Mass and weight are closely related because mass affects the weight of an object experiencing the effects of gravity. Weight is the measure of the force of gravity on an object's mass, while mass is the measure of how much matter there is in an object.

An object's weight is calculated as the mass of an object in kilograms multiplied by the acceleration of gravity exerted in meters per second squared. The resulting number is expressed in newtons, named for Newton's second law of motion. An object's mass is determined by comparing it to an object with a known mass on a balance. An object with a mass of 1 kilogram would weigh 9.8 newtons on Earth, yet it would weigh essentially nothing in space while still having a mass of 1 kilogram.