Mass is defined as the quantity of matter in an object which determines the strength of its mutual gravitational attraction to other bodies, its resistance to being accelerated by a force, and in the theory of relativity gives the mass–energy content of a system. The SI unit of mass is the kilogram (kg).

Mass is not the same as weight, even though we often calculate an object's mass by measuring its weight with a spring scale instead of comparing it to known masses. An object on the Moon would weigh less than it would on Earth because of the lower gravity, but it would still have the same mass.

Mass can be measured in several ways including: Inertial mass (measures an object's resistance to being accelerated by a force), Active gravitational mass (measures the gravitational force exerted by an object), Passive gravitational mass (measures the gravitational force experienced by an object in a known gravitational field), and  Mass–energy (measures the total amount of energy contained within a body).

vCalc's Mass folder contains equations and constants that pertain to the measurement of Mass.