Although often used interchangeably, weight and mass are two distinctly different units of measure. Mass is the measure of an object’s resistance to changes in either the speed or direction of its motion. The mass of an object is not dependent on gravity and therefore is different from, but proportional to its weight.  Weight is the measure of the force generated by the mass of an object being acted on by gravity. In other words, the difference between mass and weight is that mass measures how much matter comprises an object, while weight measures gravitational pull.

A crucial difference between mass and weight is that weight varies by location and mass is constant. For example, a 100-kilogram test weight on Earth will have the same mass property on Mars. A 100-kilogram calibration weight will weigh 100 kilograms on Earth, but only 37.7 kilograms on Mars. The matter, or mass, of the weight did not change; the gravitational pull acting on it did.

Additionally, weight and mass are often thought to be measured in the same way. Because weight and mass do not have a significant difference in commonplace use on Earth, people often use the same unit of measure to express them. However, another difference between mass and weight is that because weight is a force, it should be measured in Newtons, and mass should be measured in pounds or kilograms.

Weight (w) = Mass (m) x the acceleration of gravity (g)

The numerical weight on a scale would then have to be converted to pounds or kilograms. To avoid confusion in most sectors on Earth (besides scientific), weight is commonly expressed in pounds or kilograms.  

For more information about whether to use a balance or a scale to measure weight vs. mass, read Rice Lake’s article “Do You Know the Difference Between a Balance and a Scale?”