Mass (symbolized m) is a dimensionless quantity representing the amount of matter in a particle or object. The standard unit of mass in the International System (SI) is the kilogram (kg).

Mass is measured by determining the extent to which a particle or object resists a change in its direction or speed when a force is applied. Isaac Newton stated: A stationary mass remains stationary, and a mass in motion at a constant speed and in a constant direction maintains that state of motion, unless acted on by an outside force. For a given applied force,large masses are accelerated to a small extent, and small masses are accelerated to a large extent. The following formula applies:

F = ma

where F is the applied force in newtons, m is the mass of the object or particle in kilograms, and a is the resulting acceleration in meters per second squared. The mass of an object can be calculated if the force and the acceleration are known.

Mass is not the same thing as weight. Weight has meaning only when an object having a specific mass is placed in an acceleration field, such as the gravitational field of the earth. At the earth's surface, a kilogram mass weighs about 2.2 pounds, for example. But on Mars, the same kilogram mass would weigh only about 0.8 pounds, and on Jupiter it would weigh roughly 5.5 pounds.

When expressing large or small masses, prefix multipliers are used.The table below shows the most common alternative mass units and their relationship to the kilogram.