ J⋅s/kg)